Can we make a digital pot simulate a much larger value?

[OldMarty]

Hi All

I want to replace some old legacy analog pots with modern digital pot(chips) such as the microchip MCP4xxx range.
This ‎will then allow me to control the pot values from a remote panel or external control equipment.

Some of the original pots are high values like 250k, 500k and even 2meg, but i see that digital pots are 'upto' 100kohms, but mainly 10k or 50k are more popular.

So, is there a sneaky way i can add an external resistor network to effectivly change a 50k digipot into a 500k pot as 'seen' by the existing circuit????

I remember years ago seeing an article about changing log into linear pots (and/or vica versa) by adding a few additional resistors, so i wonder if a similar method can extend the range of a digpots total resistance????


Thanks in advance,
Marty.

[pedja089]

Not really.... You can add another resistor in series, but then you won't have full range.
But you can make your own digital pot with few (mos)fets and resistors.

[OldMarty]

Thanks for that, i was hoping "No" wasn't going to be the answer, and making a pot with several fets and resistors becomes too large and and defeats the purpose.

[pedja089]

You can get 8bit(255 levels) with just 8 resistors and 8 mosfets. If done in smd, it doesn't take very much space.

[Alberto]

You can get 8bit(255 levels) with just 8 resistors and 8 mosfets.


How can you get 255 steps with 8 resistors and 8 mosfet?

Can you post the schematic of such a device?

Alberto

[Charlie]

8 resistors go in series. Their values are X, 2X, 4X, 8X, 16X, 32X, 64X, 128X, and so on if you want more steps. an FET goes across each resistor. Turn all off for a value of (close to) 256X. Turn all on for a value (close to) 0. Use combinations of off and on to get all values in between. Straight binary math.

But why do you need a higher value?

[Dave]

Here is a schematic id did about 20 years ago for a dual 16 bit resistance card to be incorporated into a test system. Its maximum resistance was 131071 Ohms.

[amgen]

8363

R is max value of pot (mosfet is off), have to test range of voltage to mosfet gate to get your pot value operation, then use the digital pot to work within that range. If need isolation, need to then use optoisolators.