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I am trying to make a simple variable resistor. My first thought was an actual variable resistor but it is high current so not ideal. New thought is a 'digital' resistor. If I put three resistors in series, each with a switch across it then I can effectively turn each one off. If the resistors are in the ratio 1:2:4 then I get a nice linear scale and can make 0, 1, 2, 3, 4, 5, 6, 7. Perfect, except that is not quite what I want. I want a little more at the bottom and less at the top eg 0.6, 0.9, 1.4, 2, 3, 4.5, 7. So, is there a similar simple circuit and low number of resistors to achieve this? I'm thinking one in series one in parallel but I can't quite get it. Can this work or are there any other ideas?
This is the simple 1- 7 circuit:
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Add another in parallel across all three, or any two? Can't be bothered to do the maths to be honest. 🙁
My first thought was an actual variable resistor but it is high current so not ideal
What's the context?
Just need to drop a little voltage (a sort of clunky light dimmer) but it can't be switch mode so needs to be a resistor and its about 3 A, max resistance around 12 ohms. It is possible with a variable resistor but I can't find a suitable (except from china or crazy expensive) and I quite like the dip switch style solution so it can be set and forgotten
Wont a switchable 1 Ohm in parallel with the first drop it to a combined value of 1/2 Ohm?
EDIT Forget that. You couldn't get 1.5. Doh!.
I think you're going to need to re-think this. You're proposing to drop more than 100W though three or four resistors...
Add another in parallel across all three, or any two?
Well, yes. That is what I have been trying but I can't make any of the combinations I have tried work. They all lose a bit or two, which I can live with, but none give anything like the right curve. It is actually quite an interesting puzzle which I am happy to carry on with but I don't actually know if there is a correct answer waiting for me
I think you’re going to need to re-think this. You’re proposing to drop more than 100W though three or four resistors
Not quite. The current drops off very quickly with resistance so the peak is only 4 or 5 W, although I would like a bit of margin
Is this for a fixed load or something with a variable power consumption. Does it run on a DC or AC supply?
If it is a fixed DC load (like a 12V light source), you could implement a constant current source using any old low power potentiometer (well perhaps not any old one but most out there would work), a low value, high current resistor, a beefy MOSFET and an op-amp (possibly throw in a couple of small resistors and caps too, just to make it nice and stable)?
If necessary/useful I can draw pictures (or find a suitable web link).
A bit like this:
