Limiting an unknown resistor to a specified lower resistance

[John Egerton]

Hey guys...

I'm still on my quest to make a touchpad resistor using the synthstick design and I have done it...

However at present the maximum resistance of the strip is unknown (no multimeter yet).

I was wondering ho best to go about limiting the maximum resistance of the strip to 5k?

Thanks....

John

[petemoore]

I don't know the layout or implementation but it seems a 5k resistor would do that.

[Rich G.]

Right, a 5k resistor in parallel will limit the value to 5k.

[John Egerton]

I see...

So this will allow me to use the strip to vary resistance and the resistance will never exceed 5k? Even if the strip was greater than 5k to start with?

do i put the resistor in parrallel with the input or the output of the variable resistor to achieve this?

[Mike Burgundy]

if you parallel two resistors Ra and Rb, the end value of the combo is
1/R=1/Ra + 1/Rb
If you think about this, this means that:(let's assume Ra is 5k and Rb is unknown)
-the only way R can reach 5k (the value of Ra) is when Rb is infinite (1/R=1/Ra+0)
- maximum resistance will always be lower than Ra, or Rb for that matter. If Ra=Rb=5k, the two in parallel will yield R=2.5k
You should parallel it *from input to output* to use your terminology.
Hope it helps.