power supply error amplifier

Hello all, I'm working on a home brew regulated high current linear power supply. it uses an NPN transistor array driven by a 3 terminal regulator.
The error drop is about 1.8 volts at 18 amps. I would like to reduce the drop to .2 volts or less at 13.8 volts. Vo is 0-20V Vin is 33V
I realize an opamp is probably the way to go, but I'm wondering is anyone has a really clever high reliability circuit that they can point me toward.
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Do you mean you're feeding the output of a 3 terminal regulator into the base of a (number of) emitter followers?
What do you mean by the error drop? The base-emitter voltage of the NPN emitter followers? (sounds a bit high)
You could compensate for the base-emitter voltage by adding some voltage drop in the ground line of the 3 terminal regulator, such as 2 or 3 series diodes.
But you are right, an op-amp is the way to go.
--
Andrew Gabriel
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Andrew Gabriel wrote:

Yes
it includes a .5 ohm resistor for each transistor. Vbe is .7 the resistors are 30W wire wound and were originally intended to provide a voltage drop for a current limiting circuit in addition to equalizing the current in the pass array.

The drop varies dependant upon load currents. It is not a problem at light loads. It also is not a problem at continuous loads. This 'brute force' supply is my answer to all low voltage DC needs from ammeter calibration to winch motor operation.

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TimPerry wrote:

You will find one way of using a three terminal regulator with an external series pass transistor array, here:
http://www.national.com/ds/LM/LM117.pdf
This technique will give you the voltage regulation you need - at the point where R4 is connected to the output rail. Voltage losses in the cabling after that point will not be included in the feedback loop. So make sure that your cabling is up to the job!
--
Sue




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Palindrome wrote:

Thanks Sue, The emitter resistors are followed by a high current meter shunt. I can sample directly to the output terminals. The cable is 10 AWG stranded copper.
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TimPerry wrote:

Sounds good. I would be tempted to put the meter in the input side of the regulator, rather than the output side. It should decrease output noise.
Another minor mod is to use a non-linear resistor (OK, a lamp...) as the current demand sensor ahead of the three terminal regulator main input. This will reduce the dissipation in the three terminal regulator at medium and high current demand, improving the regulation.
--
Sue

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Palindrome wrote:

It is a very quiet shunt, I have not heard it complain once <g> I believe it drops 50 uV @20 A

A fine point I had not considered. I assume the lamps would have to operate an nearly full brightness to be effective. Full load base current is about 350 mA.
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wrote:

Full brightness only at full load. Dimming as loading decreases to get the effect desired.
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wrote:

How long are the leads between your supply, and the load, and where are you measuring the drop?
Also, what gauge are said leads?
Another thing to consider is taking your error amp's test point out to the load location.
Of course the correct means is to utilize an op amp. They are cheap, ya know.
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JackShephard wrote:

I can't agree that this is the "correct" (ie best engineering) solution.
The OP already has an excellent op amp optimised for power supply regulation - built into the three terminal regulator. He also has a high current series pass element to use with it. All he needed was a way of connecting the two to achieve the desired result.
If "another" op amp is to be used, there is little point in using a three terminal regulator as a voltage reference, where a resistor and zener will do.
Whilst re-inventing the wheel is one thing, re-inventing the wheel, when you already have an expertly-designed one to hand, does seem a little OTT.
--
Sue


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Palindrome wrote:

about 12 inches on the pos, somewhat shorter on the neg.

10 awg stranded copper, silver soldered and crimped to lugs.

I built this in the late 70's and have used a variety of fixed and variable supplies to drive it. It features 8 parallel T03 transistors on a large finned heatsink, large, mirror scale voltage and current meters. It is rack mountable and has dual front panel carry handles. The filter cap is a large electrolytic, 100,000 uF. The bridge rectifier is a matched pair of 30A units heat sunk to the chassis. I don't normally parallel diodes, however either one should be more then enough to handle full load. the test load at the time of construction was a pair of aircraft landing lights. If I recall correctly they were about 10A each.
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