My design of a pedal power supply with short circuit protection+indication

[jasperoosthoek]

Some years ago I designed and built a pedal power supply. Now I've used it for two years and it has worked like a charm. Therefore I like to share it here for people to use it.

Before I designed it I bought a couple of cheap dual 12VAC transformers. The idea was to give every effect its own separate power supply.

Because of the low power rating of the transformers I was limited to 35mA DC per pedal. Enough for most  pedals with the exception of some digital pedals/tuners. As I didn't want to replace transformers in case of a short circuit I designed some simple circuitry. I could have just loaded the 78L09 with a resistor but would have required a much high value resistor (say 440 Ohms with 15 volts DC after the rectifier) to keep the short circuit current as low as possible. Then the maximum current at which the regulator works would be much smaller.

Instead I designed circuit which has a turning point at around 35mA. I used a 2n3906 that is normally kept open by a 10k resistor which feeds current to a 78L09 regulator. But when the current exceeds about 35mA the voltage drop over the 27 Ohms resistor causes the 2n3906 to turn off as the LED starts to conduct and uses up the base current. This in effect turns the transistor into a regulated current supply. The voltage feeding the regulator now starts to drop very fast as more current is being drained.
The beauty of circuit is that up to say 30mA the transistor is fully open giving the 78L09 enough voltage to regulate. Different currents can be achieved by changing the value of the 27 ohms resistor. At about 0.95 volts it turns off the transistor meaning that a required resistor needs to be 0.95/current. Just be careful that smaller resistors might require a higher power rating. The 27 ohms resistor only dissipates 0.033 watts maximum.

Of course 78L09 regulators do have short circuit protection built into them but they do not have a nice LED indicating a short. Also, this circuit protects the transformers more than it protects the regulators.

Any comments would be appreciated!

[spud]

Cool idea!  Great to hear it has been working so reliably for you - it's not common that people test their creations for 2 years before telling us about it - just kidding!     

One thing and sorry for the dumb question but If I raise the current of the 12v adapter what do I need to change?   Lets say I get a 12 500mA wall wart - will that cook the regulator or anything else?   With that I could power a number of effects on a pedal board - cool.

Thanks -

Jim

[jasperoosthoek]

Hi Jim

Nice to see you here!

The biggest problem is that the 78L09 is rated at 100mA max, unlike the 7809 which can handle 1 Amp. But your transformer does not.
At the time I designed that the first power supply I also designed a higher current version. But I never built it. I didn't post it here as it is untested and I first wanted to get some reactions on the first one. The original was for 1 amp but the 78xx has that feature already built into it. So I changed it to 500mA:

For 1A you have to change the 2 ohms resistor into 1 ohms/3W. You may want to upgrade the capacitors a bit. Especially the 100u. But on the other hand, how much current are you really going to need. What I like about this design that it is really stable and protects the transformers well. My Rebote Delay 2.5 actually works on my supply even though the over current/short circuit LED starts to blink. So it is operating at a voltage below 9 volts but works fine.

But don't forget it is untested and right now I have too much projects going to have time to test that one (I just ordered tunnel diodes on Ebay...). Still it should work the same way as the lower current version. Especially the chunky power resistor has to be fine tuned. It will turn off around 500mA but it could also be 400 or 600. When the resistor is sorted it should work too on other units: The 10 identical units I built in my power supply all have very comparable short circuit currents: 35mA+/- 2mA.

[spud]

Hey Jasper,

I found this list of power consumption for various pedals - seems like a pretty exhaustive list. 

http://www.stinkfoot.se/andreas/diy/power/list.htm

I was surprised that so many pedals require so little current but then there are a few that are high as well.  Seems like it varies quite a bit. 

Jim

[jasperoosthoek]

I think that designers of most boxes tried to keep the current as low as possible to extend battery life. That's just not possible for digital effects therefore the difference is so big. I bought an Ibanez 200mA pedal power supply some time ago for my TC Electronics Polytune. It drawed just a bit to much for home built supply. The music shop had various options. Some 1.7 amp supply was the cheapest! I didn't get that one because it is a switching power supply. They are normally extremely noisy and didn't want to test it in my setup. The Ibanez was one euro more and the 200mA was more than enough.

[jasperoosthoek]

This is what it looks like:

Ten outputs and a power cable. I used high quality microphone wire with two identical connectors at each end:


The circuit board, as you can see: 


Everything in place, super tight fit:


Top of the board showing the five double output transformers:


Demonstration of shorting one of the outputs:


(Sorry for the messy table)

[Earthscum]

I dig the ease of that supply. I've been looking for something like this for a pedal board. I didn't want to have a bunch of transformers adding weight. It looks like you are running 2 supplies off of each transformer? I should be able to (12V/1A trans), using about 1/2 of the power rating, set up 6 50mA jacks, and 2 100mA, from one transformer, correct? And have some headroom after total circuit dissipation, I hope?

Where would you recommend I split out the tranny lines? Is it better practice to have a separate rectifier for each channel, or can you split them out after the rectification? I imagine the rectifier acts as a kind of buffer, but I don't know a whole lot about power supplies at the moment. I have my adjustable bench supply that I made from R.G.'s design (with dying battery resistance).

Sorry for being so buggy about this. I may order a transformer (or a couple flatpacks) tonight if I can run multiples from a single tranny. I have too many pedals, and it's starting to look like an aluminum caterpillar crawling across the floor. At our show on Saturday they almost saw the wrong parts getting stomped by my guitarist.

Oh yeah, btw... Thanks for sharing. VERY nice build! I love seeing builds that look so clean! I'm thinking now that I may use my old Sentrek amp case for a PS.

[milkypostman]

So, basically there are pairs of connections that share the same ground?  I assume that means you could run half with positive ground?

[jasperoosthoek]

Just a quick answer before I go to bed (more tomorrow)
The transformers have two completely separate secondaries. So four output wires. So the secondaries are split in the transformer. Each secondary has a rectifier and the circuit you see above. I designed it with a minimum number of components. Worth the effort if you plan to build 10 fold...
There are no connections between each output. Adding different regulators to a single secondary is not the same. If you are planning to order something but you are not sure: please wait a day ;-) I can do a quick search on mouser/small bear tomorrow to find something that works. 

[davent]

Jasper,
That's a very cool project, thanks for posting it!

dave

[Earthscum]

I re-read your post. I think I misread a couple things, one being that you said they had split secondaries.

I'm looking to be able to have multiple isolated circuits off of the same 12V supply (transformer). I'm trying to avoid having a brick of flatpacks, if ya know what I mean. I do like the split packs, though. Smallbear carries a decent tranny that I'm considering that would work, should work, quite well with your circuit.

http://www.smallbearelec.com/Detail.bok?no=597

I remember coming across a power supply (think it was for pedals specifically), where a guy used a mosfet to regulate the AC current to the rectifier, and just had like 8 or 9 7812's straight down, and took another 9V regulator off the output of those. His mosfet current limiter, I remember, had a setup kinda like yours, where when current was near the limits, the LED would glow. Maybe he used a neon lamp...

I searched, and can't find it again. I figured if it was viable, it would be a nice front-end to your circuit. Maybe someone here has come across it.

[jasperoosthoek]

Thanks guys for the big ups . I only got one reaction when I posted the schematic... I knew I needed to show some images of the actual box

@milkypostman The transformers have two completely separated secondaries, four wires. They will only share a ground when you connect them to a pedal chain. What I never expected was that the 78L09 positive voltage regulator (100mA version of 7809) works without a problem on positive ground effects (my Ge Fuzz Face).

@Earthscum When I started work on the power supply I stumbled across that transformer from SB. I would have bought it but I'm located in mainland Europe that runs on 230 VAC... Therefore I got the blue ones from an online electronics shop (conrad) that is well known here. I don't know how the mosfet would have worked with AC. My bipolar as you can see is located after the rectifier and switches DC. But it sounds like it is somewhat similar.

My first design with the single transistor actually cannot handle much more than 35mA because it will fry the 2n3906 transistor when the output shorts. It has to dissipate all the power and cannot handle more that 625mW. The second design has a TIP42 that can dissipate 65W with a proper heat sink. The extra 2n3906 is needed to drive it. I have not built the second design but I could breadboard it. I just don't know the exact value of the power resistor connected to the TIP42 emitter yet. That needs a bit of trial and error, just testing a few values that are close.

I hope this link works: http://nl.mouser.com/Power/Transformers/_/N-5gc5?P=1z0wb5b Mouser has a couple of nice double secondary transformers. Similar to mine. But I think that the small bear transformer is a much better deal.

And for anyone who attempts to build something like this I have some additional advice:
Do yourself a favor, don't use tiny box that just fits (like mine). I had to spend a lot of time getting everything to fit in the box. Look at the third picture, it's madness! Because of this it took me one and a half years to complete (!!). It had it working in a very dangerous open state for most of that time. Then I finally decided to finish it.

And, watch out that you buy isolated cable sockets. I got the pretty metal ones from small bear (http://www.smallbearelec.com/Detail.bok?no=92): hence the PVC piece I needed to fit and isolate them. I could have bought brand new ones but my local electronics shop charges me 2.50 euros a piece... I had originally bought these for my pedals but the metal part that you screw inside is the positive supply voltage which you cannot connect to the outside of the box! So I could only use them for positive ground effects (Or you need to isolate them some way with epoxy).

Good luck building! Keep me informed if you plan to build the short circuit protection