+/- 33V outputs - What opamp could I use?

Thread Starter

atferrari

Joined Jan 6, 2004
4,016
I wonder if is it possible to do something similar to what is proposed here

http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/125091-pwm-signal-lm317-adjust-voltage-output.html

but this time to get two control signals: one from 0 to +33V to control an LM317 and another from 0 to -33V to control an LM337.

I imagine two separate opamps, one powered between 0 and +38V and the other between -38V and 0V. (I have those +/-38V DC available from an old power supply).

What could be the most suitable opamps?

Searching through Farnell I can at least compare some but the prices of the suitable(?) ones is scary. To come to my hands I have to think of them multiplied around 4 times!!

Maybe I am overlooking something...

Any suggestions? Gracias
 

t06afre

Joined May 11, 2009
5,934
Microchip have some very cheap 10 bit DA converters in 8 pin DIL package. I would have used that and a transistor. To control the output voltage of the LM317 in a digital way.
 

Thread Starter

atferrari

Joined Jan 6, 2004
4,016
Hola t06,

Could you show the actual schematics to get Vout =+34 V (LM317) and =-34V (LM337)?
 
Last edited:

t06afre

Joined May 11, 2009
5,934
Hola t06,

Could you show the actual schematics to get Vout =+34 V (LM317) and =-34V (LM337)?
The best I can get is by using 2N5459 and TL431. The voltage span will be from abot 2.42 to 36 with a control voltage span from -0.4 to about -1. The design is only simulated
 

Thread Starter

atferrari

Joined Jan 6, 2004
4,016
Sure I am, Sir!!

I was considering something with a 2N5461 but had no time to develope anything. Too busy at work.

Interested. Gracias
 

t06afre

Joined May 11, 2009
5,934
Does the output need to to be linear? I can manage a 36 to 2.5 volt sweep. Using a 0- about 4 volt control signal. And cheap components 2n5460 and TL431. But the output is not linear in respect to the control voltage
 

Thread Starter

atferrari

Joined Jan 6, 2004
4,016
Well, THAT was my idea. To have it linear, worth the effort of doing it digitally.

I suspected that a JFET should be quite hard to convince.
 

t06afre

Joined May 11, 2009
5,934
I found a cheap opamp that is rated up to 44 volt supply in single supply mode. It is the MC33171, and it is not exotic in any way. But I do not know how well it can sink current so I came up with this conceptual design (stress on conceptual). It should do what you want. But this will not solve your negative voltage needs. So my suggestion is that you put together two boards of this type to create a dual supply. And then use an optocoupler to transfer the control information to the other board, that will function as a slave board. You PWM (a thought that not really thrill me) or a cheap 8 pin 10 to 12 bit DA. Controlled by SPI. For controlling the output voltage.
A thing I have not evaluated in this design. Is temperature drift in the FET
 

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