t said:
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
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
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.
From a practical, one-off stand-alone circuit, it would probably be
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
