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Transformer voltage for dual power supply (+12V / -12v)

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belzrebuth:
I've built the power supply with a standard bridge rectifier (although I've bought the diodes mentioned by Ian.M).
I can draw about 0.6A without any voltage drop but at after 1.0A I get a voltage drop of about 0.2 to 0.5V which may or may not matter for the modules I'm going to power.
I better be on the safe side and have 12,000 at all times if possible.

I was thinking instead of replacing the diodes and mess with the outboard wiring and heatsinking couldn't I just find a pin to pin replacement LDO regulator for the LM317/LM337?
I need low-noise low need for regulation voltage (about half of the that of the LM317 would be nice) and about the same or more max current rating..
Is there something that fits these specs?
I'm not particularly concerned about the cost since it's only two pieces I'm interested in.

Ian.M:
Have you scoped the output rails under load?  Your 0.2V to 0.5V voltage drop may actually be a ripple breakthrough of double that!  If you don't have a scope, put a 1uF non-electrolytic cap (or nearby value) in series with your DMM on AC V and multiply the result by 2.8 to approximate the peak to peak ripple voltage.

--- Quote from: belzrebuth on August 31, 2018, 03:14:06 pm ---I was thinking instead of replacing the diodes and mess with the outboard wiring and heatsinking couldn't I just find a pin to pin replacement LDO regulator for the LM317/LM337?

--- End quote ---
You could, but you'll have a hard time finding one that will 'drop in'.  Even if the pinout is correct, LDOs typically have far tighter constraints on input and output capacitance and especially the ESR range required, than LM3x7 regulators and if you get it wrong there's a risk of them breaking into oscillation, which I've seen bad enough cases of in the HF band to disturb other electronics over 30m away. 

Depending on the load you are powering, it may be acceptable to tweak the Ajd pin to Gnd resistor values down a bit to drop the regulated voltages to a symmetrical 11.5V to give the regulators more headroom.  That's certainly better than letting them droop on their own as it avoids ripple breakthrough.

The diodes I proposed wont get as hot as the existing bridge rectifier.  Bolt them down on a  sheet of Aluminum that's big enough for a 1" margin round them and they'll do fine.

Another option if its an open center toroidial transformer would be to add two overwinds to put in series with the existing secondary windings to boost the voltage.   Start by putting 10 turns of hookup wire round its core, through the center.  Measure the AC voltage across the hookup wire ends, then calculate how many turns you need to get the voltage headroom you need.  Post a photo and we can suggest how to wind it.

David Hess:

--- Quote from: belzrebuth on August 31, 2018, 03:14:06 pm ---I was thinking instead of replacing the diodes and mess with the outboard wiring and heatsinking couldn't I just find a pin to pin replacement LDO regulator for the LM317/LM337?
I need low-noise low need for regulation voltage (about half of the that of the LM317 would be nice) and about the same or more max current rating..
Is there something that fits these specs?
I'm not particularly concerned about the cost since it's only two pieces I'm interested in.
--- End quote ---

The LT1086/LM1086/LD1086 is a low dropout improved 1.5 amp replacement for the LM317.

The LT1085/LM1085/LD1085 is a low dropout improved 3 amp replacement for the LM317.

The LT1033 is a low dropout improved 3 amp replacement for the LM337.

floobydust:
I found no benefits to increasing the filter capacitors past 10,000uF because the noise of the LM317/337 dominates. Some manufacturers LM317's have a lot of popcorn noise. LT I found the best and Fairchild the worst for noise performance.

There is a performance limit with these convenient 3-pin parts. To do much better, you need more than 3-pins in an IC Vreg package, in order to connect a reference filter capacitor.

Adding huge output capacitors 1,000uF-4,700uF to the LM317's is what I see commonly done, as a compromise.

David Hess:

--- Quote from: floobydust on August 31, 2018, 06:12:05 pm ---There is a performance limit with these convenient 3-pin parts. To do much better, you need more than 3-pins in an IC Vreg package, in order to connect a reference filter capacitor.
--- End quote ---

Bypassing the lower resistor in the divider with a capacitor goes a long way to reducing the output noise but I agree; better performance requires a different part or circuit.

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