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30V PSU with current limiting

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Frosty_47

New Member
I designed a relatively simple +30V PSU. Output current can be adjusted via Rsense resistor. A -30V PSU can be build by use of complementary transistors and LM337.



I was thinking of adding a DPDT relay. One contact will be wired to high power low Ohm resistor that will be in series with the circuit right after the bridge. This is to limit the inrush current. Once the capacitors are at their peak voltage, a timer circuit will toggle the relay and bypass the series resistor.


Please comment, as your advice is valuable to me
 
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Hero999

Banned
Have you tested or simulated it?

That won't work, the n-channel MOSFETs on the output will never turn on because they're permanently biased off as the gate will always be below the source.
 

Frosty_47

New Member
Have you tested or simulated it?

That won't work, the n-channel MOSFETs on the output will never turn on because they're permanently biased off as the gate will always be below the source.
Darn it! Thanks for pointing that one out.

I mixed up the position of zenner diode with resistor.

Oh, and I am yet to test this circuit
 
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crutschow

Well-Known Member
Most Helpful Member
The circuit also will have poor regulation because you can not use a remote sense connection (as you show) to eliminate the drop from the Rsense and output transistors (see the 317 data sheet section about "Load Regulation" ). The 317 adjusts the output to get a 1.2V drop between the output pin and the ADJ input. If you connect the adjust resistor to the load point and not the 317 output pin, any voltage drop on the output line will just be subtracted from the reference voltage, degrading the regulation.
 

Hero999

Banned
You should measure the current going into the LM317 and perform current limiting as required.
 

crutschow

Well-Known Member
Most Helpful Member
I agree with Hero999. Place all the current limit circuitry at the LM317 input.

But you can't have any resistors or circuitry on the output or you will degrade the regulation, so you need to also remove the emitter follower booster from it's location. Use the "High Current Adjustable Regulator" configuration shown on page 17 in the National LM117/317 data sheet.
 

Frosty_47

New Member
I agree with Hero999. Place all the current limit circuitry at the LM317 input.

But you can't have any resistors or circuitry on the output or you will degrade the regulation, so you need to also remove the emitter follower booster from it's location. Use the "High Current Adjustable Regulator" configuration shown on page 17 in the National LM117/317 data sheet.
Thanks for your suggestion. However I am abandoning this design. I am working on a +/- 40V supply design that does not rely on LM317 for regulation. 317 will only be used to set a voltage reference. The rest will be taken care by an op-amp circuitry. I will try to upload the new design within two hours
 
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Frosty_47

New Member
Oh and if I cascade several LM317 in series, will that increase noise rejection? My goal is 30V reference. Input is 55V with less than 100mV input ripple.

Thanks
 

Hero999

Banned
Why not use the HV version?

What's this power supply for?

How much current do you need?

Does it really need to be regulated?

Do you need current limiting?

The LM317 is fine providing the input-output differential is below 40V. A 39V zener can be connected across it to protect it from transient short circuits. If over current protection is still required then it can be done before the LM317.

I don't see any need to cascade a load of LM317s unless you've got no choice in voltage you want to drop. In this case since you have a choice, make the voltage drop as low as you possibly can.
 

Frosty_47

New Member
Why not use the HV version?

What's this power supply for?

How much current do you need?

Does it really need to be regulated?

Do you need current limiting?
PSU is for audio AMP

4 Amps per rail

I will put current limit before...

I have two 337HVK versions (got em for $4 ). However, 317HV is expensive.
 
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Hero999

Banned
A properly designed audio amplifier doesn't need a regulated supply.

Current limiting is normally done with fuses.:D

If this is an experimental design I would advise using PTC resistors (poly(fuses/switches)).
 
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