Simple low current variable power supply.

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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

Re: Simple low current variable power supply.
On 10/11/2018 03:37 PM, George Herold wrote:
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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.

Re: Simple low current variable power supply.
On Thursday, October 11, 2018 at 3:51:47 PM UTC-4, bitrex wrote:
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I know not the LM13700.
http://www.ti.com/lit/ds/symlink/lm13700.pdf

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.  

Re: Simple low current variable power supply.
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

Re: Simple low current variable power supply.
On Thursday, October 11, 2018 at 7:01:25 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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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.  
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So swamping the output means you exceed the current limit of the opamp?  
I'm going to have to play around more.

George H.

Re: Simple low current variable power supply.
On 10/11/18 10:52 PM, George Herold wrote:
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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

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Simple low current variable power supply.
On Fri, 12 Oct 2018 09:12:56 -0400, Phil Hobbs

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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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Re: Simple low current variable power supply.
On Friday, October 12, 2018 at 5:06:27 PM UTC-4, John Larkin wrote:
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rote:
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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.
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?
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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.

Re: Simple low current variable power supply.
On Friday, October 12, 2018 at 7:48:33 PM UTC-4, George Herold wrote:
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 wrote:
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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.
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mp?
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ry  
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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  

Re: Simple low current variable power supply.
On Saturday, October 13, 2018 at 12:13:05 AM UTC+11, Phil Hobbs wrote:
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ote:
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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.
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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

Re: Simple low current variable power supply.
On Thursday, October 11, 2018 at 1:12:38 PM UTC-7, George Herold wrote:
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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.

Re: Simple low current variable power supply.
On Thu, 11 Oct 2018 12:37:39 -0700 (PDT), George Herold

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LM8261 is a nice RRIO c-load opamp.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Simple low current variable power supply.
On Thursday, October 11, 2018 at 6:54:58 PM UTC-4, John Larkin wrote:
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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,
http://www.ti.com/lit/ds/symlink/lm8261.pdf

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

George H.


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Re: Simple low current variable power supply.
On Thu, 11 Oct 2018 19:44:40 -0700 (PDT), George Herold

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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  


Re: Simple low current variable power supply.
On Friday, 12 October 2018 04:21:38 UTC+1, John Larkin  wrote:
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20v 5mA is 0.1 watt out. At 0.15W thermal protection is unlikely to be needed.


NT

Re: Simple low current variable power supply.
On Thursday, 11 October 2018 20:37:42 UTC+1, George Herold  wrote:

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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

Re: Simple low current variable power supply.
On Thursday, October 11, 2018 at 8:50:29 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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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.  

Re: Simple low current variable power supply.
On Friday, 12 October 2018 04:02:35 UTC+1, George Herold  wrote:
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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

Re: Simple low current variable power supply.
George Herold wrote...
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 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

Re: Simple low current variable power supply.
On Thursday, October 11, 2018 at 9:51:39 PM UTC-4, Winfield Hill wrote:
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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.  
  

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