Skip to main content

Rather than continue asking about this question as part of this thread, I am starting a new one.

I have replaced the circuit breaker and the whistle controller diode and resistor on my KW.  The whistle diode works, but if I short the A and U terminals with a jumper and slowly turn up the speed control it does not trip the breaker and the light does not come on.  It looks like this:


I tried resoldering the lamp resistor wire.  Using a VOM, it seems to have some resistance (although I don't know what the specific resistance is supposed to be.  The lamp does not appear to be burned out (there is continuity across it).  I changed the automotive circuit breaker to see if it was defective. (Littlefuse 10A) I used pigtails with spade terminals to make changing it easy.

I have the service manual for it which includes pictorials, but I have been a little reluctant to completely disassemble it.  There was a smoking problem when I was messing with the whistle diode / sound activation button described in the other thread.

The transformer works on both sides.  The whistle controller works as well and the smoking has stopped.

What would be the "most likely" reason for the circuit breaker not tripping?



I have attached the complete service manual for those who may want it and don't have one.


Last edited by Craftech
Original Post

Replies sorted oldest to newest

What is the rating of the circuit breaker?  Do you have specifications for it?  Where did you get it?

I got it from an auto parts store a few years ago (four of them).

Littlefuse Universal plug in Circuit Breaker 10 Amp, 12 Volt DC type, automatic reset (has break off tabs for length on each leg.

Like this one:


Last edited by Craftech

Not much information on the trip current or delay, maybe you're just not drawing enough current through it.  A 10A thermal breaker needs a lot of time at 10 amps to trip, some will tolerate a 50% overload for some minutes before tripping.

I will test it longer, but the first time I held it for 10 seconds or more the transformer started smoking.  I used that breaker after doing a bunch of forum searches.  Is it the right one?


Last edited by Craftech

Well, 10A is about what is bandied about, though I'd likely use a 7 or 8 amp breaker in a KW for the reasons I mentioned.  The KW is rated at 190 watts, but that's input watts, not output watts.  In reality, getting about 140 watts out of the KW is about all you can expect.  So, that's about 8 amps at 18 volts.  Obviously, short circuit current is considerably higher, which is what they're counting on to trip the breaker.  Another point is the rollers and the sliding contacts where the throttle shaft connects to the transformer wiring may have high resistance and start heating up with high current.  I used to rebuild transformers for a spell, and I'd use Deoxit-D5 on the throttle components to minimize the voltage drop, I've seen them dropping over a volt with a 8 amp load, that's 8 watts being dissipated in the junction.  That will get HOT and indeed smoke before the breaker trips in that case.

Thanks John.  I'll look for a smaller breaker.  I think I only have 5A, 10A, and 15A right now.

DeoxIT D5 was used along the top of the windings where the rollers travel.  I assume that is where you meant.

Does the lamp resistor have anything to do with a potential failure of the breaker to trip?

How would one do a bench test for those breakers?


Last edited by Craftech

Here is a quick test without ruining the carbon rollers. that is what is smoking. Directly short out posts C and D this is 18 volts direct and the circuit breaker should pop in 3 seconds and the red warning light should glow bright.  If not it is the wrong calibration of circuit breaker, or wired incorrectly. Remember, the circuit breaker only meant to protect the transformer core, not the trains. Use a Lionel # 91 or aftermarket circuit breaker to protect the trains.

Last edited by Chuck Sartor
@ADCX Rob posted:

Anything, really, even a wire between C-U, or a train on the track with the throttle turned up some.

OK.  8 ohms across A-U  , Throttle up 1/4, Jumper between C-D.  Plugged in.  10 seconds later breaker cuts out and light comes on.

I guess it is OK.  But do you think I should get a 7.5A breaker instead of the 10?


Last edited by Craftech

No, remember it is designed to only protect the transformer core. If you go down in value, the transformer may start cutting out after a while running trains.

OK.  Thanks eveyone.  Seems like the transformer is working properly.

I will buy a spare #51 bulb and a spare lamp resistor.

Do you think I should get the nichrome wire resistor or substitute a wirewound like I did for the whistle controller and if so what size?


Add Reply

OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
Link copied to your clipboard.