# Dual voltage supplies

Discussion in 'Electronic Basics' started by t porter, Apr 3, 2006.

1. ### t porterGuest

Gentlemen

Noob question...

I have a board w/ a number of 5V logic chips & a a couple of 9-12V
op-amps. I hope to use a wall wart to power this. I typically just
power boards w/ a simple LM317 or similar 5V regulator. What is the
simplest or most common way to split out these voltages in a hobby
environment?

Thanks, Tom

2. ### John FieldsGuest

---
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

3. ### t porterGuest

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

4. ### John FieldsGuest

---
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).

5. ### t porterGuest

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

6. ### DeefooGuest

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

7. ### ChrisGuest

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\$

| ____
| | |
| -. ,----->|--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 www.tech-chat.de)

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

8. ### Rich GriseGuest

.
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