The use of Digital Potentiometers.

I have a big old analogue meter from an ancient multimeter like an Avo 8. I'd like to use it in a volt/ohms setup controlled by a Nano or Micro. My question is can I use a digital Pot in the feedback network of an Opamp, and can I use one in series with the 1.5mA meter movement. I was thinking of about 6 volts as the Opamp output and using an MCP603.

A digital pot is primarily for use as a volume control.

Just what do you actually want to do?

I suspect you may be simply wanting a DAC.

My question is can I use a digital Pot in the feedback network of an Opamp,

Yes, but and it is a big BUT
You must not exceed the voltage rating of the pot which in most cases is the supply rails of the Arduino, so if you run the MC602 from 6V then no.

Can you draw a schematic of the proposed circuit on a blank sheet of printer paper, take a photo and post it ?

Are there any markings on the meter that might be used to find reference documentation ?
You haven't posted the meter specs so how can we answer your question ?
How does this help ?

What is the FS rating ?

Alright, here is a rough sketch. It’s not meant to be a working model but it shows my ideas. I was thinking that perhaps a digital pot might replace the gain defining resistors R6, R13 and R19. and also the meter current resistors R12 and R16.

I imagine that the former will not have a suitable voltage across it, while the latter might work - there’s no problem lowering the Opamp output voltage. Oh, and the Opamp will be a AD822, not an MPC603.

The meter needs about 1.5mA FSD and has an internal resistance of about 67 ohms. There are no specs because it was made between the wars!

I don’t think I’m wanting a DAC. The function of the pot at the positions I give is a simple attenuator, which will be controlled by the Nano via the I2C bus.

To make things a little clearer, I am thinking of using Reed switches for the range selection and a Reed relay to double (or half) the gain of the Opamp, because the meter scale is marked 0-150 and 0-75 - see what I mean?

The second opamp is probably unnecessary but there are two on an 822 and the output voltage swing is limited to +/- 10mV of the supply rail, so the second Opamp is aimed at supplying the compensating 10mV and giving a true zero reading.

You cannot power relays from an Arduino output.

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I understand that each output on a Nano is rated at 40mA. A reed relay with a 500 ohm, 5V coil should draw 10mA. Am I wrong?

20ma can be safely sourced per pin.
40ma is absolutely max is the maximum rating, not the same thing.

At 10 ma you should be OK.

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The input to A0 will be <= 5volts? (0 to 5)

IMO I prefer your hardware layout instead of a digital pot.

The input to A0 will be <= 5volts?

No it won’t, I mentioned that a lot earlier.

Okay, so now we agree they are not necessary I send a schematic with transistors. The question remains though, can I use digital pots in the positions I mentioned?

Put the 5/6 volt question to one side, the new schematic has a 5 volt power supply.

I forgot! Thanks for the complement LarryD. The reason for the digital pots is to get the Nano to auto-range and auto calibrate.

For auto range I have used H11F2 OPTOFETs.
http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/H11F1,2,3.pdf

Being old, I like relays too.

2016-06-22_15-29-33.png

For auto calibration, do some experimenting to see if, 1st it is necessary and 2nd if a digital pot works.

Thanks LarryD. I had no idea an Optocoupler would display linear resistance - I'll look into it. It looks promising!

I had a look at the Fairchild H11F2 datasheet - not much on it! It seems the device is obsolescent but I think there's an alternative NTE3085.

Unfortunately the Nano has no analogue output which necessitates the use of PWM and an integrator. I get the feeling the result will not be reliable enough for a meter circuit.

However, the H11F2 datasheet actually shows the device in the feedback network of an opamp just as I imagined, so perhaps a digital pot like DS1803 might work.

I suppose your point LarryD that auto calibrate might be unnecessary is valid and I'm getting a bit too carried away!

So now it's time to get a few bits and pieces together.

LarryD:
20ma can be safely sourced per pin.
40ma is absolutely max is the maximum rating, not the same thing.

At 10 ma you should be OK.

Only if you have a free-wheel diode. Otherwise you'll destroy the pin.

MarkT:
Only if you have a free-wheel diode. Otherwise you'll destroy the pin.

He has the diodes shown in his schematics.

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H11F1M

About $4.50
http://ca.mouser.com/Search/Refine.aspx?Keyword=512-H11F1

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