Simple low current variable power supply.

So I want a little 0-20V, few mA (say 5 mA) adjustable supply, I do want it to go right down to zero. So I'm leery of just an opamp, because I want to stick ~0.1 to 10 uF on the supply. I made a simple high side pass element driven by an opamp thing. Works fine, but is there something simpler, better... (stronger, faster, able to leap small buildings) Maybe some opamp that can drive C-loads? (+/-18V rails) Or just do the trick where you compensate the C with the right series R and fast (C) feedback around opamp only.

George (many questions) Herold

Reply to
George Herold
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For small currents could you use something like an OTA/LM13700 with feedback arranged so it behaves like a unity-gain (or whatever) voltage amp? It kind of has the high-side pass element PNP "built in" if you think about it.

Reply to
bitrex

I know not the LM13700.

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So I'd first have to figure out how to use it. (not a bad thing.) It won't oscillate with feed back and a C load?

George H.

Reply to
George Herold

LM8261 is a nice RRIO c-load opamp.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

That's a natural for a C-load amp such as an LM8261. I was a big fan of its predecessor, the LM6361 bitd. Most op amps will work fine as long as you swamp their outputs with enough capacitance, or use the split-feedback app roach, where you put a resistor in series with the op amp output--use an RC to take the HF feedback from the amp side and the LF feedback from the loa d side.

Cheers

Phil Hobbs

Reply to
pcdhobbs

People have mentioned opamps. It would also be trivial to do a discrete regulator. With such low current you could use just a resistor before the pass tr to limit s/c i.

NT

Reply to
tabbypurr

I suggest that you use an honest linear power-supply-regulator IC. They're simple and ready-to-go with big output capacitors. Many are available having low 600mV FB feedback reference voltages. Use a few tricks to get them to work down to zero volts. First add a series output diode to help insure the output stage works down to zero volts. Add a current sink (to a negative voltage) so a zero-volt output is working class A with non-zero current. Third, add a 1.2V reference with a resistor to drive the FB pin, so the circuit can balance down to zero volts and a bit below.

Alternately, connect the ground pin of the regulator IC to -600mV.

Alternately, find a linear regulator meant to work to zero volts. Then let us know which part that is.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Thanks, looks nice... It's got enough current to drive the output faster than input? I'm going to have to remember why opamp -> C oscillates... phase lag here and there and before you know it. Hmm note 3 of table/ figure 6 looks sticky,

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no short circuit protection.... ? I might be better with the classic solution, R's and C's here and there.

George H.

Reply to
George Herold

ts predecessor, the LM6361 bitd. Most op amps will work fine as long as y ou swamp their outputs with enough capacitance, or use the split-feedback a pproach, where you put a resistor in series with the op amp output--use an RC to take the HF feedback from the amp side and the LF feedback from the l oad side.

So swamping the output means you exceed the current limit of the opamp? I'm going to have to play around more.

George H.

Reply to
George Herold

OK, price is not an object, and an opamp has built in protections. In this case, I also need a supply that doesn't mind having it's output shorted... I didn't mention that in the beginning. I guess you can do that with transistors. I'm mostly more comfortable with opamps.

George H.

Reply to
George Herold

You can do all that with a single transistor, with output down to zero. It's as protected as any opamp. The only downside is less perfect regulation.

NT

Reply to
tabbypurr

Thanks Win, Sounds more complicated than the compensated RC opamp solution that I know. (always better with the devil you know.) Regulators are noisy too. not that I think it matters much here.

George H.

Reply to
George Herold

It doesn't have a thermal limit. Since you don't need much current, you could put some resistance in the positive supply rail.

Most opamps can drive capacitive loads if you add a few cheap passives.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

20v 5mA is 0.1 watt out. At 0.15W thermal protection is unlikely to be needed.

NT

Reply to
tabbypurr

No, just using a lot of capacitance. Your garden-variety op amp oscillates with load capacitances within some range. Below the lower limit, it's okay, and above the upper limit it's okay too, although very slow of course.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

It'll saturate first (no gain), which usually is an oscillation killer. That doesn't help the regulation, though... your slow VOM will show good voltage stability, but when a current pulse is drawn from the circuit, it's ALL capacitor and no regulator.

A HV-capable op amp that senses near one rail, is a good driver for the pin with the output voltage; some resistance (or R paralleled with antiparallel diodes) would work, but I'd prefer to just put a bit of resistor degeneration in series with the capacitor. If a problem arises afterward, that resistor can take a differential probe to show me what the HF disturbance looks like.

Reply to
whit3rd

Yeah, many opamps are happy when loaded by a huge amount of low-esr ceramic cap. I need to characterize a few of my faves for that.

I think some do go into a limit-cycle sort of oscillation into a big capacitive load, as the internal compensation node winds up. RRIOs are less likely to do that.

"Very slow" doesn't matter when the output caps dominate; the opamp just has to wake up and keep them charged.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

rote:

of its predecessor, the LM6361 bitd. Most op amps will work fine as long as you swamp their outputs with enough capacitance, or use the split-feedba ck approach, where you put a resistor in series with the op amp output--use an RC to take the HF feedback from the amp side and the LF feedback from t he load side.

?

I was playing with the opa2134 today. At a gain of +5 it was good with any C... well some ~few us jaggies at 0.01 to 1 uF. (1 k load R, 4k FB R.) At unity gain it oscillated with 0.01 and 0.1 uF I didn't explore the edges. I added 10 ohms of output R and 330 pF of C_f and it looked OK... maybe I need more R?

George H.

Reply to
George Herold

n of its predecessor, the LM6361 bitd. Most op amps will work fine as lon g as you swamp their outputs with enough capacitance, or use the split-feed back approach, where you put a resistor in series with the op amp output--u se an RC to take the HF feedback from the amp side and the LF feedback from the load side.

mp?

ry

I should try the opa2192. Hmm you know what I need to spin..little pcbs with mouse bite edges and a dual opamp soic8 (with bypass caps) in the middle. (riffing on JL's thing.)

GH

Reply to
George Herold

ote:

f its predecessor, the LM6361 bitd. Most op amps will work fine as long a s you swamp their outputs with enough capacitance, or use the split-feedbac k approach, where you put a resistor in series with the op amp output--use an RC to take the HF feedback from the amp side and the LF feedback from th e load side.

You have to be a bit careful with that approach. At least some op amps keep on oscillating when loaded with big capacitances. The amplitude of the osc illation can be too small to be easy to see, but the op amp is spending mos t of its time saturated, so the gain is very low.

--
Bill Sloman, Sydney
Reply to
bill.sloman

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