# Multi-output regulated PSU

Discussion in 'Electronic Basics' started by Wally, Nov 19, 2004.

1. ### WallyGuest

I want to make a regulated bench PSU with three adjustable outputs
(LM317/337). Do I need to use a separate smoothing circuit for each output,
or can I connect the inputs of the regulators to a single smoothing circuit?

2. ### Peter BennettGuest

You can connect the inputs of all the regulators to a single smoothing
circuit.

If the regulators will provide different output voltages, you may have
to worry about excessive power dissipation on the lower-voltage
regulators.

3. ### WallyGuest

In the sense that they might be damaged, or that power is being lost? The
transfomer is rated at 24-0-24v, 1A, and the spec of the regulators is 20W,
1.5A.

4. ### Peter BennettGuest

Power will be "lost" in the regulators in the form of heat. The power
dissipated in each regulator will be the voltage across it, times the
current through it.

Full-wave rectification from that transformer will give you about 33
volts DC. If a regulator is set to give you 12 volts out, it will
have 21 volts across it. If it has to deliver 100 mA, it will be
dissipating 2.1 watts. Without a heatsink, it will get HOT!

Although the LM317 is spec'd for 20 watts, you do have to provide an
adequate heatsink to achieve that performance.

5. ### Rich GriseGuest

And just for the sake of monkey wrench, if you stack them, the high ones
have to pass _all_ of the current.

Cheers!
Rich

6. ### WallyGuest

So, if I want the transformer to be able to deliver its maximum current of
1A, then I might be approaching the limit for the regulators. The (vague)
plan is to have panel voltmeters on the outputs and an ammeter between the
rectifier/smoothing and the regulators - maybe a current limiting circuit
here would be a good idea.

7. ### WallyGuest

What do you mean by stacking them? Using more than one device to get around
the input voltage limitation, and get a higher o/p voltage?

8. ### David L. JonesGuest

circuit?

You can connect the inputs to the same filter cap, but it's *MUCH* more
useful to have three electrically isolated outputs. i.e. each regulator
circuit has it's own transformer (or transformer tap), it's own
rectifier and filter cap, and it's own adjustable regulator. The
grounds are not connected and the three circuits are totally isolated.
You could veen put in a switch to connect the ground when needed.

It's also very useful to have a direct adjustable AC output. Simply put
a muti-tap transformer in the box and feed the output straight through
through a selector switch.

Dave

9. ### WallyGuest

Is having three rectifier/etc circuits hanging off the same secondary a
no-no? Methinks a multi-tap transformer might be more expensive than three
of the single centre-tapped secondary that I have in mind (I'll do some
searching and see what I can find).