Hi.
I wish to use Trimpots as series resistors. I shorted the middle pin of a trimpot to its outer leg, then I took the outer legs to be two legs of one usual resistor, I could trim the resistance.
But if I try to do that for 2 trimpots, and connect the outer pins in series, multimeter won't show the sum of two resistances.
How do I get them to act as series resistors?
The reason that I try to do this, instead of using ordinary resistors, is to get exact value of resistance to one-tenth of 1 ohm at least.
Thanks.
I have attached a Fritzing drawing with this post. Please check. I need to get sum of two resistances in the green wires.
I tried using wiper and one outer leg alone, I could trim resistance value, but I still couldn't get them to work as series resistors, I don't really understand the reason behind this
Yes, each individual 200R trimpot gets trimmed to 100 Ohms (read with multimeter), but with the connection in the image above, the multimeter doesn't show any output.
GrumpyMike, here is a photo, I don't have a higher resolution camera on my phone... i.imgur.com/CLejC4s.jpg
Hope you can see clearly at this image quality .. Thanks.
Crossroads, I will try your method and get back to you. Thanks.
Thanks for the photo but it is hard to see, and it doesn't show where all the wires go. Those are helical trim pots and not the trim pots in the Fritzing diagram, check that the wiper is where you think it is.
In the olden days it was done as a rule of thumb, also if the pot was noisy the maximum resistance (for an intermittent wiper) would be the resistance from outer leg to outer leg.
Never mind the olden days, it's still good practice when using a pot' as a rheostat to connect the wiper to one end of the pot. As you rightly suggest, if the wiper should lift or be noisy the maximum resistance is the end-to-end value of the pot rather than an open circuit.
In the imgur pic, the pot on the left has "1" and "2" jumpered and the pot on the right has "2" and "3" jumpered.
One is increasing when turned CW and the other is decreasing with CW.
Jumper them consistently.
Take them out and "pre-adjust" them to the middle.
There's no point putting trimpots in series with each other, you can't gain any
real accuracy - put one low valued trimpot in series with a 0.1% fixed resistor, so that
the low accuracy of the trimpot is diluted by the ratio of resistances.
A 5% 100 ohm trimmer resistor in series with a 0.1% 10k resistor will actually
contribute 0.05% error whilst allowing a 1% trim range.
If you want a high accuracy trimmer over a large range you need a 10-turn cermet
trimmer which are mechanically and thermally more stable.
Sometimes thermal stability is more important than value, in which case low tempco
resistors are needed, not necessarily 1% or 0.1% ones. This means metal film for
fixed and cermet for trimmers IIRC.
I thank all of you for your replies.
Runaway pancake's method worked and I got resistance as sum of two values. Thank you, I should have tried that earlier.
Mark T, I want to use trim pots because I couldn't purchase 1% or 0.1% metal film resistors in smaller quantities here.
I have 5% tolerance metal film resistors but I think they will not be good for quarter bridge Wheatstone for strain gauges. So I thought, maybe, if I could use accurate values set in trimpots as the resistors in bridge and it could be better
There's no point putting trimpots in series with each other, you can't gain any
real accuracy
True if the resistors are the same value but if they are of different sizes, say in the ratio of ten to one then one becomes the coarse control and the other one is a fine control.
.. Thanks.