Problem with motorhome lighting

[Tparker]

Having a problem with a friends motorhome panel lighting. Recently he found that his lighted switches and gauge illumination lights (house 120 volt generator panel, holding tank level, etc.) were not turning off when he turned off his headlight switch. The engine gauge lights do not have this problem and work normally. It appears his push/pull headlight switch with built-in reostat/pot directly feeds the engine gauge lights but also feeds 2 2N3771 NPN transistors thru their base connections. The output of these transistors feed the illumination lighting of his upper and lower electrical panels ( one for the upper panels and one for the lower panels). No other componets other than the transistors are found in this dimming circuit. I suspect one or both of the transistors are shorted (when you remove the transistors from the circuit, the lights go out). Does this make sense?

[russlk]

Makes sense to me.

[sundancer87]

Tparker, those are 30 amp transistors. 30 amps services a lot of lamps and gauges. I would suggest before you jump in to replace the transistors, you do some serious troubleshooting. Sounds as if there is a problem causing those big hockey pucks to smoke.

[dyarker]

Even big transistors need resistors in series with the base.<p>Or,
If the wires from the transistors to the lamps have worn insulation, going over bumps could have caused a short. That could have caused an emitter/collector short. The lamps stay on when the vehicle is still or on smooth road.<p>Or,
How well are the transistors heat-sinked? When the reostat is set to half bright, the transistors have to dissipate a lot of power; 6V times how much current the lamps draw on the other 6V.<p>Or,
Incandescant lamps have a VERY low cold resistance compared to their on resistance. If the turn-on surge of the lamps is a little greater than the transistor rating, they would eventually die. In this case to E-C short.<p>[ October 17, 2003: Message edited by: Dale Y ]</p>

[Tparker]

The transistors are mounted on 2 seperate heat sinks that seem adequate. One transistor did show signs of high heat and its phenloic mounting socket came apart when I removed the transistor. I also noticed that no mica insulating "washers" were used and found no evidence of any heat sink compound (the heat sinks were mounted on a piece of plywood). Think I will replace them with new ones using mica insulators and heat sink compound and see if that cures the problem. Maybe I should use 2N3772's which have a little higher rating.

[Tparker]

The transistors are mounted on 2 seperate heat sinks that seem adequate. One transistor did show signs of high heat and its phenloic mounting socket came apart when I removed the transistor. I also noticed that no mica insulating "washers" were used and found no evidence of any heat sink compound (the heat sinks were mounted on a piece of plywood). Think I will replace them with new ones using mica insulators and heat sink compound and see if that cures the problem. Maybe I should use 2N3772's which have a little higher rating.

[sundancer87]

According to my book the amperage rating is the same, 30 amps. The source is NTE #9.<p>I have to wonder if you don't have a problem in the wiring causing the transistors to smoke.

First question would be, how long has the transistors been in service. Has this been working for a long time without problems? Did the owner recently add some additional lamps or something that would cause an overload? Has there been any changes to the coach that could cause the problem?<p>You could be lucky and just replace them as you suggest and come out looking good. However, they probably didn't die of old age.<p>One point to consider if you do use a bigger transistor. Will the wiring handle the load?

[dyarker]

I agree with sundancer87, there is a problem. Those transistors are large enough to drive head lights.