Simple low current variable power supply.

Started by George Herold October 11, 2018
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
On 10/11/2018 03:37 PM, George Herold wrote:
> 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 >
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.
On Thursday, October 11, 2018 at 3:51:47 PM UTC-4, bitrex wrote:
> On 10/11/2018 03:37 PM, George Herold wrote: > > 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 > > > > 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.
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.
On Thu, 11 Oct 2018 12:37:39 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>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
LM8261 is a nice RRIO c-load opamp. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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 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 the load side. 

Cheers

Phil Hobbs
On Thursday, 11 October 2018 20:37:42 UTC+1, George Herold  wrote:

> 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
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
George Herold wrote...
> > 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.
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
On Thursday, October 11, 2018 at 6:54:58 PM UTC-4, John Larkin wrote:
> On Thu, 11 Oct 2018 12:37:39 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >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 > > LM8261 is a nice RRIO c-load opamp. >
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.
> > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
On Thursday, October 11, 2018 at 7:01:25 PM UTC-4, pcdh...@gmail.com wrote:
> 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 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 the load side.
> > Cheers > > Phil Hobbs
So swamping the output means you exceed the current limit of the opamp? I'm going to have to play around more. George H.
On Thursday, October 11, 2018 at 8:50:29 PM UTC-4, tabb...@gmail.com wrote:
> On Thursday, 11 October 2018 20:37:42 UTC+1, George Herold wrote: > > > 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 > > 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
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.