required potentiometer power rating?

Question

I have a train set transformer that puts out 16VDC rated at 7va. I want to use a pot as a rheostat to control the current. how many watts does the potentiometer need to be able to handle?

Answer 1

If the transformer can put out 7 VA, then you'll need a reostat with a rating of at least 7 watts. It won't hurt if you get a higher rated reostat.

I'm not sure why you need a reostat. I thought most train set transformers have a built-in adjustment. My old Lionel one did...

Assuming a fixed resistance of the train engine (and other accessories), you alter the voltage and Ohm's Law alters the current. V ÷ R = I

Answer 2

For a start the power if the train takes the full load from the transformer would be 7 volt amps (which is about the same as 7 watts). So if you get a 10 watt pot you should be in perfect shape.

Are you sure you meant to say 7VA and not 7 amps?

Answer 3

7va in other words 7 watts.

You're going to have a hard time finding one over 3 watts.
Digikey has some 12watts but they're like 30 bucks.

My old trains never had the much control over the speeds and it wasn't a problem. What about setting up a few resistors range on ohms. You should be able to get at least 5 speeds with 3.. 10-15 watt resistors are less than buck.

Answer 4

Well, 7va is 7watts for DC, so a 7 (or 10) watt rating would be reasonable, since it is unlikely that _all_ the power will ever be dissipated in the rheostat, some will always go to the load except in the case of an accidental short circuit, you might get by with a 5watt unit.
If you are in fact running a model train you will , however, get better results using a solid state train speed control. 7va is not much power, is this "N" gauge? a small power supply is not very expensive.

Answer 5

7va/16v=.435amps
P=.435*16=7 watts or higher