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Digital adjustable resistor

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eslavko:
Hello...

Is it possible to make digital adjustable resistor based on attached schematics?
I can't find proper resistor values to have for example 100 to 1600 ohm values in linear range.

...and resistors are of range tens of watts...
[attach=1]

Kleinstein:
Switching with the MOSFETs only works well for 1 direction of the current and low voltge in the other direction.
Finding the right values may be tricky, but with high power one could use parallel / sereis connections. This can also help reducing the BOM.   Power resistors are often not very accurate anyway and may need some trimming / added parallel resistor for fine adjustment.

eslavko:
The 'resistor' will see DC voltage. I know that values for each resistor will be strange. The problem is that I fail to calculate R1...R4 values.

jwet:
You don't say but this but this looks a bit like an electronic load type circuit- unipolar.  You can make a constant R type load from a current sink by using the input voltage to program the sink current to a standard op-amp FET type current sink.  The higher the volts, the higher the current, just like an R.  You adjust the fraction of the voltage that take to adjust the R.  If this is just a one time need, most electronic load will do constant R mode.  Also at your resistances this wouldn't be too bad.  The confusing part is 10 watts across a 1600 ohm resistor is about 125 volts.  The benefit is this is that you don't need a lot of high power R's (expensive) just a bank of FET's on a good heat sink.

If I was making a more universal type "floating" resistor, I would consider using something like Photomos parts like the Panasonic AQY211EHAX.  These can do an amp continuous, conduct in both directions, etc.  The FET's are actually turned on by light from the LED's so you don't have to mess with biasing, etc.   This would allow you to make network the resistors in a 100,200,400,800 series string.  I would take 3 Photomos, the base 100 ohm would be unswitched minimum a Photomos across each of the upper 3.  You might buy 200's and series and parallel them to get your values- cheaper by the dozen.  Opto isolation is a nice side effect.

Don't overlook plain old relays or latching relays if power is tight.  They still have their place but at these resistance values, their best attribute- low low R is kind of wasted.  Have fun.  Also add a fuse to this rascal- if my calcs of the volts and amps are right, this could be dangerous during failures.

Kleinstein:
The get a large range and about equal steps it would be test to have resistors in 1,2,4,8,16,...  ratio. The steps would be equal for 1/R.
Depending on the intended use is may be also OK to have several equal resistors for the smallest value as this helps with the BOM.
With mechanical realys one could even consider unsing the same high resistors in series or parallel depending on the needs. This works well with 3 resistors in series and 2 additional links to switch to a parallel  configuration.

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