Looking for replacement lamp resistor for KW

[Birds]

I would like to replace the lamp resistor in my KW, but I am having difficulties tracking down the correct part.

The exploded diagram for the KW lists the part as "20-45".  I can't find this part (or even sub. ZW-185) in the catalogs of the various parts places (Olsens, Train Doctor/Brasseur, Hollash, French's, Stan Orr).

Can this resistor be replaced with a modern one from Radio Shack?  If so, what would I be looking for because I can't find specs for the original. 

Thanks,

Chris 

[lionelsoni]

This lamp is supposed to be a number 51, which is rated at 220 milliamperes at 7.5 volts.  Assuming that the transformer setting is 18 volts when the circuit breaker trips, the resistance should be about 47 or 50 ohms.  The power dissipation is about 2.5 watts; so I would use at least a 5-watt resistor.  Radio Shack has 10-watt resistors, 271-133, two for $1.79, which would not be too much of an overkill.

[Birds]

Bob,

Thank you for the reply.

I was able to unsolder one end of the bulb resistor and measure it with a digital VOM.  I got a reading of 61.2 ohms.  It appears that it is still working, but I have to remove the entire resistor to replace a rusted bulb holder.  I am not sure what the condition will be after this given the glob of oxidized solder on the end still connnected.

If I wanted to keep 60 ohms of resistance there, could I combine the 50 Ohm resistor you specified (RS 271-133) in series with a 10 ohm 10 watt resistor (RS 271-132) to make 60 ohms?

Thank you,

Chris 

[lionelsoni]

You certainly could.  However, the power dissipated in the 10-ohm resistor will be only 1/5 that in the 50-ohm one; so you could use the much smaller 271-151 1-watt resistor if that's easier for you.

Actually, I don't think 10 ohms will make much difference in practice.  That corresponds to just a slight change in the transformer setting that we assume will be in place when the breaker trips.  Since the lamp will be on only briefly, its lifetime will still be quite long even with a couple of extra volts.

[Birds]

Bob,

Thank you for your responses.

I was looking over schematics of the ZW transformer last night and noticed that it has a 60 ohm resistor specified at the same location as the resistor in my KW (the KW's schematics didn't give an ohm rating).  That resistance matches what my KW's resistor wire tested out at.  The part for the ZW is still available.

There was also a note on the ZW schematics that said the 60 ohm resistor wire is only needed if a 6 volt lamp is used as the short circuit indicator lamp, but that if an 18 volt lamp was used as the indicator lamp then the 60 ohm resistor is not needed.

Do you think moving to an 18 volt lamp in the KW would also eliminate the need for a resistor in line with the lamp socket?  The voltage on the KW maxes out at about 19.22 volts.  The circuit breaker in the KW is rated for 12 volts (someone installed a 15 volt ZW breaker in mine, so that will also get changed out to a 12 v KW version)

It appears there may be three possible options if one has to work with the lamp assembly:

1. order a reproduction ZW-183 resistor wire
2. use modern resistors as you specified
3. change the lamp to an 18 volt BA9 base lamp and remove the inline resistor

Chris 

[lionelsoni]

Yes, you could use an 18-volt lamp.  I think that Lionel may have had the idea to make the lamp brightness vary less with the transformer setting by using the lower-voltage lamp with the series resistor.  This would be desirable; since the breaker might trip at a low or a high setting.  The resistor would act somewhat like a current source, to increase the liklihood that you would see the light under all conditions.  But there is no harm in using the higher-voltage lamp, except that it won't light up much when the breaker trips at 6 volts.  I personally would go with the number 51 lamp and the 50-ohm resistor.

Don't you mean amperes, not volts, when you talk about the circuit-breaker ratings?

[Birds]

Bob,

Thanks for explaining about the brightness issue with an 18v lamp.

Normally I would have said "amps" in reference to a circuit breaker, but the parts list at Olsens lists the KW breaker as "20-150  circuit breaker KW, 12v", but does not give an amp rating.  At this time I am not sure if this is a typo or not on their parts list.

From what I can find in the KW schematics the breaker should carry 10 amps before tripping (as opposed to the currently installed ZW breaker which trips at 15 amps).

I am guessing that I could also use a 10 "auto reset" breaker for a car as long as it can carry 18-20 volts.

Thank you again.

Chris