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I have an RS-485-like (not a standard RS-485, but close to it) circuit with 35V+ voltages in it. I need to change resistance of the resistor, that installed parallel to my device data connectors A and B (in the range of ~120Ohms..5kOhms).

The main problem is that I need to do it in the run-time and do it with MCU. I know, that digital potentiometers aren't suitable for it, because voltage range of the resistor terminals should be in (Vss, Vdd) range. Are there any other ways to do it? Or it's impossible/useless and I should play with changing my circuit overall impedance (seems like impossible to me).

Update:

schematic

simulate this circuit – Schematic created using CircuitLab

As shown on the picture, I have U1 -- RS-485 similar optoisolated driver and need to control the resistance of R1 with MCU. The typical voltage ranges in the network can reach up to 35V+.

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    \$\begingroup\$ Relays can be used if you only need a few different choices of resistor. They can be almost perfect in terms of on and off resistance. Otherwise it will depend a lot on the exact circuit you have and the performance you need, so more information or we'd just be guessing. \$\endgroup\$ Commented Mar 5, 2014 at 19:11
  • \$\begingroup\$ @SpehroPefhany I was thinking about relays, but they will make resulting board too big, as I think. I've tried to provide more information. \$\endgroup\$
    – Darkkey
    Commented Mar 5, 2014 at 19:23
  • \$\begingroup\$ There are a lot of small telecom style relays such as G6L from Omron. Some are latching so they retain the setting even after power is cycled. \$\endgroup\$ Commented Mar 5, 2014 at 20:36
  • \$\begingroup\$ High voltage considered more than around of 50V, depends of which regulation followed by the country \$\endgroup\$
    – GR Tech
    Commented Mar 5, 2014 at 22:02

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If you can get away with a limited number of values and, providing there is always a net positive voltage on one wire with respect to the other, you could use a MOSFET for switching an extra resistor in circuit. This could be done via an opto-coupler but it has to be a low power mosfet else the open circuit (MOSFET off) drain-source capacitance could be a problem with the data edges being corrupted.

I'm not sure what resolution you can get away with (and still obtain the desired functionality and performance) but I think you can obtain octal packages and you could arrange the resistors so that the biggest value is 100R, next value 200R then 400 etc.. Basically you would create a DAC with dial up resistor values. Maybe 6 bits would be enough?

It's probably feasible to use Darlington transistors too (providing of course there is the DC bias on the circuit lines you mention. They definitely come in octal packages and would have significantly smaller capacitance than a MOSFET.

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  • \$\begingroup\$ Thanks! Nice idea, I will go this way (array of resistors), if the idea with relays fail. \$\endgroup\$
    – Darkkey
    Commented Mar 5, 2014 at 19:46

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