Help modifying a power supply

[gil1]

I salvaged a PS from a dead receiver that I would like to use for a preamp, the preamp runs on 18+ and 18- DC, the power supply I recovered from the dead receiver runs at 35+ and 35- DC

My question is how can I bring it down to 18+ and 18- ?

Please excuse me if this sounds like a dumb question but when it comes to power supplies Im pretty much a newbie.

Thanks in advance

[jitter]

There are many ways to do it. You write the preamp runs on +/- 18 V, but it's also helpful to know what the current drain from the PSU is in order to decide on how to bring the +/- 35 V down.

If the current drain is low or modest, a simple but very effective way is to use the ubiquitous 7818 and 7918 voltage regulators. Even better are the LM317 and LM337 voltage regulators. You'll find plenty of circuits on the internet with these...

[jackand08]

You might have to use a pre-regulator when using voltage regulators though, as the maximum input voltage for the 18V version of the LM series is 35V, pushing the limits of the regulators.

Rod Elliot has a suitable design for a pre-regulator here. It requires a few calculations to work out the values for the resistors, but is otherwise an extremely simple and functional circuit.

[jitter]

Originally Posted by jackand08

You might have to use a pre-regulator when using voltage regulators though, as the maximum input voltage for the 18V version of the LM series is 35V, pushing the limits of the regulators.

Oh, I forgot about that. Using a 7818/7918 directly on +/-35 V is indeed pushing it to its limit.

Rod Elliot has a suitable design for a pre-regulator here. It requires a few calculations to work out the values for the resistors, but is otherwise an extremely simple and functional circuit.

This could indeed be used in front of the 7818/7918 VRs.

But the LM317/LM337 could be used without this prereg. It's specified Vin-Vout = max. 40 V. Since your Vin = 35 V and your Vout = 18 V, Vin-Vout = 17 V, so that's well within the limit.

[jackand08]

I didn't think about the maximum voltage limit for the 317/337 but yes obviously, that would be fine.

[Speedskater]

Can you wire the power supply primary to the 230 V setting, but plug it into 120V ?

[random007]

Pre-Regulator but use 1N4746 18V zener diodes.

[ChrisA]

Originally Posted by gil1

I salvaged a PS from a dead receiver that I would like to use for a preamp, the preamp runs on 18+ and 18- DC, the power supply I recovered from the dead receiver runs at 35+ and 35- DC

My question is how can I bring it down to 18+ and 18- ?

Once you know the current draw of your preamp then select a resistor that will drop about 10 volts at that current and place it in series with your three terminal regulator. That resistor will take a lot of the heat off the three terminal regulator. Use a resistor that is rated in watts for about 4 or 5X more than you need.

But such a waste of a nice 35V power supply. It is likely 100+ times bigger than you need

[random007]

I don't hugely like the resistor idea. It's workable if the overall output wattage is, well, tiny, as is usually the case with pre-amps, granted, but using a resistor to drop voltage is... a bit ugly.

You want lower voltage but as much current as required: bypassing current across a low wattage voltage regulator via a transistor is the best of both worlds. I've yet to find any solution to this that's better in terms of control (regulation), noise suppression, maximum power, and/or overall energy efficiency.

If wanted, the Zener diode in post 7 can be replaced with a low-wattage regulator network.

Then again, the OP seems to have vanished!

[ChrisA]

Originally Posted by random007

I don't hugely like the resistor idea. It's workable if the overall output wattage is, well, tiny, as is usually the case with pre-amps, granted, but using a resistor to drop voltage is... a bit ugly.

You want lower voltage but as much current as required: bypassing current across a low wattage voltage regulator via a transistor is the best of both worlds. I've yet to find any solution to this that's better in terms of control (regulation), noise suppression, maximum power, and/or overall energy efficiency.

If wanted, the Zener diode in post 7 can be replaced with a low-wattage regulator network.

Then again, the OP seems to have vanished!

Maybe ugly. But you have to look at circuit requirements. He is powering at most a few op amps. We are talking about milliamperes. He has this 35V PS that is maybe 100X over sized for the job. He will get less noise and lowest cost with the power resistor between the 7815 regulator than if he puts a transistor in parallel with it. The transistor is in effect an amplifier with the 7815 in it's feedback loop. It is the "correct" solution if the 7815 can not supply enough current. We don't have that problem. Our problem is voltage, not current.

In terms of efficiency there is no difference, exactly as much power is wasted in heat with every proposed solution. The difference is in where the heat is generated and if you need a heat sink. I like the resister because it is it's own heat sink. You can buy even a 10W ceramic power resistor for 50 cents

The resisor also doubles as a filter. It would act as the top of a "pi" type CRC filter and also reduce voltage at the same time. Always when youhave extra voltage you should take advantage of it an use it to run a filter. In fact with so much voltage to burn you could ues two resistors and one more capasitor and build a CRCRC filter for about $1 and change.

The pass transistor does nothing to reduce noise, in fact adds some

It would be different if the preamp needed more then 1A of current.