I am working on a power supply right now for a guitar pedal, and I have a question regarding the transformer. The power supply has some 9volt outputs and a 12volt output. Now is a 9v AC transformer big enough when rectified and filtered to run a 12volt regulator? I am using a 7812 to be precise. Should I go to the next transformer up (which is 12V AC)?...but then I would get less current output.
Thanks,
Christan
Thanks,
Christan
Hi Christan,
A 9-volt AC RMS transformer output would probably NOT work.
Even if you managed to get 12.72-volt AC peaks out of the transformer (i.e. 9VACRMS x sqrt(2)), a full-wave rectifier bridge would drop that by up to 1.4 volts or so (or maybe by only half that, if you're lucky, if you used Schottky diodes). Then, the main problem: regulator headroom (i.e. allowing for "dropout" voltage plus some margin).
A regulator's input needs to be higher than its output by at least its dropout voltage spec, and preferably somewhat higher. There do exist what are called LDO (Low DropOut) regulators, with dropout specs of less than one volt. But you should probably plan on having a few volts to spare, for the regulator.
Note that the LM7812 datasheet from http://www.fairchildsemi.com/ds/LM/LM7812.pdf shows the typical dropout voltage as 2V, at a 1A output-current level. So at least 15V at the regulator input might be good, for 12V out.
A 12 VAC RMS transformer output should give you roughly (12 x 1.414)-1.4 volts after the rectifier bridge, which would be about 15.6v (or 14.3V with line voltage 10% low: Don't forget to design it so that it's still OK if the line voltage sags by 10% or so, which would make your transformer output sag by the same percentage.)
I would go with a 12-volt transformer. Then, almost any type of rectifier bridge and any type of 12V three-terminal regulator should work OK, to give you a 12-volt DC output.
Note that you could also simulate your power supply, very easily, using the excellent, free LTspice (aka SwCadIII) software, from http://www.linear.com . Then you could quickly and easily actually see what happens with various transformer output voltages. It should only take a few minutes to set up a basic circuit, if you use an ideal voltage source to simulate the transformer output (using ideal sine voltage source with amplitude = 1.414 x the VAC RMS rating of the transformer's output). The program already includes various three-terminal regulator models, as well as diodes, capacitors, resistors, inductors, etc, plus a whole bunch of other stuff (which you wouldn't need for this particular circuit). It allows you to easily draw a schematic and then just click the "Run" button to simulate it. Then you just click the "probe" (mouse pointer) anywhere in the circuit, to see the waveform (or frequency response, if doing AC Analysis instead of Transient Analysis). It's coool.
I can post (or email to you) a ready-to-run LTspice circuit file for your supply, if you want to give it a try.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
A 9-volt AC RMS transformer output would probably NOT work.
Even if you managed to get 12.72-volt AC peaks out of the transformer (i.e. 9VACRMS x sqrt(2)), a full-wave rectifier bridge would drop that by up to 1.4 volts or so (or maybe by only half that, if you're lucky, if you used Schottky diodes). Then, the main problem: regulator headroom (i.e. allowing for "dropout" voltage plus some margin).
A regulator's input needs to be higher than its output by at least its dropout voltage spec, and preferably somewhat higher. There do exist what are called LDO (Low DropOut) regulators, with dropout specs of less than one volt. But you should probably plan on having a few volts to spare, for the regulator.
Note that the LM7812 datasheet from http://www.fairchildsemi.com/ds/LM/LM7812.pdf shows the typical dropout voltage as 2V, at a 1A output-current level. So at least 15V at the regulator input might be good, for 12V out.
A 12 VAC RMS transformer output should give you roughly (12 x 1.414)-1.4 volts after the rectifier bridge, which would be about 15.6v (or 14.3V with line voltage 10% low: Don't forget to design it so that it's still OK if the line voltage sags by 10% or so, which would make your transformer output sag by the same percentage.)
I would go with a 12-volt transformer. Then, almost any type of rectifier bridge and any type of 12V three-terminal regulator should work OK, to give you a 12-volt DC output.
Note that you could also simulate your power supply, very easily, using the excellent, free LTspice (aka SwCadIII) software, from http://www.linear.com . Then you could quickly and easily actually see what happens with various transformer output voltages. It should only take a few minutes to set up a basic circuit, if you use an ideal voltage source to simulate the transformer output (using ideal sine voltage source with amplitude = 1.414 x the VAC RMS rating of the transformer's output). The program already includes various three-terminal regulator models, as well as diodes, capacitors, resistors, inductors, etc, plus a whole bunch of other stuff (which you wouldn't need for this particular circuit). It allows you to easily draw a schematic and then just click the "Run" button to simulate it. Then you just click the "probe" (mouse pointer) anywhere in the circuit, to see the waveform (or frequency response, if doing AC Analysis instead of Transient Analysis). It's coool.
I can post (or email to you) a ready-to-run LTspice circuit file for your supply, if you want to give it a try.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
By my back of the envelope calculation you'll get 12.9V into the regulator. I don't have a datasheet handy but they might be cutting it close for the 7812, but a low dropout regulator would work fine. I would go with the 12V output to make sure there's enough voltage at the input.
Pedals don't use much current, maybe around 10-15mA in the ones I've worked on.
I've seen guys with half a dozen pedals an a single 7809.
Pedals don't use much current, maybe around 10-15mA in the ones I've worked on.
I've seen guys with half a dozen pedals an a single 7809.
You can always use a voltage doubler and then post regulate if you don't want to use a low drop-out regulator.
Then it's much better to use the right transformer to start with. If you take ripple and line regulation into account a 7812 actually needs a 15v transformer to be safe, believe it or not. It needs at least 3 volts to work and at 10% low line you will go out of regulation with a 12V transformer. And that's without even considering rectifier voltage drop or ripple voltage.
Hi,
an easy to remember rule is Vac=regulated Vdc.
As usual there are exceptions at low voltages <=9Vdc add a volt or two to the Vac.
At high voltages, around >=50Vdc one can reduce the Vac by about 10%
an easy to remember rule is Vac=regulated Vdc.
As usual there are exceptions at low voltages <=9Vdc add a volt or two to the Vac.
At high voltages, around >=50Vdc one can reduce the Vac by about 10%
megajocke said:
I agree, about using the right transformer to start with. But we don't want him to to use a higher-voltage transformer than necessary. And it kind of looks like you might have forgotten to convert the mains' RMS voltage to its peak value, at least in your last sentence. But I will have to work through the math, to see if a 12VAC RMS transformer output could be too low for a 12V regulated DC supply:
First: A "15V transformer" output is 15 x sqrt(2) volts zero-to-peak, i.e. about 21.21 VAC 0-P. Allowing 1.4 volts drop for a rectifier bridge, we would be left with a little less than 19.61 volts DC +/- ripple.
The ripple voltage amplitude would be approximately = I/2fC, where I is the DC load current in Amperes, f is the AC mains frequency in Hertz, and C is the capacitance (in Farads) used after the rectifier bridge. If f=60 Hz and I=50 mA, that becomes .000417/C. For an example C of 1000 uF, the ripple amplitude would be about 0.417 V P-P.
With a full-wave bridge, that would change the estimate of the DC voltage at the regulator input downward, by half the ripple amplitude, to Vpeak-(I/4fC), which would be about 19.61v minus about 0.209 v, or 19.4v. Then, the actual minimum voltage, at the troughs of the ripple, would be that average ripple voltage minus half the ripple amplitude; about 19.19 v for that case. And with the AC mains voltage 10% low, the minimum voltage seen at the regulator input would be about 17.27v.
With a 7815 regulator, spec'd with 2V dropout voltage, 17.27v minimum for 10%-low mains voltage might be OK, for such a low load current. Or, the smoothing capacitance could be doubled, to give about 19.5 v minimum regulator input for normal mains voltage and about 17.6 v minimum when the mains voltage was 10% low. If that's not enough headroom (i.e. reg's output sags during input ripple's troughs, or worse), then an LDO 15V regulator could be used. And with a 7812 regulator, under the stated conditions, a 15 VAC RMS transformer output is obviously enough, even for 10%-low (or worse) mains conditions.
For a 12 VAC RMS transformer output, we would get about 12v x 1.414 = 17 V 0-Peak, and about 15.6 Vpeak after a full-wave rectifier bridge's 1.4v drop. Assuming the same 50 mA DC load current, 60 Hz mains, and a 1000 uF smoothing capacitor, the ripple amplitude would again be 0.417 v p-p. The average DC value estimate should then be 15.6 - (1/2)(0.417) = 15.4 v. The minimum voltage seen by the regulator's input, during the ripple's troughs, would then be about 15.2v. In that case, with a 10%-low mains situation, the minimum regulator input voltage would be only 13.68v!
That only leaves 1.68v more than 12v, versus the 7812's spec'd dropout voltage of 2v! (That would probably result in an output voltage averaging about 11.68v, but with all of the input ripple intact. Not good.)
So, if my math was done correctly, and assuming that the 7812 dropout voltage is not significantly lower for 50mA load current versus 2V @ 1A, then you are absolutely RIGHT (Thanks for raising the question, by the way!):
************************************************************
A 12VAC RMS (measured at normal mains voltage) transformer output would NOT
be sufficient for a 12 VDC 7812-regulated output, when mains AC is 10% low,
assuming a 50mA DC load and a 1000 uF smoothing capacitor!
************************************************************
And note that even if the smoothing capacitance were doubled and the load current were halved, giving AC ripple of about 0.1v p-p, the minimum voltage seen at the regulator's input would still dip to about 13.95 volts, with 10%-low AC mains voltage. That's still below the 2V dropout voltage spec of the 7812, never mind the 3V we'd be comfortable with.
For this application, I think that the OP "might" be OK, using a "Low-Dropout" (LDO) 12v regulator (but definitely not a 7812) and a 12v transformer output, assuming the DC load current is low-enough and the smoothing capacitor's value is high-enough.
But I think I'D use a 15V transformer, instead, especially since he alluded to needing significant load current.
Regards,
Tom Gootee
http://www.fullnet.com/~tomg/index.html
This is simplified, voltage will be even lower due to the high peak-to-average ratio:
A 12VAC transformer at load and low line (10%).
Vac = 12*0.9 = 10.8VAC
Vpk = 10.8 * 1.4142 = ~15,3V
A 7812 needs 3V for regulation. 15.3-12 = 3.3V. This leaves 0.3V for the rectifier and the ripple, it won't work.
edit:
Is it 2V? Still, that leaves 1.3V for rectifier and ripple. It might be doable with a oversized transformer and filter caps. But this is just due to the higher unloaded voltage of the transformer.
A 12VAC transformer at load and low line (10%).
Vac = 12*0.9 = 10.8VAC
Vpk = 10.8 * 1.4142 = ~15,3V
A 7812 needs 3V for regulation. 15.3-12 = 3.3V. This leaves 0.3V for the rectifier and the ripple, it won't work.
edit:
Is it 2V? Still, that leaves 1.3V for rectifier and ripple. It might be doable with a oversized transformer and filter caps. But this is just due to the higher unloaded voltage of the transformer.
I think you were quite right, to begin with, and he should just use a 15V transformer output (unless he wants to takes chances and use an LDO 12v regulator and a 12v transformer).
For the OP, triden:
Here are some webpages about power supply design that might be helpful:
http://www.zen22142.zen.co.uk/Design/dcpsu.htm
http://sound.westhost.com/power-supplies.htm
- Tom Gootee
For the OP, triden:
Here are some webpages about power supply design that might be helpful:
http://www.zen22142.zen.co.uk/Design/dcpsu.htm
http://sound.westhost.com/power-supplies.htm
- Tom Gootee
Wow, what an informative thread!
The reason I am bent with sticking with a 12vac transformer is because going to a 15vac gives me much less current output. I intend to run some high current 9v pedals on this at the same time. Sadly, going to the next VA up will increase the size of the transformer quite substantially. For some reason the next VA up is a big step physically which is a hinderace since I am trying to incorporate the trafo onto my PCB.
I am going to look into an LDO regulator that will do 12v...I just hope I can find one that will do about 500mA. I am sure there are some out there. If not, I will see what I can do about fitting a 15vac trafo onto my board.
*******
Update
*******
After looking I did find a 12v LDO from Fairchild. The only problem is that it is a non-stock item from digikey. Maybe I'll have to look into a different distributer. The part number is KA78R12C.
" The dropout voltage of KA78R12C is below 0.5V in full rated current (1A)."
I found the LM2940 in stock at digikey...
Regards,
Christan
The reason I am bent with sticking with a 12vac transformer is because going to a 15vac gives me much less current output. I intend to run some high current 9v pedals on this at the same time. Sadly, going to the next VA up will increase the size of the transformer quite substantially. For some reason the next VA up is a big step physically which is a hinderace since I am trying to incorporate the trafo onto my PCB.
I am going to look into an LDO regulator that will do 12v...I just hope I can find one that will do about 500mA. I am sure there are some out there. If not, I will see what I can do about fitting a 15vac trafo onto my board.
*******
Update
*******
After looking I did find a 12v LDO from Fairchild. The only problem is that it is a non-stock item from digikey. Maybe I'll have to look into a different distributer. The part number is KA78R12C.
" The dropout voltage of KA78R12C is below 0.5V in full rated current (1A)."
I found the LM2940 in stock at digikey...
Regards,
Christan
The ST KD1084 is a 5A LDO with a 1.3V drop out voltage.
Available fixed or adjustable in T0220 and DPAK
Data sheet at:http://www.st.com/stonline/products/literature/ds/8761.pdf
Available fixed or adjustable in T0220 and DPAK
Data sheet at:http://www.st.com/stonline/products/literature/ds/8761.pdf
triden said:
It depends how much current you need, and what size transformer you can fit on your board. Feel free to give us some numbers, whenever you're ready. 🙂
At http://www.mouser.com , and other suppliers, there are quite a few PCB-mount transformers. Mouser has, for example, the Hammond 183-series, which includes models that, with the dual secondaries connected in parallel, will povide 12v at 4.66A, 2.12A, 1.44A, 0.72A, 0.36A, or 0.18A. The largest of those (the 4.66A one), the Hammond 183K24 (Mouser.com # 546-183K24) is 1.81 inches tall and 3 x 2.5 inches when seen from above, but the actual footprint on the board is significantly smaller, with usable space under the "overhang". They also have a similar 14V model with 4.0 Amps rated output. That one is the same size as the 12V/4.66A model. Both of them are about $28 qty 1.
Basically, in the 183-series with PCB-mount pins, they have six Volt-Amp ratings, i.e. 56, 30, 20, 10, 5, and 2.5 VA, and for each VA rating, they have seven possible (dual) secondary voltages, i.e. (if connected in parallel: ) 18v, 14v, 12v, 10v, 8v, 6.3v, and 5v. Or, you can connect the secondaries in series to get twice the voltage with half the max current.
All of the 183-series transformers that have a particular VA rating are the same physical size, and almost all have the same price for the same VA rating, regardless of voltages and currents. These transformers also all have dual primaries, making it easy to switch them between 115v/60 Hz and 230V/50Hz, while getting the same secondary V and A ratings.
Exactly the same transformers are also sold, by other suppliers, with the "Triad" name on them. I don't know who actually makes them. But I have used quite a few of them with no problems.
OK. I just realized that you mentioned 500 mA. In that case, from the Hammond 183-series mentioned above, you could use either the 24v/12V transformer with paralleled secondaries giving 12v @ up to 0.84A, or a 28v/14V xfmr with par sec for 14v @ .72A max, or even a 12.6v/6.3v model with series secondaries for 12.6v @ 0.8A max.
Those three are all 10 VA models, each costing $15.57 qty 1, and each measuring 1.38 inches high and 1.88 x 1.56 inches as seen from above. There is also a 16v/8v 10VA model, which you could use with the secondaries in series, to get 16v @ 0.62A max.
For any of those voltages, you could also go to the next-bigger VA model, to allow for about twice the max current, for about $3 extra and a slightly larger physical size.
You can see all of the Hammond 183-series transformers by going to http://www.mouser.com and doing a search for 546-183.
If you need less than the 1.38-inch height, the 229-series has some 12 VA PCB-mount models that are all 1.07 inches tall, but are slightly larger the other way, at 2.0 x 2.5 inches. That series has some different voltages and current available, and are touted as being semi-toroidal, and their cost is slightly higher. You can also look at the 164 and 162 series. The mouser searches for all of them would start with 546- and then the series number.
Regarding 12v regulators: There are lots and lots of them. I'm probably most familiar with the 5-Amp ST LD1084 adjustable LDO version, in a TO-220 case, which also comes in a 12V fixed LDO version. Their dropout voltage goes down to about 1.06v, at 0.5A current, but can go as high as 1.3v typical/1.5v max, at 5A.
If you can't find an acceptable (e.g. high-enough voltage) fixed-voltage regulator, look at the adjustable-voltage regulators. They only need a couple of extra resistors, and an extra capacitor if you want the lowest output noise, compared to the fixed-voltage models. Whichever regulator you choose, be sure to read the datasheet recommendations for capacitors for the input and output and adjust pins, and for protection diodes across (usually) two of the sets of pins.
And note that for your application, you will not be using a regulator in a TO-92 case. It will have to be one of the larger cases. And you will need a decent-sized heatsink.
BY THE WAY: To pick a proper transformer, you will need to run through the calculations that we did above, again, but with your 500 mA DC load current, or whatever the max load current might be, to recalculate the ripple amplitude, which will give you the minimum voltage seen at the regulator input.
It would probably be a good idea to try to make sure that the regulator's input voltage will always stay above the output voltage by at least 50% (to 100%) more than the dropout voltage, even when the AC mains voltage is 10% low.
Good luck!
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
Hi,
Mega almost got to the important point overlooked by your calculations - transformer regulation!
The output from a small transformer can be 10% to 30% higher than rating when on zero loading.
Now adjust all your calculations for the new peak voltage at mA loadings when mains is 10% low.
12Vac works for regulated 12Vdc.
The other alternative is use an oversized transformer with a lower regulation value. The reduced effective loading is partly compensated for with the lower regulation. The result is once again 12Vac is adequate for regulated 12Vdc.
In all of the above it is assumed the transformer VA is adequate for the power required.
It is normal for the continuous load on a transformer to be less than or about half it's rating. In addition the transformer must be de-rated to account for the capacitor input filter that it feeds after the rectifier.
eg.
12Vdc @1A after the regulator requires about 15V @ 1Adc (15W) before the regulator.
This can be supplied by a 12Vac 2A transformer (24VA).
Note that 12W of regulated output requires as a minimum 24VA of transformer rating. Oversized would be substantially higher than 24VA.
If the quiescent current is around 100mA then the input voltage will be in the range 18.4Vdc (+6% mains, 10% regulation) down to 15.6Vdc (-10% mains, 10% regulation).
On transient current demand these voltages will drop by a further 1V depending on the value of smoothing capacitance and the length of the high current transient. Yes, it gets very close to regulator drop out, but that is only when high current demand coincides with lowest mains voltage.
Is this a single polarity supply? or dual polarity of +-12Vdc?
Dual will have extra overhead of 0.7V due to half the rectifier bridge losses (Vdrop) across each polarity.
Mega almost got to the important point overlooked by your calculations - transformer regulation!
The output from a small transformer can be 10% to 30% higher than rating when on zero loading.
Now adjust all your calculations for the new peak voltage at mA loadings when mains is 10% low.
12Vac works for regulated 12Vdc.
The other alternative is use an oversized transformer with a lower regulation value. The reduced effective loading is partly compensated for with the lower regulation. The result is once again 12Vac is adequate for regulated 12Vdc.
In all of the above it is assumed the transformer VA is adequate for the power required.
It is normal for the continuous load on a transformer to be less than or about half it's rating. In addition the transformer must be de-rated to account for the capacitor input filter that it feeds after the rectifier.
eg.
12Vdc @1A after the regulator requires about 15V @ 1Adc (15W) before the regulator.
This can be supplied by a 12Vac 2A transformer (24VA).
Note that 12W of regulated output requires as a minimum 24VA of transformer rating. Oversized would be substantially higher than 24VA.
If the quiescent current is around 100mA then the input voltage will be in the range 18.4Vdc (+6% mains, 10% regulation) down to 15.6Vdc (-10% mains, 10% regulation).
On transient current demand these voltages will drop by a further 1V depending on the value of smoothing capacitance and the length of the high current transient. Yes, it gets very close to regulator drop out, but that is only when high current demand coincides with lowest mains voltage.
Is this a single polarity supply? or dual polarity of +-12Vdc?
Dual will have extra overhead of 0.7V due to half the rectifier bridge losses (Vdrop) across each polarity.
Alright,
I've decided to go with a 15VA transformer from Amveco. I am using a TE70052 which is a sealed, pc-mount toroid. It has dual primaries and dual secondaries rated at 12vac @ 1.25 amps if they are paralleled.
I chose a bridge rectifier than can handle 4 amps and a 3300uF cap as a filter. I put a 100nF ceramic in parallel with it because I was told this helps with some high frequency rejection that the filter cap may have trouble with.
As for outputs, I have seven 78L09's which are 9 vdc regulators that can handle 100mA max. Then I also have a high current 9vdc regulator for pedals that require about 300mA of current. Finally for a 12vdc option, I went with an LM2940 which is an LDO with a 0.5V dropout at full load (1 amp). All the outputs will never all be used at once, maybe 2 or 3 at a time max, therefore the trafo should never reach it's current limit.
The transformer is rated for 12vdc at full load, which means the voltage may be a little higher most of the time...which is nice.
At full load (worst senario) the trafo should show 12 V RMS. So that is 16.92 V p-p minus 1.4v (diodes) = 15.52 VDC +- ripple (3300uF cap). Line voltage here is about 121 V RMS, but with 10% compensation, output is about 14.3 VDC so that leaves 2.3 vdc minus the 0.5 vdc drop across the regulator. That is at absolute worse and at full load...is that cutting it too close?
I have attached an example of my schematic. Feel free to comment on anything that should be changed. The cap on the input of the reg's are 0.47uF tantalum and the outputs are 0.1uF metal film. The big regulator's have 22uF tantalum on the output. I am also thinking of putting a 175vac transient suppressor across the inputs for safety.
Christan
I've decided to go with a 15VA transformer from Amveco. I am using a TE70052 which is a sealed, pc-mount toroid. It has dual primaries and dual secondaries rated at 12vac @ 1.25 amps if they are paralleled.
I chose a bridge rectifier than can handle 4 amps and a 3300uF cap as a filter. I put a 100nF ceramic in parallel with it because I was told this helps with some high frequency rejection that the filter cap may have trouble with.
As for outputs, I have seven 78L09's which are 9 vdc regulators that can handle 100mA max. Then I also have a high current 9vdc regulator for pedals that require about 300mA of current. Finally for a 12vdc option, I went with an LM2940 which is an LDO with a 0.5V dropout at full load (1 amp). All the outputs will never all be used at once, maybe 2 or 3 at a time max, therefore the trafo should never reach it's current limit.
The transformer is rated for 12vdc at full load, which means the voltage may be a little higher most of the time...which is nice.
At full load (worst senario) the trafo should show 12 V RMS. So that is 16.92 V p-p minus 1.4v (diodes) = 15.52 VDC +- ripple (3300uF cap). Line voltage here is about 121 V RMS, but with 10% compensation, output is about 14.3 VDC so that leaves 2.3 vdc minus the 0.5 vdc drop across the regulator. That is at absolute worse and at full load...is that cutting it too close?
I have attached an example of my schematic. Feel free to comment on anything that should be changed. The cap on the input of the reg's are 0.47uF tantalum and the outputs are 0.1uF metal film. The big regulator's have 22uF tantalum on the output. I am also thinking of putting a 175vac transient suppressor across the inputs for safety.
Christan
Attachments
Hi,
the maximum continuous DC output current from all your regulators should be about 600mA. (15VA/2~=7.5W, 7.5W/12V ~=625mA)
Take care you don't run the 15VA transformer too hot.
The regulation on the 15VA will be between 10% and 30%. Measure your max DC voltage after the smoothing caps when there is no load. Estimate the maximum voltage when mains is at maximum tolerance. In the UK our 230/240Vac mains ranges from 216Vac to 254Vac.
Then check the dissipation on the low voltage regulators if one of them is operating at maximum current.
the maximum continuous DC output current from all your regulators should be about 600mA. (15VA/2~=7.5W, 7.5W/12V ~=625mA)
Take care you don't run the 15VA transformer too hot.
The regulation on the 15VA will be between 10% and 30%. Measure your max DC voltage after the smoothing caps when there is no load. Estimate the maximum voltage when mains is at maximum tolerance. In the UK our 230/240Vac mains ranges from 216Vac to 254Vac.
Then check the dissipation on the low voltage regulators if one of them is operating at maximum current.
Guys, sorry to rake up an old thread but I need a sanity check
I am also trying to build a pedal power box to power my guitar pedals. 2 of my guitar pedals are digital (Eventide H9) and need 9V 500mA. The others are smaller like the Voodoo Wahzoo etc. However since I am making a box I figured I would like it to be future proof. For reasons of space I would like to keep the box height to less than 45mm something like these.
FUELTANK CHAMELEON ? T-Rex Effects
Voodoo Lab - Pedal Power® 2 Plus
If I was going to make my box I might as well make that is the mother of all boxes. Hence the plan is to build 4 9VDC outputs with 500mA each, a 12V output with minimum 500mA (this will be switchable from AC or DC), a 18V output with 500mA and a pair of variable outputs for any future use.
The idea was to use 7809s for the 9V, 7812 for the 12V, 7818 for the 18V and 317T for the variable outputs.
Variable Voltage Power Supply Using The LM317T
From what I know a 7809 needs to see 11V, a 7812 15V and a 7818 needs 22V the 317 however is ok as low as 21V for 18VDC out and the 317 is happy with just about any voltage above 20V
http://www.electronics-tutorials.ws/blog/variable-voltage-power-supply.html
Vac = 12*.9 = 10.8
Vpk = 10.8 x 1.4 = 15.3
Vrect = 15.3 - 1.4 = 13.9
Vripple = 13.9 - 0.3 = 13.6 enough to drive a 7809
Vac = 16*.9 = 14.4
Vpk = 14.4 x 1.4 = 20.2
Vrect = 20.2 - 1.4 = 18.3
Vripple = 18.8 - 0.3 = 18.5 enough to drive a 7812
Vac = 24*.9 = 21.6
Vpk = 21.6 * 1.4 = 30.2
Vrect = 30.2 - 1.4 = 28.8
Vripple = 28.8 - 0.3 = 28.5 enough to drive 7818 and 317T
For the 7809 x 2 (one transformer can drive 2) should I use
BLOCK FL30/12 Isolation Transformer, 30 VA, 2 x 115V, 2 x 12V, 1.25 A
FL30/12 - BLOCK - Isolation Transformer, 30 VA, 2 x 115V, 2 x 12V, 1.25 A | element14 India
OR
BLOCK FL24/12 Isolation Transformer, 24 VA, 2 x 115V, 2 x 12V, 1 A
FL24/12 - BLOCK - Isolation Transformer, 24 VA, 2 x 115V, 2 x 12V, 1 A | element14 India
The FL24/12 is half the price
For 7812 (both secondary coils in series) should I use
BLOCK FL 14/8 Isolation Transformer, 14 VA, 2 x 115V, 2 x 8V, 875 mA
FL 14/8 - BLOCK - Isolation Transformer, 14 VA, 2 x 115V, 2 x 8V, 875 mA | element14 India
OR
BLOCK FL18/9 Isolation Transformer, 18 VA, 2 x 115V, 2 x 9V, 1 A
FL18/9 - BLOCK - Isolation Transformer, 18 VA, 2 x 115V, 2 x 9V, 1 A | element14 India
The FL18/9 is cheaper and delivers slightly more current
For 7818 (both secondary coils in series) I am thinking of this
BLOCK FL24/12 Isolation Transformer, 24 VA, 2 x 115V, 2 x 12V, 1 A
FL24/12 - BLOCK - Isolation Transformer, 24 VA, 2 x 115V, 2 x 12V, 1 A | element14 India
For the 317T (one transformer will drive both)
BLOCK FL30/24 Isolation Transformer, 30 VA, 2 x 115V, 2 x 24V, 625 mA
FL30/24 - BLOCK - Isolation Transformer, 30 VA, 2 x 115V, 2 x 24V, 625 mA | element14 India
Hammonds are pretty expensive, are the BLOCK models I have posted above reliable?
Also I was trying to keep the input voltage to the regulator as close to the recommended voltages because the higher the input voltage the more power we lose to heat. This is not good if the box needs to be small or we are putting many regulators close together.
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The regulators do indeed need a minimum voltage but why not look at their maximum input voltages - normally 30V DC.
All the regulators could share a common input voltage as long as it is above the minimum voltage.
The only concern here is that the smaller regulators might need a little bit of heatsinking as they will run hotter with a higher input voltage.
All the regulators could share a common input voltage as long as it is above the minimum voltage.
The only concern here is that the smaller regulators might need a little bit of heatsinking as they will run hotter with a higher input voltage.
Apart from the heat sinking required (given the number of regulators in a small box it might be an issue), I wanted to isolate each regulator so that if any one pedal hums we do not have all pedals humming and also sometimes we do get center positive pedals (most are center negative).
I intend to add a center negative / positive switch for the 9VDC and 18VDC outputs and a AC/DC switch for the 12V outputs.
Cosmetically there will be 3 holes I need to drill for each output, one of the output itself, one for an LED indicator, and one for a switch or a pot (for the 317T driven variable outputs).
Meanwhile do the transformers look ok? They are expensive hence I need to be sure before ordering. Thanks.
For the 7809 x 4 I am thinking of using 2 Nos. BLOCK FL24/12 Isolation Transformer (24 VA, 2 x 115V, 2 x 12V, 1 A)
For the 7812, the BLOCK FL18/9 Isolation Transformer (18 VA, 2 x 115V, 1x18V, 1 A)
For the 7818, the BLOCK FL24/12 Isolation Transformer (24 VA, 2 x 115V, 1 x 24V, 1 A)
For the 317T x 2, the BLOCK FL30/24 Isolation Transformer (30 VA, 2 x 115V, 2 x 24V, 625 mA)
Can I connect all the primaires in parallel?
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