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Discuss 60s lamp overheating in the Electrical Forum area at ElectriciansForums.net

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Simon47's no-load test is a good one. Switch the mains on to high with no bulb in and check the temp after 30 mins. If the transformer remains cool - perhaps just barely warmer than when switched off - and does not buzz, it's probably 100% fine. Normally, when a small transformer has shorted turns it self-destructs fairly quickly in a cloud of smoke that smells of burnt varnish.

We don't know how hot it is getting. For comparison, we have 50W transformer-fed bench lamps here with the transformer in the base like yours. They run hot - 60° or so - it's uncomfortable to hold your hand on the base for long. They are designed like that for compactness; none of the transformers have ever failed.

What is its rated voltage? If it is 220V or lower, it will run hot on average UK mains. The more compactly a transformer of a given power rating is made, the closer to magnetic saturation its iron core runs. This makes it more sensitive to high incoming voltage and it will dissipate extra heat regardless of whether it has load.

Was the 15V measured with the lamp in circuit or not? If in circuit, that suggests the voltage is indeed wrong. If the lamp was removed, 15V off-load is a reasonable voltage to expect due to the poor regulation of small transformers.

Reducing the lamp wattage, and hence load current, will reduce the resistive loss in the windings and make it run cooler. This could mitigate high iron loss although it won't actually reduce the iron loss (if high incoming voltage is the cause). There will not be any harm to the transformer from the lower load.
Thanks so much for this. Loads of useful info. The output voltage was tested without any load.

I haven't run it unloaded for more than about 10 mins but the transformer seemed pretty cool both unloaded and with the bulb in and I tried it with the bulb on HIGH setting for around 25 mins. The bulb plus surround was getting hot but the transformer (in the main housing) was nice and cool. It hums a little bit but not ridiculous.

As M-Ty noticed the bulb that it currently has is too high in terms of wattage. The lamp is designed to take a 13W (number 93) bulb. Mine has a 21W modern brake light in it.
 
The LED bulb you've found should work, since It will run from AC, but it will be a bit dimmer than the car bulb, and the brightness switch probably won't do anything.
Personally I would fit a filament car bulb of the correct wattage, and then the lamp retains its 'originality'.
 
The LED bulb you've found should work, since It will run from AC, but it will be a bit dimmer than the car bulb, and the brightness switch probably won't do anything.
Personally I would fit a filament car bulb of the correct wattage, and then the lamp retains its 'originality'.
Yes, it would certainly look better. Trying to source one now.
 
Simon47's no-load test is a good one. Switch the mains on to high with no bulb in and check the temp after 30 mins. If the transformer remains cool - perhaps just barely warmer than when switched off - and does not buzz, it's probably 100% fine. Normally, when a small transformer has shorted turns it self-destructs fairly quickly in a cloud of smoke that smells of burnt varnish.

We don't know how hot it is getting. For comparison, we have 50W transformer-fed bench lamps here with the transformer in the base like yours. They run hot - 60° or so - it's uncomfortable to hold your hand on the base for long. They are designed like that for compactness; none of the transformers have ever failed.

What is its rated voltage? If it is 220V or lower, it will run hot on average UK mains. The more compactly a transformer of a given power rating is made, the closer to magnetic saturation its iron core runs. This makes it more sensitive to high incoming voltage and it will dissipate extra heat regardless of whether it has load.

Was the 15V measured with the lamp in circuit or not? If in circuit, that suggests the voltage is indeed wrong. If the lamp was removed, 15V off-load is a reasonable voltage to expect due to the poor regulation of small transformers.

Reducing the lamp wattage, and hence load current, will reduce the resistive loss in the windings and make it run cooler. This could mitigate high iron loss although it won't actually reduce the iron loss (if high incoming voltage is the cause). There will not be any harm to the transformer from the lower load.
Good call Lucien! Just tested the voltage with the bulb in and it is 11.5V as oppsoed to 15V unloaded so it looks like the transformer is ok. Just trying the 30mins no-load test...
 

Simon47

-
Esteemed
Arms
The regs don't require you to replace a plug with unsleeved pins. They are just not allowed to be reused on new installations. I would not replace it myself.
That was if he replaced the flex - then the regs wouldn't allow the plug to be refitted to the new flex.
 
Hi again,

My friend wants me to replace the cable and the plug. Would it be ok to use 6amp rated 2-core cable like this: -
https://www.ebay.co.uk/itm/2-Core-F...146026&hash=item2a94bb19d4:g:9vgAAOSw7z1aAcuf

Or will the 6A max current be a problem. Not sure how much current a transformer (220V to 12V) like this would draw...

The fuse in the plug is 13A
On a technical note, the current taken by your lamp would be in the order of tenths of an amp (If that), so the flex you suggest would work. The 13A plug must have a 3A fuse in it.

On an ethical / legal note, if you are updating, hence modifying, the lamp, you are personally taking over responsibility for its safety. The lamp would have originally gone through a (hopefully) rigorous design and testing regime, and shown to be safe according to the standards of the time. By changing anything now, you invalidate that original verification. If it's your lamp, to be used by you, you might feel you are happy to take the risk of your own handiwork backfiring. But if its going to someone else, or be used by them, it's totally your responsibility to make sure what is done is safe (and prove it).

This really means technical competence and workmanship done to the proper standards, and then testing the updated item (eg PAT). Not knowing if "replacing the cable" involves screw terminals, or soldering, or modifying strain relief cos the flex is different, or if adding an earth will improve safety, it's impossible to advise.

Be sure to keep safe, and keep the lamp safe too!
 

Simon47

-
Esteemed
Arms
I would temper that warning with my feeling that replacing the flex on a "like for like" basis, assuming that's practical (as you say, we don't know how the thing is constructed) would be "routine maintenance" rather than "modification". So the legal risk is much much lower - but you still need to be able to test it (especially IR since it has unearthed conductive parts).
I would be tempted to replace the flex with 3 core and earth the metalwork, upgrading it from a questionable Class II to a known Class I device - but again, that's without knowing how practical that would be as I've not seen anything but the two photos. I also have the means to test it, which I'm guessing the OP doesn't.
 
On a technical note, the current taken by your lamp would be in the order of tenths of an amp (If that), so the flex you suggest would work. The 13A plug must have a 3A fuse in it.

On an ethical / legal note, if you are updating, hence modifying, the lamp, you are personally taking over responsibility for its safety. The lamp would have originally gone through a (hopefully) rigorous design and testing regime, and shown to be safe according to the standards of the time. By changing anything now, you invalidate that original verification. If it's your lamp, to be used by you, you might feel you are happy to take the risk of your own handiwork backfiring. But if its going to someone else, or be used by them, it's totally your responsibility to make sure what is done is safe (and prove it).

This really means technical competence and workmanship done to the proper standards, and then testing the updated item (eg PAT). Not knowing if "replacing the cable" involves screw terminals, or soldering, or modifying strain relief cos the flex is different, or if adding an earth will improve safety, it's impossible to advise.

Be sure to keep safe, and keep the lamp safe too!
 

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