Voltage source design problem

Hello!

Could anyone push me to the right direction with a following design?


I need to build an voltage source that has an output by conrollable percentage from a variable reference voltage.

I need to control the voltage source with a CPU (with PWM, DC voltage or external DAC), potentiometers are not the solution for controlling, except digital pots.

For example:
Vout = x * Vref
Vref constantly varies somewhere between 12 -14V (voltage from car battery with alternator on/off)
x = 0.25 to 0.75


I've simulated many kinds of op-amp diagrams and such with no solution.
The changing Vref is the problem, gain changes with it and suddenly 0.5*Vref isn't 0.5*Vref anymore...
(I'm not sure if there is an simple op-amp design for this kind of situation)

Could I use MCU+DAC to easily implement this kind of voltage source?

One solution I thought was to 'read Vref with ADC - process MCU - Output an correct voltage with DAC and buffer the output with OP-AMP' This seemed unnecessary complex...

I'm more familiar with digital bus systems/protocols and programming, but analog electronics is quite new to me.


Thanks for any advice!

What is your load current specification?

Hi

It's a high impedance load, draws current 0,25mA (Z at 0,5*Vref = 12K)

If you use Vref as one input of a OP-amp and Vout as the - input, the op-amp then drives the series power transistors. The circuit will stabilize with Vout = Vref. If you put your digital pot across Vref and take its output to the input instead of Vref. The circuit should then stabilize at K X Vref, K being set by your pot.
Frank

For a digital voltage regulator i would use a LM317 combined with Microchips digital pots; they're awsome. The LM317 is a Linear regulator and can provide only 1A of current with not so good efficiency, especially if your input voltage is high. If you want 80+ efficiency then look at some switching regulators like LM2576. They need few external components and are quite cheap.Everithing to set the output voltage is documented in the datasheet.Hope this helps.

I drew for you one principle circuit to solve
the problem described in your original post.

View attachment Digi_Volt_Divi_Pot.pdf

Regards KAK

You can use a digital potentiometer here.

The important point is to retain the ratios of the three resistances.

The output will be any value between 25 and 75% of Vref.

The voltage follower assures that the divider isn't loaded. You can scale the input and/or output as you see fit.

Thanks kak111 and Syncopator!!

That's a simple and working solution!
Thanks!

dumdum123 said:

Thanks kak111 and Syncopator!!

That's a simple and working solution!
Thanks!

Click to expand...

A pleasure to be of help. It's quite satisfying to find a simple solution for something which at first seems more complicated.