[SOLVED] Variable Current Devices

[crysotyle]

So I have a variable current device, as I'm calling it. This controls the brightness of a bulb in a 1930's 1940s projector it works on 110v AC and controls the bulb wattage to between 250w and 500w, 500w being the upper limit of the bulb. As you can see its got a bit hot at some point in its life, I will find out what caused it, as I repair. I need to fix this, I think the only way to do it properly is to rewind, which is hard to do with this thickness of wire and get it smooth, so that the arm will travel across. I also need to make a new copper part to the arm which has melted. I could wrap it in a softer wire (resistance wire or copper?) to make it easier to get a better finish but I'm concerned that new wire will change the resistance values and potentially blow the bulb (new original bulbs very expensive £30-£40). Any thoughts on whether I have the right idea or is there something else I could do. I'm aware that the core is probably asbestos so might need a bit of Bakelite (phenolic resin sheet). Any help would be great thanks.

 

[barry]

That’s probably nichrome wire, it’s not just plain copper. Maybe you could just replace this with a triac dimmer?

 

[std_match]

If you don't want an electronic solution, get a variable transformer. It will generate a lot less heat than the old variable resistor.

 

[betwixt]

Agreeing with earlier posts - it is supposed to run very hot, it isn't a fault.
However, unless you need to keep to the original design for aesthetic reasons, a small triac phase control circuit will work just as well and barely get warm.

Brian.

 

[crysotyle]

That’s probably nichrome wire, it’s not just plain copper. Maybe you could just replace this with a triac dimmer?

Yeah it was Nichrome wire which will have a different resistance, than the steel currently in use. I have a multi meter with OHMS which starts at 200, 2k, 20k, 200k, 2m. Set at 200 each single wind gives me a jump of 0.1-0.2. so the first wind is 0.5 then 0.6 then 0.8 then 1.0 then 1.1 and so on. 1 Metre of wire gives me a reading of 1.3. So I understand the numbers on the dial 100s , k (thousands), m (milloins) but not sure how to workout which wire might have the same resistance in nichrome.

--- Updated Aug 15, 2020 ---

Agreeing with earlier posts - it is supposed to run very hot, it isn't a fault.
However, unless you need to keep to the original design for aesthetic reasons, a small triac phase control circuit will work just as well and barely get warm.

Brian.

Unfortunately the original design will need to be kept as this component sites in the casing with the bulb which gets very hot even with the fan running, and the space is very tight.

 

[betwixt]

The picture explains how hot it should get - the panels are asbestos and the insulation on the wires is ceramic beads, all designed to run at extreme heat.

It still should be possible to use a triac, it will get hot but if bolted down to a flat metal surface it should be OK. Your only problem will be mounting the control if it has to be in the casing for mechanical reasons. A triac circuit cuts the power to the lamp rather than converting some of the power to heat so it is far more efficient and only needs a few inexpensive components.

Brian.

 

[std_match]

Put a new box next to the "lamp box" and bypass the old overheated variable resistor.
In the new box you can put a TRIAC dimmer or a variable transformer.

 

[crysotyle]

Thanks all for your help with this, I decided to go with re-wraping it with nic chrome wire of the right resistance, not tested it yet but it looks good.