Dual voltage supplies

--- Are the opamps dual or single-supply?

That is, are they like this; view in Courier:

+12 | | ----|-\\ | >--- GND---|+/ | | -12

or more like this:

+12 | | ----|-\\ | >--- 6V----|+/ | | GND

-- John Fields Professional Circuit Designer

They are dual supply (that is, like your top image)

--
I\'m afraid I don\'t have a good answer for you.

I think you\'ll be hard-pressed to find a wall-wart with a triple
output, or even a dual output, so you might have to go the route of
a couple of 12V wall-warts for the analog stuff and then a 7805 on
the + output wall-wart to get your 5V.

But maybe there\'s a better way.  

Tell us more about what it is you\'re trying to do and something
about the current you\'ll need from the supply(ies).

Well, the circuit I'm trying to build is on page 367 of "Digital Electronics with PLD Integration" - a two-digit counter. There is no way I can draw it in ASCII here.

The circuit has a couple of 555's, a 74157 mux, a 7447 binary/7 segment decoder, and a couple of 74196 timers. A dual op-amp (they recommend a 1458) is used as a comparator and an inverter.

The circuit shows the op-amp powered from a +12/-12 volt supply, and the logic powered by 5V. How would most people, who know what they are doing (as opposed to me), power this circuit?

Best, Tom

Let me guess: you have two cascaded 74196 (obsolete?) bcd counters that are multiplexed by the 74157 to one 7447 decoder that drives both digits. One

555 is used for the digit multiplexing and another 555 is used as a clock for the bcd counters (you could combine these two, I guess). The opamps are the input and gate the counters and thus determine the value displayed. Is that about right? Or is a 555 used as the gate?

Anyway, depending on what the opamp has to do it may be possible to use a (5V) single supply opamp or run it from a single supply. Need more details. What is the input for your circuit? What is it supposed to count?

--DF

Hi, Tom. You have a circuit that's primarily digital, with a relatively heavy current draw on the 5V supply, and you also need 20mA or so for a +/-12V supply.

Using one transformer with one secondary is probably a pretty wasteful solution. If, say, you got together a 25.2VCT secondary transformer, with a bridge rectifier to create + and - 17VDC unregulated sources, and then used a 7812 and 7805 for the + voltages and a 7912 for the -, each watt of power from the 7805 output would require it to dissipate

2.5 watts in heat. Not very efficient.

One possible solution if you wanted to stay with a linear supply would be to get a transformer with dual secondaries. Signal Transformer, among others, makes transformers specifically for this problem. You just use two diodes on one secondary to get the +5, and a bridge with the center tap to get the +/- analog voltages (view in fixed font or M$ Notepad):

| ____ | | | | -. ,----->|--o------o---|7805|--o----------o | )|( | | |____| | | )|( | --- | --- | )|,--. | --- | --- | )|( | | | === | | )|( | | === GND === | )|,--)-->|--' GND GND | )| | ____ | )| | .-------. | | | )|,--)------o~ +o-----o--|7812|--o----o | )|( | | | | |____| | | )|( | | | --- | --- | )|,--o | | --- | --- | )|( | | | | | | | )|( | | | | | | | -' '--)------o~ -o -. o----o-----o----o | | '-------' | | | | | | | | | | | === | | | | | | GND | === | --- | --- | GND | --- _|__ --- | | | | | | | '--o--|7912|--o----o | |____| | | | (created by AACircuit v1.28.6 beta 04/19/05

All of this is a little much, though. First, you really don't need any of the advantages of a linear supply for the 5V supply. You'd have to look at the circuit to see if you can use a switcher for the +/-12V.

best to use a wall wart or a tabletop adapter for the power. Jameco has a wall wart that will give you a regulated linear +5V at half an amp, and +/-12V at 130mA. Their P/N 154624CB is available for $12.95 USD. These have the advantage of relieving you of the necessity to work with line voltage, and their convenience is worth it.

But for a hobby environment, you really want to have something that will work for you on the next circuit, too. I'd recommend looking around for a surplus triple output linear open frame opwer supply. You can mount it under the bench, and they're very reliable. They provide almost lab quality regulation, the +5V typically has overvoltage protection, and they're cheap enough that if something goes wrong, you can just junk it without a second thought. Just mount it under the bench, with a fuse and an accessible switch, and a jiffy box with binding posts on the top of the bench, and you're good to go.

Good luck Chris

On Thu, 06 Apr 2006 11:05:45 +0200, t porter wrote: ...

If it's a one-off, or a demo of some kind, I'd stick with the 7805 for the 5V stuff, and use a couple of 9V batteries to get +9-0--9, which will probably operate your opamps and comparators just fine. Or maybe just the one - it's not written in stone anywhere that it's forbidden to run an opamp or comparator off +12 and -9V.

Or you could go for something like a 24V wall wart, and emulate a split supply with a virtual ground in the middle, or, you could bite the bullet, and get a DC-DC converter to make -12 out of +12, but still use the 7805 off the +12 for the +5.

Good Luck! Rich