50 Farad 18 volt capacitor. Yoicks! - Page 2 - diyAudio
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Old 20th May 2003, 03:25 AM   #11
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Lesson 5
In our last lesson we learned that caps actually store charges on their plates. And of the 1960 joules stored in a 20 Farad cap that 1000 of them sit at a potential below 10 volts. This means there is no way they can ever be used by an operational audio system. Today we will look at another loss factor. It has to do with the loss factor due to the ESR of the cap. We have already studied voltage drop due to ESR but now lets view it from an energy/watts standpoint. Lets clarify things. The power delivered to the stereo by the battery and alternator bypass the cap. They merely flow by it's terminals. If the cap charge is lower than the battery/alternator potential current will flow INTO the cap until it reaches equilibrum with the Batt/Alt. If the B/A potential is lower than the charge potential of the cap current will flow OUT of the cap to the battery and or the amp. Always remember that voltage always flows from the highest potential to the lowest potential just like water. Current does not however flow into the alternator even if it is lower than the battery and cap because it has diodes on it's output that only let current flow FROM it's output. Now whenever any current flows into or out of the cap it must pass thru the ESR of the cap. The resistance is really distributed throughout the cap but it behaves just like it were right on the output terminal as in a series circuit location in the circuit loop does not matter. Now suppose our 20 farad cap is charged to 14.2 volts and we place a load on it's output. This load is the same one that we used in lesson 2 to cause 100 amps of current to flow from our unlimited capacity cap. Only now we have our smaller 20 farad cap. We know that if 100 amps of current flows out of our cap that 1.7 volts will drop across the ESR of .017 ohm. This will cause the output to drop to 12.5 volts just like it did with the unlimited cap. This means that the load (100 ohm res)will be consuming 1250 watts from our cap. 12.5 volts x 100amps = 1250 watts. The total wattage output produced by the cap is 1420 watts. 14.2 volts x 100 amps = 1420 watts. Unfortunately 170 watts of power will be lost in heat in the ESR of the cap. This represents a loss of 13% of our total usable joules (960) at this point. Now tonights question is if we increase the current draw to 300 amps (300amps x 14.2volts = 4260 watts), how many watts will be dissapated in the ESR of the cap and what percentage of the total 4260 watts does it represent? Of our total usable 960 joules, what percentage will be left for the stereo?
Well i finally made it back from the west coast and i will try to get back on track. For now a few comments. Guys like David said---we are not pushing the frontiers of science here. Yes it is true every day that there are some really bright people discovering a lot of things about physics. These guys are studying the behavior of black holes, top quarks, and superconductors at ablsolute zero. They are discovering NEW physics laws. The stuff we are playing with the smart folks figured out over 100 years ago. Just because some of us don't understand the oldest laws on the books doesn't mean anything. I'm sorry but I just have to feel sorry for someone who thinks that an amp that doesn't exactly double it's power when it's load is halved is a contradiction of physics. Believe me it is an iron clad example of physics. In the future perhaps I will cover that, but for now I still have the cap thing. Just remember when we do get to the amp thing it will be interesting when I show how much understanding ESR is a part of that issue. It is easy to see how someone who doesn't understand this fundamental subject could be wrong about caps AND amps. Now for your comments--------Indysubaru. Your comments about infinite current are true but only in a lab exercise. Ideal caps in theory have no ESR. A real cap has esr. The voltage drops across the ESR are instantaneous. It can be modeled exactly like a resistor in series with the output. Of course there are no perfect resistors and all of them have minute amounts of ESL which makes things even worse but that is another sesson.------genIIgn----It is true that the farad ratings of these giant caps is really what they say it is. At least for the ones I have checked. But that is part of the problem. In order for that much capacatance to be in such a small space the physical design leads to high esr and that defeats the purpose in high current applications like a car. Remember in my lessons that the loss in the esr goes up by the square as current draw is increased. If you look inside many cell phones there are capacators the size of pencil erasers that are 1,2 or even 3 farads. They are of course usually only 5 volt devices but their size is dimunitive. This is achieved by giving up what isn't important in low drain applications----low esr. Earthquake-------your analogy is so off base I don't think it is worth responding to. If any one is even half way following me they should be able to see that you are totally lost. You don't seem to get it. There is no need to be defensive about the caps. I haven't started my conclusion yet. All I have done to date is teach the basic electrical theory. I want everyone that really cares to understand this issue so it will be settled once and for all. Sorry that the obvious is coming to light before I get to tie it all together. As far as trying to tell me about caps I guess you haven't been around car audio very long. You look pretty silly telling me about power supplies and caps. Do you not know why everybody uses caps to start with. Heres a clue------Tech Briefs april/may 91---dec/jan 92----april/may92----aug/sep 92----dec/jan93---oct/nov93 just for a start. Dawright----don't get so concerned with the battery vs cap thing. I am only at this point trying to put the things that matter in perspective. You are jumping to conclusions and they are wrong. Pay close attention and be patient. Your analogies are seriously flawed. Power companies use capacators to correct the power factor of the lines. Usually this calls for placing them in series with the ac lines so they will act as conjugate elements for inductive loads (such as motors) that are more common than capacative loads. This corrects the phase angle relationship between the voltage and current to make the load appear more resistive in nature to the generator----making the generation and transmission more efficient. This has nothing to do with a stiffening application in car audio. As far as the bridge application that is remotely similar. But it requires that the cap at sometime charge to the highest voltage peak and that it can hold enough energy to fill in the ripple. In this application it is critical that the ESR not cause a voltage drop equal to or greater than the charge--------think about this---it will hit you like a ton of bricks when it sinks in.-------And overkill----I know from calls and faxes that you are not alone but of the folks that are posting you do seem to be one of the few that are following this thing and keeping up, kudos---- stay tuned it will only get better---------------RC
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Old 20th May 2003, 03:26 AM   #12
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Lesson 6
Ok before the next lesson lets review lesson five. When I checked the posts no one had the correct answer of 56% but some were close. The important part is that everyone seems to understand the loss mechanism. From lesson five we see that the energy we can get out of a cap is inversely proportional to the rate that we try to take it out. This is because the ESR that is in series with the output stays constant regardless of the load. At very high power levels, this ESR can amount to a sizeable amount. In an earlier lesson we learned that the ESR causes a voltage drop proportional to current flow. When voltage is dropped across a resistance heat is created. Lesson five taught us that with 100 amps (flowing from a cap with .017 ohm ESR) we lose 13% of our joules as heat when we try to remove them. If a cap has an ESR of .017 ohms, and 300 amps flows we will lose 56% of the stored energy when we try to remove it. In our giant cap example with 300 amps of current, we will lose this as 1530 watts of heat. This is the same loss mechanism that causes a battery or amp or powersupply to get hot when they are delivering high power levels. Virtually all voltage sources have at least some ESR. At this point we should have a good understanding of how ESR affects a component. The next logical thing to cover is ESL.
ESL stands for equivalent series inductance. Just like the ESR it can be modeled as an inductor in series with the output of our capacitor. Now everyone in car audio knows what inductors do. They resist a change in current flow. Their most common use is in speaker crossovers. When used in series with a woofer they let the slowly changing low frequencies pass, but stop the fast changing high frequencies. The reason an inductor does this is because it behaves like a resistor that changes value with frequency. Unlike a capacitor that decreases in value with increasing frequency an inductor decreases in value with decreasing frequency. Now I have been told that the ESL value of the giant cap is 0.2 mh. Somebody check my math but I think this would put the reactance of the cap near .063 ohms at 50 Hz. This means that if we wanted to refresh our amps at a rate of 50 Hz (seems reasonable if we were playing bass real loud) our ESL of .07 ohm would be in series with our .017 ohm ESR for a total value of .08 ohms. Now we know from ohms law that if we try to get 100 amps through .08 ohms we will have a voltage drop of 8 volts and at 300 amps the drop would be about ………………..well it’s pretty clear that we will be left with less than a fraction of a volt if we start out with only 14.2. Everybody still with me?????????? I know it’s not good news but I’m not making this stuff up.
Now for tonight’s lab lesson to prepare us for lesson 7 Tomorrow I will post the results of the following test. If you want to check me go to Radio Shack and buy the following. Bulb # 272-1127. Socket # 272-360, and a nine volt alkaline battery. For the battery a Radio Shack is ok but a Duracell is better. Make sure it is fresh!!!!! Wire the socket and connect it to the nine volt battery and record how long the bulb stays lit. Be prepared to wait for a couple hours. Charge a giant cap to 14.2 volts and do the same with it. Be prepared to wait about an hour. Charge a 1 or 1.5 Farad cap to 14.2 volts and do the same. This will take only a few minutes. Record the times and we will discuss the importance of this in our next lesson.
Lesson 7
Ok in last lesson I left everyone with instructions to duplicate the results of the test I am going to post tonight. The purpose of this test was to put the capacity of even a giant cap in perspective. As I have pointed out in earlier lessons storing electrons in the form of a charge on a plate is not really very efficient. Some folks think we should stand in awe of a value like 2000 Joules. Well our test tonight puts some reality in this value. If we perform a test like described in the end of lesson 6 we come up with the following results.
1.5 Farad cap lights the bulb for about …………5 minutes and 28 seconds
a giant cap lights the bulb for about……………. 54 minutes
a nine volt alkaline does so about …………………. 2 hours and 14 minutes
did anybody get results similar to these…….are we in agreement on these numbers ?
as for the relationship of these numbers, each of these units has a higher ESR than the previous one. The highest esr in the group was the nine volt battery. It actually has enough energy to light the bulb far longer but since its esr is fairly high it loses a lot of its energy as heat internally. But even still it should be apparent that it holds more energy than the giant cap and a whole lot more than a 1.5 farad unit
For now I do not care to concern ourselves with the meaning of this ---we will cover it in the closing. Before going on let’s review a few facts. In lesson 3 we learned that a giant cap can hold 1960 joules at 14 volts. In lesson 4 we learned that only 960 of them sit at a potential above 10 volts. In lesson 5 we learned that if we want to use them at a rate of 100 amps we will lose 13% of the 960 that are left. If we use them at a rate of 300 amps we will lose 56% of the 960 which will leave us with only about 500 usable joules. And these losses are only for the ESR mechanisms—they do not include the ESL mechanisms that could actually be higher if the demands are quick enough.
It has been suggested that the purpose of these giant caps is to provide quick energy. It has also been suggested that they are for slow energy. I am not sure what is being claimed so I guess I need to cover both situations. As for slow energy I think the previous test could put that thought out to pasture. For long term energy one of these units is less useful than a nine volt battery and to compare it to a car battery is really useless. After all what good is 500 useable joules when we have over 2 million in the car battery? It should be obvious if one of these devices can be of any use at all it will have to be able to provide energy faster than a car battery. But before we get to that issue lets cover the behavior of alternators and batteries under dynamic load conditions.
Tomorrow is Saturday and I will have time to measure the response time of a few alternators. This will enable me to model my closing explanations more exactly. I will post the results of these tests tomorrow night.
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Old 20th May 2003, 03:29 AM   #13
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Lesson 8
For this lesson I have done some actual measurements. Here are the test conditions: To measure voltage we used an Audio Precision with a DCX module. It is accurate to three decimal places. For sample time we chose 40 samples per second. For the non audio system test I used a KAL carbon pile load tester. It can do power tests on 12 volt charging systems up to 1200amps continuous. The audio system consisted of a couple of Rockford 1100 amps bridged into four ohm nominal speakers. The alternator was a stock Delco 80 amp CS type unit. Its case temperature was monitored by a Raytek ST2L IR sensor. Engine speed was regulated with a Thexton #398 IACV tester. The music material was the SPL track # 30 from the IASCA competition disc. The battery was a Stinger spb-1000. All voltage measurements were done directly at the terminals of one of the amps.
Chart 1 Alternator/cap/batt test with 200 amp dummy load
For this test we monitored the voltage of the car with the stereo turned off. With the car running the voltage can be seen to be stable at about 13.7 volts. After 22 seconds (The horizontal scale is 100 seconds-10 sec per major division) we applied a 200 amp load. The voltage can be seen to drop to 11.6 on both traces. This test obviously exceeds the ability of the alternator to keep its regulation set point so its voltage falls. The drop can be seen to be nearly instant (steep curve) until about 12.5 volts where the battery starts to supply a significant amount of the power. Ultimately the voltage drops to 11.6 and at 26 seconds we turn off the load. The voltage then starts to rise to the regulator set point as the battery is recharged (yellow curve) and as the battery and cap (green curve) are recharged. At a time of 50 seconds I turn the motor off so the alternator stops. The voltage then droops down to the float voltage of the battery—about 12.7. The only reason for the small difference at 50 seconds is because I couldn’t get the timing of the engine shut-off exactly the same every time. I did it several times and these two are within one second. That was as close as I could get it.
I am able to see no difference from these measurements. There are microscopic differences but I believe they are due to the alternator temperature. Alternator regulators are usually temperature sensitive. As they get hotter they tend to fold back. For this reason we let the unit cool off between each test and closely monitored the case temp throughout the tests. For this reason I believe that none of these measurements are meaningful to more than a couple tenths of a volt.
Chart 2 Music tests with a audio system
Note: Between each test the alternator was allowed to cool and the battery was charged until an automatic charger said it was topped off.
Purple curve---
For our first test we played the system with the engine off and no cap. The result was the purple trace at the bottom. We played the system as loud as we could get it that seemed to produce no audible distortion. This was track 30 of the IASCA disc. It starts off with fairly low level sounds for the first 34 seconds. In order to insure the electrical system was stable we did not start the measurement until we were 20 seconds into the song. This means that our 0 starting point is :20 on the CD counter. The battery was able to maintain it's voltage just below 12.5 until the loud bass hits at 34 seconds (14 seconds into our chart) At this time it dropped to about 11.5 and had a few large variations due to the music. According to the computer calculations (third chart) the average voltage for this test was 11.7volts. This test was done as a baseline for the following tests.
Yellow curve—no cap
For this test the volume was left as it was for the baseline test. The engine was started. Notice that at low volume the alternator was able to maintain about 14 volts. When the loud music hit the voltage dropped to about 12.5 where it remained except for a few short moments where it actually climbed back to over 13.5 volts. The computer averaged calculations for the average voltage during the 100 seconds of this test was 12.973 volts.

Red curve—cap added
This test was identical to the previous test except the cap (15 farad type) was added 6 inches from the amp with 4 gauge wire—no relays or fuses. The red curve seems to overlay the yellow except that the actual peaks don’t rise as fast or as high during the brief quiet moments. I feel this would be due to the alternator having to recharge the cap. The voltage on loud passages hovered around 12.5 volts. The computer averaged calculations for this test show the average voltage to be 12.878 volts. I see no meaningful differences with or without the cap. I certainly don’t see the voltage sitting solid at 14 volts.
One note I might add is that this was a two thousand watt system driven right to clipping and the average voltage stayed above 12.8 with a stock 80 amp alternator. Under these conditions the battery would never discharge!
The green and blue curves were done just for kicks while we had the system set up. In both these tests we turned the volume up until the system was very distorted. This placed a severe load on the alternator and caused the voltage to dip as low as 12 volts. The curves seem to follow each other so closely that unless you have a good monitor it is doubtful you can tell there are two curves. The average voltage for these two curves were both 12.277 and 12.295 volts. If this volume were sustained for very long periods of time this battery would discharge.
Any questions? Please ask -- I will give everyone a chance to ask them before I sum this all up in lesson 9
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Old 20th May 2003, 03:31 AM   #14
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now for some of the questions--lambchop--how big an alternator does someone need--in more than one way I think I have shown that almost any car can do with it's factory alternator with systems up to as much as a couple thousand watts. There are always accesories that are not being run and the duty cycle of music (10%-20%) is such that this is easy to believe. Add to this fact the new generation of switching amps for bass use, and this is more true than ever before. Go back and look at one of the articles where I have done test drives in cars with equipment to monitor the charging system. MARCOS--yes the drop does increase with increased current flow--as the amp draws more power the load it presents on the charging system naturally gets greater--the effect is that the impedance/resistance of the amp's power supply gets lower. The voltage drop in the power source esr is proportional to how much power the amp is drawing--the more power it tries to draw the less it can get because the losses increase--STACKHOUSE--your question about caps filtering noise is academic--of course they filter noise--WHAT IS NOISE ?--it is a moment of change in the voltage value in an otherwise stable voltage reading--usually considered an unwanted one but a change in voltage nevertheless. And yes the cap acts like a shunt, and in theory should act like a low impedance path for those momentary changes to route to ground--so to answer your question --noise filtering IS voltage stabilization. MAX88 as for as the esl/esr ratio of importance this is dependent on the frequency of interest--at very low frequencies and DC the esl in a nonexistant parameter. As we go up in frequency we reach a point where even small amounts of esl can dominate the esr. Even so the two are always in series and they BOTH play a role at high frequencies. What really matters is that unless we can define at what frequency we are trying to remove energy from the cap we cannot define the relative importance of the two parameters--but if the esr is high enough to limit things it will do it at all frequencies--in the tech briefs we used to say that almost any esl--even that present in a piece of wire could be detrimental--and yes a lot of the current crop of caps do have esl too high to be of any good but that doesn't stop them from selling to those who don't know better. But I doubt that I will be able to do such a test as I have really ruffled some feathers with this cap thing. CHEV2--if a cap had 10mh of esl it would be useless for anything in a car--there is no need to do any figuring-heck to have that much inductance the thing would just about have to have coils of wire in it--i've seen crossovers with coils that were no more inductance than that--how much current you gonna get through several ohms at 12 volts? OVERKILL--yes a battery can produce enormous currents across it's terminals if it is loaded enough. If it is charged and you place a large copper bar across the terminals (do't try this at home it could be dangerous) the terminal voltage could be only a few tenths of a volt but the current could be a couple thousand amps---you see all the voltage would be dropped across the batteries internal esr but all the current would flow through the esr AND the copper bar as they are in series and as we know in a series circuit the current is equal everywhere, but not the voltage--the voltage is the total in all the parts. Sure short the giant cap-it may produce hundreds of amps but what good are they if the voltage is too low for them to be used--get it ? BRAD--if a 12 volt car battery had a esr of 1 ohm it would only be good for a couple of amps before the voltage dropped below 10 volts--it couldn't even run a factory head unit-- I suggest you go back to the first two lessons and review this --caps, batteries, amps, or any voltage source- the formulas are still the same. As for as the esl of the giant cap I used Cades numbers--I wasn't going to complicate things--besides the esr was so high we didn't even need to consider the esl. BRAD-the need for large wire is for low resistance --the caps in the input of virtually amps integrate the pulses so that they appear to be DC in nature so the current flow is somewhat continuous and at least at a relatively low frequency--if the caps were removed even small amounts of inductance would prevent the amps from working..These type caps are selected for low reactance at high frequencies..... Overkill--the reason the esr is not a problem for the rest of the system is because the esr actually serves to isolate the cap from the system--to understand this carry the thought to the extreme--suppose you had a load of some sort and you paralled it with the system--but you isolated it with a series resistor of i meg ohm--the system would not even know it was there--suppose you carried this thought to the ablolute extreme and you isolated the load by an infinite ohm amount--say it was sitting in the next room on a shelf and not even hooked up--how much problem could it cause? see the thought pattern?--it is the esr/esl that isolates the device from the system/not hurts it.--------tomorrow--we wrap things up


Now that we have had time to study theory in each of the 8 lessons and the results of the actual tests on a real system it is finally time to bring this discussion to a close. Unfortunately, when this thread started I was unable to explain the concept, as it was obvious that many of the people posting responses just didn’t have a good grasp of the way things really work. Those of you who have taken the time to follow the lessons should know by now why I was so frustrated at the arguments that were so illogical. It is important to keep in mind that this is a technical forum, not a marketing forum. I do not care or want to know about companies or brand names. Nothing I have said was ever meant to disparage a particular product or company and I would appreciate it if in the future we could always keep that in mind. We should be able to discuss the merits of radial vs bias ply tires without caring if they are made by Michelin or Goodyear.
In car audio we have little choice of how we are going to power our systems.
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Old 20th May 2003, 03:32 AM   #15
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Default last one.

Presently we have only four things that are practical. Each of them has it’s own characteristics that incorporate good points and bad points. Lets review them. The battery--this device has the ability to provide a very large amount of current. But due to it’s nature the current is provided at a voltage that is less than optimum –at least for a high powered stereo. Since it’s float point is 12.8 volts if fully charged, it can provide current only at voltages that are proportionally lower than 12.8 v. The alternator—this device is electronically regulated at a point that allows it to recharge the battery. The alternator is usually designed to output voltage in the 13.8 to 14.5 volt range. Because it’s output is actively regulated it attempts to maintain this voltage with varying load conditions up to the point where it’s output cannot keep up with the load at which time it’s output drops off very rapidly. While relatively tight regulation is the strong point of the alternator it’s weak point is that it simply is not practical to obtain one that can provide large amounts of current like a battery is capable of. The third device we can use is a cap. The advantages of a cap are that it can charge up to whatever the highest voltage source in the system is, (in a car this would be the alternator) and provide current at this elevated voltage. The down side of a cap is that it cannot store very much total energy and only a portion of this energy is available at a usable voltage potential. The fourth type of device is an electronic voltage regulator. These devices have not been part of this discussion so I will pass over them for now.
Now modern car audio amplifiers are capable of consuming enormous amounts of power. Even with efficiencies in the range of 60% to 90% an audio system is capable of drawing hundreds or thousands of amps from the cars electrical system. Typically, the audio system is larger than any other electrical device in the car including the engine starter. Fortunately for the car, the demands of an audio system are rarely continuous in nature. The very nature of music rarely demands more than a duty cycle of 10% to 20% from a power standpoint. This means that the audio system is demanding short term, but repetitive peaks of current from the electrical system.
The primary source of this power is the alternator. It should be considered primary for two reasons. The alternator is the only first generation source of power. It ultimately provides all the power for the system either directly, or indirectly by restoring power to the battery or cap. It is also primary as it is the power source with the highest voltage potential. In an electrical system current always flows from the source of highest voltage to circuits of lower of lower potential.
All three devices can be used in a system to great advantage. But the dynamic conditions present in a music system determine the role each device plays and to what degree. To understand this lets consider a low current drain condition. In this scenario the alternator will be at or near it’s set point. This voltage is designed to be high enough to charge the battery meaning it will be one or two volts above 12.8 volts. This means that the battery will actually be a continuous load on the alternator and provides no power to the system. The size of load it presents is determined by the state of charge of the battery. The higher it’s state of charge the smaller the load will be. A cap if present in a system in this state will present a load for a finite amount of time until it’s charge voltage reaches equilibrium with the alternator. Unlike the battery, the cap will cease to be a load after it is charged except for a factor known as dissipation, which for all practical purposes can be ignored in this application unless it is excessive. Under these circumstances, as long as the alternator can maintain its set point, it will provide all the power for the music system and the rest of the cars accessories. The battery and cap may as well not even be in the car.
Now if we increase the current demands of the music system to an amount that taxes the alternator it’s output voltage will begin to drop. Even so the alternator will continue to be a source of current to the system –ie. the car, music system, and battery. It is at this time that the cap will begin to discharge and begin to augment the alternator as a source of current. The degree to which it provides current to the system is dependent on the actual voltage at the alternators terminals. Only when the alternator begins to drop below the caps charge potential does current flow out of the cap. This is a continuous process and the current provided by the cap tries to maintain the voltage at it’s charge potential. The degree to which it can do this is dependent on two things. The current provided by the cap is limited to the total capacity of the cap and any series reactance’s (resistive or inductive components) that are part of the cap. The instant the cap starts to output current it’s charge potential begins to drop.
Now just what can we expect the cap to provide. Suppose we happened to have a cap charged to 14 volts, with a total reactance (made up of either resistive or inductive components) of about .017 ohm. We could figure that at the first instant of discharge it could provide ten amps at 13.83 volts. Of course if we were playing the system at a level enough to load our alternator, ten amps is not likely to provide much relief. But perhaps 30 amps might help—at this modest level our cap could begin to provide current at a potential of 13.5 volts. (lesson 2). Of course this voltage level would drop at an exponential rate commensurate with the discharge curve that is standard with caps. No doubt the cap could help out a hundred amp alternator with the addition of an extra 30 amps even though it might be for only a brief instant. But it is sort of interesting that at even this modest power level of 130 amps (100 amps alternator + 30 amps cap) the cap is unable to maintain the voltage at 14 volts. Of course in this scenario we are sitting at 13.5 volts for a brief instant and our poor battery is unable to help at all as it’s potential is at a lowly 12.8 volts. In fact the battery is still a load on the system!
Now what if we get serious with our stereo, and we really crank it up. Lets say we have something like a manufacturers demo van with lots of amplifiers that can draw hundreds of amps on musical peaks. Lets pick a nice round number like 500 (Cade said 490) amps. Lets say we have a 200 (Cade said 190) amp alternator. Typically such an alternator can maintain a voltage near it’s set point up to perhaps 80% of it’s rating-after which it’s voltage begins to drop as it provides large amounts of current. As I am not familiar with all the different alternators lets just assume these assumptions are close and our alternator is putting out 200 amps. Well our amplifiers in an instant are asking for 500 amps so what happens? In any constant voltage system when the current capability is exceeded the voltage drops. So lets say our alternator voltage starts dropping. What does our cap do? Since it’s charge potential is at 14 volts it starts to discharge and provide a source of current. Since the cap is now sharing the load with the alternator it is called on to provide what the alternator can’t—that would be 300 (see footnote) amps. What happens to the terminal voltage of our cap when 300amps is flowing? Well for starters, the voltage tries to drop nearly 5 volts inside the cap before it can even get out. Not in a short time but instantly. There is no time constant in the formulas for ohms law. They are instantaneous calculations! But wait. The voltage doesn’t really drop to 9 volts because we have our battery sitting in reserve waiting at 12.8 volts. Our cap lets our poor alternator down as the voltage plummets and when things hit 12.8 volts our battery jumps in and starts to take over. The battery with its enormous storehouse begins to provide vast amounts of current until things lighten up for our poor cap and alternator. Of course we could add another cap to halve our ESR loss to only 2.5 volts but that would still cause the cap terminal voltage to drop to 11.5 volts. Lets see how many caps of this spec we would have to add to keep the voltage at 13.5 for even a few milliseconds. We would need a cap bank with a total ESL of about .001 ohm. Gee it looks like it would take over thirty caps paralleled to maintain 13.5 volts at 300 amps for even a brief instant. And lets hope we don’t need to do this for long, as the total power contained in thirty units is only about what is in a dozen 9v alkaline batteries! (lesson 7)
It should be clear that if the voltage doesn’t drop the caps don’t do anything. The voltage MUST drop for them to start discharging. A steady 14 volts because we added caps—I don’t think so.


footnote: in this model for simplicity I am not considering the regulator lag time of the alternator-if we were to consider this the demand would be over 300 amps demanded from the cap--for kicks do the math on 400 or even the full 500 amps---can we say thank goodness for batteries?
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Old 23rd May 2003, 01:03 AM   #16
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Default Re: Re: 50 farad

Quote:
Originally posted by zx3chris
Lesson 1
Ok powertrip how about we have a discussion in basic electrical theory. At the end of this thread...

Whoa, sorry couldn't spare the time to read all the way through the end.

Two questions? Did you take into account the ESL of the cap, or only ESR? How about the duty cycle and switching frequency of the power converter in the amplifier?

Anyway... ...my thoughts on the subject.
A power converter running at 50 Khz with a 50% dutycycle during peak demands will draw current much faster than most large capacitors will be able to supply it. This can be a misleading factor when measuring the input voltage to the amp. When the switching converter actually draws current, series inductance and resistance in the power supply to the amp can cause voltage drops, that will not be detected by the by most DMMs.

I have measured input voltage on amplifiers, with scopes and DMM's. At times the DMM a will read 11 volts (Average), when the input voltage is actually only 9 volts (steady state low of the waveform) when the switcher is actually drawing current. This is because the current draw from the battery is a high frequency current draw (50Khz or so) In this case, the capacitor would need to be fast enough to supply current at the switching frequency, not the music frequency. (40 samples per second will probably not catch the transients)

Most large can type capacitors don't do much good, because they cannot supply power at the frequency of the swithcing currents. The best solution is to add more capacitance to the output side of the switcher just before the input to the amp.

-Dan
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Old 23rd May 2003, 02:24 AM   #17
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Default ooohh



very interesting i read the first two,il come back for the rest later..

makes sense..lol at the dent in the exhaust pipe story..

i need to know the conclusions of this all,il read when i ahve the time :-D:-D

cos everyone in car audio seems to think a huge cap makes it much more better,and i dont buy/do things for no proven reason

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Old 24th May 2003, 04:21 AM   #18
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Default Great read

zx3cris..thanks for posting it. What was the source for this? While I know you said that it was written for folks who know nothing, I get the feeling either they were supposed to know it, or thought they did.

Secondly, If this was open, it would be neat if you could get permission from the author to put it in the wiki. Then folks could just point to it when the next newbie came by asking about "stiffening caps"..

Finally, it was great to see someone actually spend that amount of time to teach the mostly unwilling it seems. I remember my days trying to sell audio. I was not any good at it since I could not let my ethics go so low as to sell useless stuff like these caps. (or any of a thousand other snake oil items..)
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Old 25th May 2003, 05:51 AM   #19
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this was posted years ago in the carsound.com forums.. richard clark (electrical engineering who worked in audio for years and years, and has quite a history---look him up), wrote it.. he has a forum just for him on that site, and will occasionally answer questions so long as they havent been asked 1000s of times.. the oringal post had 100s of replies--i just cut out what he said, and composed that.. u can put it whever it can be useful--its a great explanation in my opinion ;D

-chris
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Old 25th May 2003, 08:31 AM   #20
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Hi!

Well, OK, I understand a lot of the explanantions, and generally I despise everything in the HiFi sector which even faintly smells of "Voodoo", but how do you explain this:

Without my 1F (not those overrated 10+F thingies) cap my lights are actually flashing like a strobo during strong bass bursts.

With the simple addition of the cap (and now here is one thing which I think was not mentioned: the cap has to be put NOT MORE THAN 30cm (cable length, that is) from the main load, in this case my bass amplfifier), the lights stopped flashing entirely!

And while this cap got older, the flashing grew stronger again. Two months ago I exchanged it with a new one, and the flashing was gone again...

That can't be just my imagination, so physically adding such a cap must make a difference.

Just my 2 cents...


Bye,

Arndt
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