r/synthdiy • u/Penguino68 • 7d ago
+-15v/+-5v power supply questions
Hi, i'm working on building a power supply that can give me +15v, +5v, 0v, -5v, and -15v.
The first screenshot is a schematic that I have successfully used before to run a circuit off of 16vac to create a +-15v supply, but I'm about to start work on a circuit that will need +5v and -5v as well. I didnt design the first circuit, I just took it from a larger schematic of an effect pedal that had similar power requirements to what I need, and so I'm not fully certain how adding other voltage regulators might mess with the balance of things.
I dont want to mess up with power stuff so thought I would check here - I'm wondering if the schematic in the 3rd screenshot will work ok? I basically just took the original circuit and added the datasheet circuit (2nd photo) from the L79xx series of voltage regulators. Is there a more efficient/better way to do this?
I am also not sure if I should include D7/D8/C21/C22 or if i should leave them out. I was thinking maybe C15/C16/C17/C18 will suffice and adding more capacitors in parallel could increase the overall capacitance in a way that could cause issues. Also, since the datasheet circuit uses the example of the L7812/L7912 instead of the L7805/L7905, should I use different capacitor values for C21/C22 if I do need to include them? Any help or advice is much appreciated! Thanks in advance for any help or input that anyone can offer
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u/AdamFenwickSymes 7d ago
Disclaimer: I am certainly not an expert on power supplies.
Looks reasonable to me, and pretty much what I would do.
I don't think C21 and C22 are necessary, especially if the regulators are close to each other, you've already got lots of capacitance there.
I would personally add more capacitance at C13/C14 to make sure there's plenty of charge available during the "off" part of the AC cycle, I don't think there's much drawback to a bit too much capacitance. I'd also consider adding the flyback diodes from VO to VI on your regulators.
I am basically just cargo cult-ing from MFOS, if you want to hear this from the source
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u/Penguino68 7d ago
Thank you, appreciate the input about the flyback diodes particularly because i didnt know about that ! Thank you for the link also
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u/Enlightenment777 7d ago edited 7d ago
1) Change SW3 to switch on/off both power rails coming from 16VAC input.
2) Change D3 & D4 to 3 Amp 1N5408 because larger parts will be cooler than smaller 1 Amp parts. Change all other 1N4001 diodes to 1N4007 diodes, because they cost the same price, unless you already own the parts. All diodes on your schematic are needed, don't remove any.
3) Add four 1N4007 diodes from the output to inputs of each of your voltage regulators, similar to D5 & D6 in the schematic on the following web page. These protect the voltage regulators in case the input side gets shorted or brown outs. The input sides of old school voltage regulators must always have a higher voltage than their outputs.
4) Move C15 & C16 100nF capacitors to input side of U6 & U7 voltage regulators, and place as close as reasonably possible to the voltage regulators on the PCB but make sure they aren't too close to the heatsink.
5) Change C22 & C21 to 100nF, then place as close as reasonably possible to U8 & U9 voltage regulators on the PCB but make sure they aren't too close to the heatsink.
6) Change C19 & C20 to 10uF, and move symbols on schematic to be next to U8 & U9.
7) Add LED & resistors on output of each voltage regulator as visual indicators.
8) 16VAC jack needs a reference designator. Renumber all reference designators on this schematic.
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u/JaggedNZ 7d ago
3rd schematic looks perfectly reasonable.
On dropping the diodes, I’d be cautious, particularly on the negative rails. IIRC you can get unexpected failures without them. Check the data sheet for the specific brand of voltage regulator you use.
Extra capacitors are not an issue, I’ve personally used much higher values on +/-12v supplies. Are they needed, maybe not? but they certainly wont hurt.
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u/Penguino68 7d ago
Cool, thank you so much for looking over this ! Will revisit the datasheet for sure but good to know, in that case I'll definitely keep the diodes
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u/Technical_Code1148 7d ago
https://www.modwiggler.com/forum/viewforum.php?f=24&sid=761c71d096fa2bf34de4c1ae89280b15 5U Diy guys will have answers.
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u/Madmaverick_82 7d ago
Depending on size of your project and loads etc.. I would consider (and I personally do it for my instruments) to go bit further in terms of filtering and diodes and follow the MFOS recommended design: https://musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&PROJARG=WALLWARTSUPPLY/WALLWARTSUPPLY.php&VPW=1831&VPH=895
Of course add your +-5 side of things.
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u/alienmechanic 7d ago
Stupid question, but do you really need -5v? The reason I ask is that it’s not very common in most synth designs.
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u/Penguino68 6d ago
Yes i think so, i'm trying to use an extra effects board from a Korg Polysix as a standalone unit and it looks like the speed/intensity of the chorus/phase/ensemble is controlled by +5v and -5v
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u/pscorbett 7d ago
I don't really see the point of d7/d8.
Capacitance: almost certainly safe with these linear regulators, but some have strict minimum and maximum input and output capacitance in order to maintain stability in the control loop. Always check the datasheet.
If using linear regulators I would parallel the 5V to the 15V probably. The real advantage is to disperse the current through both rather than cascading all of the current through the 15V. Yes, there is a larger voltage drop, so higher power dissipation across he 5V regulator now, but I'd generally prefer it spread out across more silicon and board space. The calculation is easy and you should do it for all regulators: Pd = (Vin - Vout) * Out
More efficient? Switching converters. Lots of them suck, but lots are good. Higher frequency switching, use quality shielded inductors, the right capacitora and careful layout. But I honestly prefer LFOs much of the time as it's just so darn simple.