+/- 15v and +5v PSU???

[Jaicen_solo]

Anybody have any idea of the best way to achieve a 1amp PSU that simultaneously produces +15v, -15v and +5v??
I need one for my TR-808 clone, but I don't have a lot of experience with PSU design. Is it possible to generate + and - voltages from a single 15v supply? If so, I could use a center tapped 15v transformer, and rectify one half to +5v.
Anybody got any ideas?

[GibsonGM]

Hi J., you might find something like this for a computer...I have some of those lying around with nothing to do, lol.   Have you considered finding something like a 30V, 1 A wall wart?  I did that using (of course) an old computer PSU...just created a virtual ground and split it.  From there, you can have + and - 15V, and use a voltage regulator off that leg to get +5v.   Do you need the entire 1A?  Might cost a little for that high a current rating, but can also be found for free or recycle.  I get mine at the dump, ha ha.

[idlechatterbox]

Those are the outputs of a standard PC clone power supply. So you could power your pedals AND your dvd burner 

[GibsonGM]

You might like to try this out.  Above the top resistor is +15v, and below the bottom one is -15V.  Use the junction of both as the ground point.  From there, you could tap +15v to a regulator to get your 5V    The value of the resistors sets the current in the loop from + and - at your PSU, you'd want this to be a bit more than what you anticipate your ckts to draw.  Remember to use proper wattage resistors!  Ohm's Law...

[R.G.]

Anybody have any idea of the best way to achieve a 1amp PSU that simultaneously produces +15v, -15v and +5v??

This is such a standard power supply that there are many, many commercial solutions. "Best way" implies that you have some way of evaluating what is most important to you. Cheapest? Best regulation (that is, is 0.01% better than 0.1% variation)? Most current? Lowest wasted power? Best power factor in the drain from the AC line?

If you want cheapest, go to Goodwill or equivalent and get one DC wall wart that will do over 7V @1A,  and two that will do over 17V @1A. Then hook them up to a 7815, a 7915, and a 7805.

If you want smallest, lightest, etc. buy one from Mouser. It'll cost some money, though.

Finally, if you want to build one yourself, read "Power Supplies Basics" at GEO.

Those are the outputs of a standard PC clone power supply. So you could power your pedals AND your dvd burner

PCs run on +/-12V, not +/-15V. The -12V in a PC power supply is only good for maybe 100ma. The +5 will do 30-40A, and the +12 is good for a few amps.

You might like to try this out.  Above the top resistor is +15v, and below the bottom one is -15V.  Use the junction of both as the ground point.  From there, you could tap +15v to a regulator to get your 5V  Smiley  The value of the resistors sets the current in the loop from + and - at your PSU, you'd want this to be a bit more than what you anticipate your ckts to draw.  Remember to use proper wattage resistors!  Ohm's Law...

As shown, the resistor setup pulls 1A/30W all the time. You really can't pull more than 0.1A difference from either side without unbalancing the +/- 15V. And there is no regulation on +/-15 - the total is whatever the "30V" power supply says it is.

[Paul Perry (Frostwave)]

First I's want to know exactly (or approximately) what current really is needed at each voltage. All I know is, it isn't going to be an amp!!!

If it was me, first I'd check ebay, because there are a LOT of surplus +5, +-15 supplies (used to be standard for mixed analog & digital systems before everyone wne low voltage and DSP).

If I had to make one, dependign on the currents... I'd have a 15V AC transformer, ground one side, half wave rectify + and - & regulate with a + and a - 15V regulator, also run the + to a +5 reg.

[Jaicen_solo]

That's pretty much exactly what i'd hoped to do Paul.
As I said, i'm a bit of an idiot when it comes to PSU issues.
When I defined 'the best' PSU for my needs, that should have read cheapest, most stable option. As you say, I'd planned to rectify a 15VAC adapeter into +/- 15VDC, the problem then is achieving +5v, but if the transformer had a pair of 7.5V AC secondaries, it would be pretty simple to use one half for +5VDC would it not? I wouldn't really want to rectify 15v down to +5V, i'm sure the regulators would run pretty hot doing that!
Im going over to GEO, i'll be back!

[Paul Perry (Frostwave)]

As you say, I'd planned to rectify a 15VAC adapeter into +/- 15VDC, the problem then is achieving +5v, but if the transformer had a pair of 7.5V AC secondaries, it would be pretty simple to use one half for +5VDC would it not? I wouldn't really want to rectify 15v down to +5V, i'm sure the regulators would run pretty hot doing that!

Sure, no problem.
And you are right.... the power dissapated in a regulator is voltage drop across it X current drawn from it. So if you have appreciable current, it's going to heat up plenty!

[Jaicen_solo]

To be honest i'm not sure of the exact current draw on the +5v line.
I know it will definitely be powering an 18Fxx PIC, and a handful of CMOS chips, which should be no problem really. The thing i'm worried about is the 30 odd LED's I was planning to use as well, which could add up to a few hundred milliamps.
In my original post, I specified a 1A supply because I'd like to have more current handling than I'm likely to need. I think a 500mA supply might be a bit too close to the edge and could run hot which is what i'm trying to avoid.

[R.G.]

The thing i'm worried about is the 30 odd LED's I was planning to use as well, which could add up to a few hundred milliamps.

Ah. That's much easier. You probably have control of exactly how much current is used there by determining the size of the current limiting resistors in series with the LEDs.

It's easy to guess at an upper limit. LEDs are usually specified for 20ma maximum current. We often use them at less, but the 20ma is set by thermal limits. So if you're using 32 LEDs, the max current you will want will be 32 * 0.02 = 0.64A. It will probably be less, as most LED uses don't need to pump them up to a full 20ma because modern LEDs are so bright.

The PIC is specified for 200ma max through the power supply wires, taking into account that it may be driving up to 25ma each on output pins. The CMOS is trivial. So you're looking at 0.84A max that you could use before burning something out.

You have control of making this smaller by limiting the currents out of the PIC pins and through the LEDs with current limiting resistors.