Have AC-DC +12v psu, need +-12v! Noob mistake!

[Kowalski]

Hey everyone,

So this is the situation: I was looking to acquire a cheap function generator, something to emulate low-voltage AC, mess around with the oscilloscope or otherwise play with a pulsing current without killing myself, that is either electrocuting myself from the wall or banging my head on the table trying to build a safe alternative from scratch. So I did what many electronics noobs do and looked through the Won Hung Lo section of Ebay. Ordered myself a function generator kit(what, for soldering practice and whatnot: http://www.ebay.com/itm/291481351716)

What I needed next was a source of power, so I took a quick look at the listing and saw 4 power pins: +12, -12, +5, GND. I figured: Simple, just get a +-12 supply for the amp and use a 7805 for the micro. I went ahead and opened a listing (http://www.ebay.com/itm/371117456686) and, seeing how this is a fairly low power device, I picked out the smallest, 1-amp option. Of course, before ordering I asked them if there were actually outputs for +12v and -12v, and they replied something along the lines of: "Yes, there V+ and V-, see picture!"

Of course, when I got the supply I was a bit disappointing. Turns out the only real ground is mains-earth, which is isolated from the DC side. V- is just that, negative. Not -12, but the negative for the +12. Oops. Anyhow, that leads to my question! I currently have a +12 switching power supply(the IC is a HT2203C) and no real means of easily getting -12v. My question is, what can I do to get the -12v output that I need, along with the +12v from the supply? Can I link two of these cheap supplies in series the same way it can be done with a bench supply? Or will I have to explore other options?

I included a picture of the supply below. Any tips, advice and insight would be greatly appreciated!

Thanks

[michaeliv]

Edit: Not sure that putting them in series would work -- perhaps someone with experience on this can comment on weather the lower voltage supply can sink the current.

[mikerj]

Putting them in series should be fine since they are isolated supplies.  However, for something like a function generator a linear regulated power supply would have been preferable; no switching noise and no potential leakage currents through EMI suppression caps.

[AlxDroidDev]

You're better off building a small linear psu yourself, using a center-tap transformer, a rectifier, and 7812, 7912 and 7805 linear regulators (plus all the other bits such project requires).

Or use an ATX PSU.

Or add an ICL7660 to the SMPS you already have.

[Zero999]

The problem with the ICL7660 is it might not be able to provide enough current.

I agree, a linear PSU would be ideal.

Given what you've got, how about a buck-boost SMPS: +12V in, -12V out? You can probably find one on ebay.

[AlxDroidDev]

The problem with the ICL7660 is it might not be able to provide enough current.

Actually, I was wrong. The ICL7660 (or TL7660) can only output down to -10V. Maxim's ICL7662 can go down to -20V, and according to its curve, it can supply up to 50mA at -12V. For some linear circuits, that can be enough, but I wouldn't count on it either.

[Kowalski]

Thanks for the input so far, everyone.

A buck-boost circuit is definitely one of my options. The pre-made circuits online are mostly from China, (no US warehouse = hellish shipping)but assuming the PCB's are double sided I might be able to reverse engineer them based on photos and cobble together a circuit of my own from digikey parts.

I like the idea of building my own power supply! I just don't like the idea for buying the transformer, as far as I know, it can be incredibly expensive.....however, I do have some spare computer PSU's lying around. Now I don't want to use an entire PSU for a little 10x10cm board, so I was thinking of scrapping one of those for the transformer. I can probably pull out most of the necessary components to get a clean, rectified voltage off the secondary, and then use the 7### voltage regulators and their datasheet-recommended discreet components to get my +-12 and +5v rails. Is my idea feasible, or is there some site for buying relatively-cheap transformers that I have not heard of?

[AlxDroidDev]

Computer PSUs use high frequency switching transformers, which aren't adequate for linear PSUs. A small linear transformer is cheap, specially because you don't need more than a few hundred milliamps. You can even find an used one in old devices, specially sound equipment.

I would give the 7662 a shot: 50mA goes a long way in modern linear circuits.

[alsetalokin4017]

If you're taking up electronics as a hobby you definitely need a proper power supply. If you like building, there is no need to reinvent the wheel.

The Elenco XP720K in kit form would be ideal for your present purpose and for a lot more in the future.

By the time you've built up a proper bipolar linear PS from scratch, including regulation and short-circuit protection, you will probably have spent more than the Elenco kit costs. Check EBay  for XP720K        ...

(No, I don't work for Elenco, but I have an XP581 that has served me well for many years.)

[Kowalski]

I found that a 7662 from Digikey comes out to around $3.50, and then it's also a matter of getting the discreet components from the same site with dirt-cheap ~4 day shipping. That's not too bad...I just have to double check the maximum current draw I'd expect from my function generator. According to the datasheet, drawing too much over 1mA results in the voltage changing, which is a big issue in a function generator.

On the way to school tomorrow I'll stop by the thrift shop and browse through their "vintage"(throwaway) electronics. I'm sure I'll find what I'm looking for provided the appliance is analog and plugs directly into mains.

Alsetalokin: Well, I do happen to have a few PLC PSU's (5v, 12v, 24v) and a few different LM317 circuits that I use for fine-tuning. In this case, however, I'm looking for a small solution that won't turn a little isolated generator into a cabling mess every time I want to power it on. However, I AM still looking for a "proper" bench supply for daily use, and I love the idea of kit-building. Is the XP581 linear or stepping on the DC side? I might just put an order for one next payday. Also, does it still have much room around the front panel after assembly? I might also wish to implement a current meter to free up as many probes as possible during testing.

[alsetalokin4017]

Well, I do happen to have a few PLC PSU's (5v, 12v, 24v) and a few different LM317 circuits that I use for fine-tuning. In this case, however, I'm looking for a small solution that won't turn a little isolated generator into a cabling mess every time I want to power it on. However, I AM still looking for a "proper" bench supply for daily use, and I love the idea of kit-building. Is the XP581 linear or stepping on the DC side? I might just put an order for one next payday. Also, does it still have much room around the front panel after assembly? I might also wish to implement a current meter to free up as many probes as possible during testing.

Ah, OK, from your original question I was thinking you were a "total Noob". But you evidently have a bit more experience than that.

Both the XP720K (which I recommended for you)  and the XP581 are linear supplies. The complete manual for the XP720K, including schematic, can be downloaded from the Elenco website:
http://www.elenco.com/admin_data/pdffiles/XP-720K_REV-H.pdf
You might be able to fit a small panel meter in there.

The XP581 has built-in analog ammeter and voltmeter (and is more expensive than the XP720K, as well.)

[Kowalski]

Well, I do happen to have a few PLC PSU's (5v, 12v, 24v) and a few different LM317 circuits that I use for fine-tuning. In this case, however, I'm looking for a small solution that won't turn a little isolated generator into a cabling mess every time I want to power it on. However, I AM still looking for a "proper" bench supply for daily use, and I love the idea of kit-building. Is the XP581 linear or stepping on the DC side? I might just put an order for one next payday. Also, does it still have much room around the front panel after assembly? I might also wish to implement a current meter to free up as many probes as possible during testing.

Ah, OK, from your original question I was thinking you were a "total Noob". But you evidently have a bit more experience than that.

Both the XP720K (which I recommended for you)  and the XP581 are linear supplies. The complete manual for the XP720K, including schematic, can be downloaded from the Elenco website:
http://www.elenco.com/admin_data/pdffiles/XP-720K_REV-H.pdf
You might be able to fit a small panel meter in there.

The XP581 has built-in analog ammeter and voltmeter (and is more expensive than the XP720K, as well.)

Great, thanks for the info!

[DivineChaos]

I have a 16.5v AC "wall wart" that came from an alarm panel. Perhaps you can look into that for very low voltage AC. (I can grab the leads and not feel anything)
Works great for powering just about anything I put into it.

[mikerj]

Computer PSUs use high frequency switching transformers, which aren't adequate for linear PSUs. A small linear transformer is cheap, specially because you don't need more than a few hundred milliamps. You can even find an used one in old devices, specially sound equipment.

I would give the 7662 a shot: 50mA goes a long way in modern linear circuits.

One thing to be aware of is that these "flying capacitor" inverters can be horribly noisy.  Lots of filtering and good PCB design required to use them in a noise sensitive application.

[station240]

I did have a circuit somewhere for this type of PSU, at least the 12V side. Naturally I don't have it handy.
Read the part number on the 8 pin IC and google for that and the word "circuit"

transformer output goes to D7 - C8 - L1 - C?
If you duplicated those components, but with the new diode and electrolytic capacitors in reversed polarity. You'd get an extra output of -12V DC.
The only connections to the existing circuit are:
1) The transformer tap D7 connects to (cathode aka band of diode goes to this)
2) The existing 0V output (which you connect the positive side of the capacitors to).

[dentaku]

You can also build a +/- 12V power supply from two AC wallwarts.
http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php

You can also salvage a center tapped transformer from old stereo equipment and build one that way.
You might be able to find one with multiple taps so you can easily  create the +/-12V and the +5V all from the same transformer.
or build something like this that only needs one tap, you could probably replace the top LM317 with just a regular 7805 circuit.
http://www.modularsynth.ru/en/2013/06/13/regulated-bipolar-psu/

Almost all modular synthesizers need a power supply just like you're looking for so synth DIY webpages are a good place to look.