Goblin Teasmade. Resistor power rating?

[Hobgoblin’s-tea]

Good morning all,

I have recently joined.

I have started to collect vintage Teasmades.

I’m trying to find out what wattage the 12K resistors
are in the Goblin Teasmades that use the Swiss clock motors.
Does anybody know where this is documented as I can find nothing on line or written on the machine.

It would be great for peace of mind to replace the resistor with a new one, although I am getting slightly above
12K as a resistance reading so think they are good.

Any advice would be appreciated.

A couple of you have mentioned they are 2 watts but I wanted to know how you know this?

Thanks a lot

[trsomian]

From memory (sorry I am not going to take our teasmade apart to look) I would estimate 2W. If in doubt, use bigger, rather than smaller. I am sure 5W would be enough, and I don't think space is a problem. Or use 2 off 2W of half the resistance in series

[paulsherwin]

There's no reason to change the resistor if it's within spec and not damaged.

[Sideband]

Most resistors are rated at either 10% (silver band) or 20% (no band). If the 12k resistors are within 1.2K (or 2.4k) then there is absolutely no need to change them. In fact if they are not significantly out of tolerance, they will still be OK.

[Lucien Nunes]

From memory, the resistor drops something like half the mains voltage and disspates about 1.2W. The original part is an Iskra 2W film resistor that is quite stable and most unlikely to need changing even if the paint bands have faded a little. I would think the clock will mechanically wear out before the resistor goes out of spec.

An important thing to remember about a resistor power rating, is that it is simply a maximum for that part at a particular ambient temperature, that depends on what temperature it can itself withstand, and its surface area for dissipating heat. The rating has no effect on how hot it gets, that depends on the power dissipated in that circuit and its surface area. A modern resistor that has a high rating in a small package, will have a higher surface temperature than an older one of the same rating in a larger package.

This can be important when a part is physically near heat-sensitive materials, and in operation 24/7 like a clock. A small 2W part pressed against the plastic of the clock housing might damage it, when a larger, older part of the same rating might not because of the difference in surface area. When restoring vintage equipment, I often choose a resistor of at least the same physical size as the original even if that means a higher rating, to avoid higher temperatures.

[Mike Phelan]

No reason whatsoever to change it - our Teasmade has been running continuously since the 1970's!

The resistor barely gets warm.

[julie_m]

I measured the motor current on my own Goblin Teasmade using a UNI-T UT210E clamp meter around one of the clock motor wires, and got a reading of 0.016A. The resistor was slightly warm to the touch after 10 minutes of operation; certainly not uncomfortable.

By my calculation, the resistor is dissipating 3.072W; so my original estimation of it as a 2W part was a bit mean. It looks like a carbon film type, so 3W is not outrageous for its size.

However, if it's working, the best course of action is to leave it alone. Film resistors are generally very reliable unless abused (and this isn't; these resistors have lasted for years in continuous operation in thousands of Teasmades). They can only possibly fail open-circuit; and if that happens, there will be no current through the motor, so the clock will stop, but nothing else will be damaged. And it's not an unusual specification, so it will still be possible to source a replacement if and when anything does happen to it.