# Preset ratings for a comparator circuit

1)What are the rating for these kind of presets? I need the maximum current they can withstand without any harm, (Imax). [data need for a 100k preset]

2)I'm hoping to use this kind of voltage divider method to set reference voltages to a voltage comparator circuit. Because ref voltage should be able to adjust. Is there any good method to set reference voltages rather than this?

• The weak link is not what current enters the top or bottom of the resistor but what enters or leaves the wiper. Oct 2, 2013 at 9:38
• How much voltage are you using that you would put enough power into a 100k trimmer to damage it? Oct 2, 2013 at 9:47
• I haven't decided the Vmax yet. To decide that I need the maximum current (Imax) that the preset can withstand. Assume that output draws no current.
– Niyu
Oct 2, 2013 at 9:53
• I'd consider using a multi-turn pot for setting a control voltage on a comparator - more reliable and easier to set the precise value because they are multi-turn. Oct 2, 2013 at 10:01

Here's one you could use: -

The important parameters are shown in red

• Wiper current = 50 mA
• Power rating = 0.2 W
• Max voltage across pot = 100 Vdc

If you use the maximum power limit (0.2 W) and the resistance (100 k$\Omega$) you can calculate the maximum allowable voltage across the device: -

$Power = \dfrac{Voltage^2}{Resistance}$ therefore voltage = $\sqrt{0.2\times 100k}$ = 141V.

However, it says in the spec the voltage limit is 100 Vdc so, what current would flow into the 100 k$\Omega$ pot if this voltage were applied? Answer is 1 mA.

This tells you that you need to ensure you don't put more than 100 V across it and you won't get more than 1 mA and your power rating won't be exceeded.

Because the value of the pot is so high, it will never pass enough current to cause a problem with the wiper. This statement is made with the understanding that the wiper takes no current as per a comment made by the OP.

However, it's very important that if you have a circuit where the wiper does draw current, you keep it well within the boundaries stated in the data sheet.

• @DaveTweed I shall add what the OP says in one of his comments about not using the wiper into a load. Oct 2, 2013 at 12:14

I used pots like this one up to 7-9mA any higher than that tends to be damaging them.

• I dont need to pass a much current It would be 5mA<. So Imax will not become a big issue I guess. Thank you.
– Niyu
Oct 2, 2013 at 10:39