Electronics > Beginners
Using TIMM pot with 2 pins
anishkgt:
Hey All,
Not sure if a trimm pot can be used with only two pins as a variable resistor. I tried as per schematic but i don't see any change in voltage with a DMM at pins 1 and 2 of the optocoupler.
Would appreciate if somebody could advise.
JS:
Using a trim with 2 terminals as a variabke reaistor is fine, you usually want to connect the unused pin to the wiper so it doesn't go open if wiper fails or skips but no problem in your app.
I've used an H11AA1 in an aplication similar to yours with a fixed 1W 100k resistor to detect 220V mains active. No external diodes. Success and reliable so far, I have 15 of them working for a few years. It does take some power but this is an industrial application so a few mW isn't a real concern for efficency sensing a 50kW installation. Kind of hard to work it to archive any sort of threshold like it looks like you want with the variable trim, if you want to know if there is under or over voltage you should probably go a different way. I've also got it to work with 3 resistors as a zero crossing detection.
Maybe you can get it to work with a resistive divider and a zenner, then to the optocoupler. Also load on the other side of the optocoupler would change your results as current in both sides has a relationship and you will get some little current even if your voltage is much lower.
JS
vk6zgo:
--- Quote from: anishkgt on July 10, 2018, 07:47:48 pm ---Hey All,
Not sure if a trimm pot can be used with only two pins as a variable resistor. I tried as per schematic but i don't see any change in voltage with a DMM at pins 1 and 2 of the optocoupler.
Would appreciate if somebody could advise.
--- End quote ---
The voltage across pins 1 & 2 will be clamped at the forward voltage of the two back to back diodes.
anishkgt:
Tried without the diodes as well, nothing happened.
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JS:
I don't know what the soldering iron has to do with this project :-//
As I said, I wouldn't expect the voltage to change much at the input of the optocoupler, the current will, they are diodes after all, use a transformer to insulate from mains and have a look with your scope so you understand what's going on, I know what you will see... If you don't have a mains insulation transformer you can take two 9V transformers, hook the 9V windings together, plug the mains side of the first to mains, do your measurements using the second one as insulated mains.
JS
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