how to tell V and A of a pot

It should have a power rating - that's the maximum power it can safely dissipate. From the resistance and power rating, you can determine the maximum safe current through the pot.

The arithmetic, of course, is left for the student.

Good Luck! Rich

Pots often have a wiper current limit that turns out to be lower, maybe much lower, than the Pdiss spec might imply.

John

Good Luck, indeed! Luck may well be needed. If the power is cranked through the entire pot, from end to end, then the pot's resistance and power rating may be relied upon. But, if the pot is used in rheostat mode, as a variable resistor, as is often the case, then the power may be dissipated in a small region of the pot's resistance element, and the true power rating will be a fraction of the nameplate value. I have seen pots repeatedly fail when improperly used in this fashion in expensive well-designed instruments (HP), so it's clearly a problem that, sadly, even highly-skilled engineers can fall prey to.

You're absolutely right, of course. Not even considering the wiper current limit that John pointed out, I should have been more clear that "that's the maximum save current through [any portion of] the pot", IOW, should have explained that if the pot's at, say, 10%, it can only do 10% of the power, and so on.

Thanks, Rich

--- You need to know the power rating of the pot and whether you're using it as a pot or a rheostat.

If you want to use it as a pot and you've got a 10k ohm 1 watt unit then, since:

P = I²R

you can rearrange to solve for the maximum current:

P 1W I = sqrt --- = sqrt ------ = 1E-2A = 10mA R 1E4R

The maximum voltage allowed across the element, then, would be:

E = IR = 1E-2A * 1E4R = 100 volts

If it's to be used as a rheostat then the power it can dissipate has to be derated as a percentage of the value of the resistance used to the total resistance of the element, like this:

PtR2 P2 = ------ Rt

Where P2 is the power dissipation allowed, Pt is the power rating of the total element, R2 is the resistance used Rt is the total resistance of the element.

For instance, using the 10k ohm 1 watt unit as a 5000 ohms rheostat:

PtR2 1W * 5E3R P2 = ------ = ----------- = 0.5 watt Rt 1E4R

Knowing that, you can use the procedure given earlier to figure out what the maximum current and voltage will be.

-- JF

Rich Grise wrote:

When I think of "pot", I think "tapped". Not saying "rheostat" explicitly (as Win did) when you mean "variable series resistor" clouds the issue.

I wonder if the googlie OP is even reading any of this.

Thanks, Rich