Digital pot for vrf drive

[aaronik19]

Dear All,

Is it possible to drive a vrf inverter and interface a digital pot such as mcp4313 to 0-10vdc input to control the speed?

[Ttelmah]

Yes, and no.

You can't control 0-10v directly with a pot like this. They are rated for
operation between Vss & Vdd, so off a 5v supply, 0 to 5v.
You can generate a 0-5v output (though with slightly more noise, and lower
accuracy), simply using a PWM output from the PIC.
Whatever method you use to generate the voltage output, you are gong to
have to simply amplify this to get 0-10v.
Alternatively, you could use an isolated MOSFET drive to give a PWM between 0 and 10v.

[temtronic]

A 3rd possiible option is to use a voltage doubler on the output of a PWM pin. I've done this for years to power 5V LCD modules from 3V PICs. You'd have to experiment with cap values, frequency and duty cycle but it should be easy enough to breadboard and test.
Jay

[smee]

Perhaps even a digital pot, in the feedback/sense line of an adjustable voltage regulator.

[Ttelmah]

That's just an amplifier.
How best to amplify would depend on the load current, but most control
inputs of this sort are quite high impedance, so almost anything can do
it. However key thing is stability...

[dluu13]

I am a fan of using the PWM. But my requirements are not exactly tight, with
a ~100 mV resolution required. This is an analog control signal for a bench
power supply.

I use the PWM fed through a passive low pass filter into a non-inverting
opamp with gain 3 to get 0-10V from my 3v3 pwm.

[temtronic]

Yes, opamp amp will work as well. I use a discrete one here to allow PIC to send +-40v signals on my energy control systems.
I'm currently looking at the LTC6090 to replace the 25+ parts with 1 !

Jay

[dluu13]

Opamps are great! temtronic's post just reminded me that if you do use an
opamp and you are powering from a single supply (V+ to GND rather than
V+ to V-) then you need to make sure that it's a rail-to-rail type or else you
won't be able to hit 0V.

Both the one temtronic mentioned (LTC6090) and the one I use (LT1784) are
rail-to-rail.

[dyeatman]

The circuit that Linear shows in the datasheet with the opamp being driven by
a DAC is a pretty elegant solution in my view. Allows controlling the VRF via
SPI or MicroWire... This would result in better stability, a finer range of
control and a set it and forget it circuit which would simplify the code.
_________________
Google and Forum Search are some of your best tools!!!!

[Ttelmah]

and, if you are a little careful with the design, it is amazing just how good both
the linearity and resolution can be.