60s lamp overheating

[Tadmeister]

Hi everyone,

A friend of mine has asked me to take a look at this vintage 60s lamp which is running a little hot on the 'HIGH' setting. It has a transformer in the main plastic body which converts the mains to 12VDC. There is a single positive wire running from the transformer through the metal shaft to the bulb (presumably via the ON (HIGH) - OFF - ON (LOW) selector switch on the main body but I can't get to this at the moment). The negative from the transformer is connected to the metal shaft which is continuous with the -ve of the bulb. The bulb is basically a 12v car bulb. The wattage on the bulb currently fitted is 21W 12V. The rating on the sticker on the lamp says 20W 12V.

The mains lead is in ok condition but has no earth and no outer sheath (as you can see in the picture - old skool!)

Couple of questions...

1. Is it safe to have the -ve from the DC output connected to the metal shaft?
2. Should the thing be earthed. If so then should I attach the earth from the mains to the metal shaft?
3. Why might it be running hot?
4. If I put in a lower wattage LED bulb instead of the filament bulb - as in maybe 5W instead of 21W - would that be bad for the transformer?
5. Should the mains lead be replaced with sheathed 2 core?

Cheers

Tom

 

[Tadmeister]

0.5mm flex is rated at 3A, so 'compatible' with a 3A fuse in the plug, though as we all know it takes a lot more than 3A to blow the fuse!
BS1363 the standard for plugs seems to say 0.5mm can be fused at 3A.
As suggested previously, the lamp is only going to take 1/10ths of an amp - it's fault current that's the issue. In practice I think your 3 core should be fine.

Thanks again. I think I can get the strain relief bush on the new cable but there is absolutely no room in the exit hole to wedge it in there so it will have to be just inside, stopping the cable from being pulled out (as it was with the old cable). However the fit is so tight with the 3-core that I don't think the cable would budge anyway.

Could I ask one more question? Inside the housing I would like to attach the new neutral wire to the end of the old neutral wire (which I have cut and you can see dangling in the vid). This is because the wire attaches to the rotary switch, which is right up inside the case, behind the transformer and so taking the old wire out completely and soldering the new one directly to the rotary switch is going to be a nightmare.

My plan was to splice (as in merge the strands) and solder the 2 wires together and apply 2 layers of heat shrink over the join. Is that the best way or is there an issue with the lasting insulation of heat shrink?

Cheers

Tom

 

[Avo Mk8]

Thanks again. I think I can get the strain relief bush on the new cable but there is absolutely no room in the exit hole to wedge it in there so it will have to be just inside, stopping the cable from being pulled out (as it was with the old cable). However the fit is so tight with the 3-core that I don't think the cable would budge anyway.

Could I ask one more question? Inside the housing I would like to attach the new neutral wire to the end of the old neutral wire (which I have cut and you can see dangling in the vid). This is because the wire attaches to the rotary switch, which is right up inside the case, behind the transformer and so taking the old wire out completely and soldering the new one directly to the rotary switch is going to be a nightmare.

My plan was to splice (as in merge the strands) and solder the 2 wires together and apply 2 layers of heat shrink over the join. Is that the best way or is there an issue with the lasting insulation of heat shrink?

Cheers

Tom

If you put the sleeving on the flex, make a mechanically sound joint and solder it soundly - i'm sure you know how to make a good joint, and then shrink two layers, I would judge that satisfactory. The alternative would be a crimp, but it's a fine gauge wire for that method.

 

[Tadmeister]

If you put the sleeving on the flex, make a mechanically sound joint and solder it soundly - i'm sure you know how to make a good joint, and then shrink two layers, I would judge that satisfactory. The alternative would be a crimp, but it's a fine gauge wire for that method.

Brilliant. Thanks again.