50 Farad 18 volt capacitor. Yoicks! - Page 2 - diyAudio
 50 Farad 18 volt capacitor. Yoicks!
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 20th May 2003, 03:26 AM #12 diyAudio Member   Join Date: Oct 2002 Location: south amboy nj 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.
diyAudio Member

Join Date: Nov 2002
Location: Michigan, USA

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

 23rd May 2003, 02:24 AM #17 diyAudio Member     Join Date: Nov 2002 Location: NZ 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
 24th May 2003, 04:21 AM #18 diyAudio Member   Join Date: Jun 2001 Location: Alexandria, VA USA 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..)
 25th May 2003, 05:51 AM #19 diyAudio Member   Join Date: Oct 2002 Location: south amboy nj 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
 25th May 2003, 08:31 AM #20 diyAudio Member     Join Date: Jan 2003 Location: Dortmund, Germany 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 __________________ ************************ A Sacrifice For Freedom

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