Rheostat question

[BatteryCharger]

I'm making a variable constant current power source from an LM317. I'll need a rheostat to do that, but I'm a little confused about their ratings. Take this one from Radio Shack for example. 25 ohm, 3 watt rheostat. Does that mean it's variable from 0-25 ohms?

 

[Lynx_Arc]

Yes, variable resistors, potentiometers, rheostats almost always start at 0 and end at the full resistance. Most have 3 pins and if you measure the outside two pins you should get the full resistance where the middle pin is usually the wiper or variable between the other two outside pins.

 

[greg_in_canada]

When using a pot as a variable resistor it is often
wise to connect the middle pin to one of the end ones
(which one depends on whether you want the R to go up
with CW rotation or with CCW rotation).

Then if the pot gets noisy the resistance jumps to the
pot resistance, not to infinity, when the wiper loses
contact.

Greg

 

[andrewwynn]

hey thanks greg i always wondered why we did that.. it's so clear to me now.

-awr