Overheating Rheostat

[Kyle]

Good Morning Everyone.

I have gone back and re-read the post regarding our reohstats. Prior to the annual mine was heating up so much that when you touched it you would burn your finger. Believing it was bad I replaced it with the Spruce, pn 0144, 25 watt, 8 ohm unit. Also to care to ensure no other wires were resting on the reohstat when it went back in

Flew yesterday and after some time in the air it still heats up (really hot). Do all of you experience the same thing or is it just mine?

Thanks...

Kyle

[Bruce Fenstermacher]

Kyle

Not sure if yo know this but that is how a rheostat works. A resistance is set by the rheostat restricting how much electricity gets through. The energy has to go somewhere so heat is created.

Of course there is a limit as to how much heat is OK and that would depend on the application and the rating of the rheostat.

First you wouldn't be able to put a rheostat of this nature in a closed area that the heat couldn't be dissipated.

Second you wouldn't want to draw more amperage through the rheostat than it was rated for. In your case of the 25 watts, ohms law tells me that it is rated at 2.08 amps at 12 volts. You need to evaluate the amount of appliances (lights) and their accumulative amp draw and see if you are overloading the rheostat.

I just measured a 12v post light at 80 milliamps. That means that I could have 25 of these post lights (.08 x25 = 2) till I overloaded your 25 watt rheostat. The closer I got to the the 25 bulb max would mean more heat or if you had the rheostat adjusted to dim the bulbs the most (equaling more resistance) would create the most heat.

So do an amperage survey on your circuit and see where you are. If your not overloading the rheostat and the rheostat is not getting hot enough to ignite or melt anything near it, your OK.

Special notice: I'm not an electrical engineer. Just a pilot, jack of many trades, who has a slightly better working knowledge of electrical circuits and Ohms law. But I don't exercise this electrical knowledge every day so I could have screwed up a decimal point or two.

[cessna170bdriver]

I just measured a 12v post light at 80 milliamps. That means that I could have 25 of these post lights (.08 x25 = 2) till I overloaded your 25 watt rheostat. The closer I got to the the 25 bulb max would mean more heat or if you had the rheostat adjusted to dim the bulbs the most (equaling more resistance) would create the most heat.

So do an amperage survey on your circuit and see where you are. If your not overloading the rheostat and the rheostat is not getting hot enough to ignite or melt anything near it, your OK.

You might just want to measure it directly by puttinig an ammeter in series with the rheostat and a voltmeter across it. Multiply the two readings, and that is the power the rheostat is dissipating. It's hard to use the ratings of light bulbs to figure this out on paper, as light bulbs have notoriously non-linear resistance (which means that if they draw 80 mA at 12 volts, they won't necessarily draw 40 mA at 6 volts).

Also, the maximum power is dissipated by the rheostat when it's resistance is equal to resistance of the load in series with it. At the brightest setting, there is very little power dissipated as there is little voltage drop across the rheostat; at the dimmest setting, there is little current through it. Somewhere in between you'll maximize current times voltage. Take ammeter and volt meter readings at a few different settings and try to find a maximum.

Special notice: I'm not an electrical engineer. Just a pilot, jack of many trades, who has a slightly better working knowledge of electrical circuits and Ohms law. But I don't exercise this electrical knowledge every day so I could have screwed up a decimal point or two.

Pretty good for a helicopter pilot. (Just kidding of course!)

[Bruce Fenstermacher]

Also, the maximum power is dissipated by the rheostat when it's resistance is equal to resistance of the load in series with it.

That's interesting about the rheostat load Miles and of course you are right I'd not thought about it and have learned something new.

My light bulb scenario was just for discussion. An actual measurement of the circuit would be best.

And yes Miles as a helicopter pilot I was having a good day.

[thammer]

You might find this interesting reading. Neal Wright did a nice write up on panel lighting issues, including the rheostat, on the 120/140. 170's might be similar, 'specially the 48.

http://www.cessna120-140.org/forum/viewtopic.php?t=2301

tye

[Kyle]

Guys,

Many thanks as always for the responses. Looks like I have my homework cut out for me next time at the hanger ....

Kyle

[marks]

Reference http://www.cessna170.org/forums/viewtop ... dim#p54544