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

 

[Avo Mk8]

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'.

 

[Tadmeister]

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.

 

[Tadmeister]

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]

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.

 

[GeorgeCooke]

That was if he replaced the flex - then the regs wouldn't allow the plug to be refitted to the new flex.

I guess you are right. I would still use the old plug anyway for something of my own. No one would ever know.

 

[Tadmeister]

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.

 

[Avo Mk8]

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]

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.

 

[Tadmeister]

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!

 

[Tadmeister]

Hi again,

Thanks for all the help everyone.
Here's a short vid showing construction of the lamp - insides plus the wiring path.

It is mainly plastic but has the metal arm which is carrying part of the 12V supply to the lamp. There are 2 bolts attached to this and there is also a screw holding the base on.

There is also a retractable metal hanger which I forgot to mention in the first vid. I've shown it in this one...

The hanger is inside a plastic channel which is separate from the main compartment.

Should I earth the metal arm?

Cheers

Tom

 

[Avo Mk8]

Hi again,

Thanks for all the help everyone.
Here's a short vid showing construction of the lamp - insides plus the wiring path.

It is mainly plastic but has the metal arm which is carrying part of the 12V supply to the lamp. There are 2 bolts attached to this and there is also a screw holding the base on.

There is also a retractable metal hanger which I forgot to mention in the first vid. I've shown it in this one...

The hanger is inside a plastic channel which is separate from the main compartment.

Should I earth the metal arm?

Cheers

Tom

All credit to you Tadmeister for a thorough video. I just need to absorb the information!
Well done for that - I'm sure you'll get some suggestions!

 

[Tadmeister]

All credit to you Tadmeister for a thorough video. I just need to absorb the information!
Well done for that - I'm sure you'll get some suggestions!

Thanks so much!

 

[Avo Mk8]

Hi Tadmeister Tom

Just a few thoughts:

I guess the first task is to find some 3 core flex that will feed through the base! The strain relief bush you have is a type that I thought, once clipped on the flex, was supposed to push into the exit hole from the inside. But it doesn't look if there's room for that. I did wonder if some other original strain relief occupied that rebate where the wire comes out of the base.
If the bush you have is a type that clicks closed and stays closed (around a new flex) it should be OK to use. If you need another, Google is your friend.

I don't think it is practical or necessary to try to earth the sliding hanging bracket. It seems to have its own insulated pockets to sit in.

I suspect it is worth earthing the metal arm of the lamp, as it's connected to the secondary of the transformer to complete the circuit to the bulb. Opinions anyone? Transformer is providing isolated supply, presumably built with robust isolation pri to sec. But if transformer fails, earthing a good thing!

You need to somehow get a good connection to the arm, eg via a ring tag to the screw connecting to the transformer, or soldering to the existing tag. It's good practice to make the earth wire longer than the other mains connections, so that if the flex strain relief fails, and the cable were to pull out of the lamp, then the earth wire would be the last to disconnect/ break.

Looks like you are happy to do some soldering, so good luck with the restoration. If you can find someone with a Portable Appliance Tester, or equivalent test equipment, to check earth resistance and insulation resistance when you've finished that would be great.

 

[Tadmeister]

Hi Tadmeister Tom

Just a few thoughts:

I guess the first task is to find some 3 core flex that will feed through the base! The strain relief bush you have is a type that I thought, once clipped on the flex, was supposed to push into the exit hole from the inside. But it doesn't look if there's room for that. I did wonder if some other original strain relief occupied that rebate where the wire comes out of the base.
If the bush you have is a type that clicks closed and stays closed (around a new flex) it should be OK to use. If you need another, Google is your friend.

I don't think it is practical or necessary to try to earth the sliding hanging bracket. It seems to have its own insulated pockets to sit in.

I suspect it is worth earthing the metal arm of the lamp, as it's connected to the secondary of the transformer to complete the circuit to the bulb. Opinions anyone? Transformer is providing isolated supply, presumably built with robust isolation pri to sec. But if transformer fails, earthing a good thing!

You need to somehow get a good connection to the arm, eg via a ring tag to the screw connecting to the transformer, or soldering to the existing tag. It's good practice to make the earth wire longer than the other mains connections, so that if the flex strain relief fails, and the cable were to pull out of the lamp, then the earth wire would be the last to disconnect/ break.

Looks like you are happy to do some soldering, so good luck with the restoration. If you can find someone with a Portable Appliance Tester, or equivalent test equipment, to check earth resistance and insulation resistance when you've finished that would be great.

Thanks again for all your help. I can squeeze some 3 core 0.5mm 3 Amp flex cable through the aperture in the base. Do you think that'd be ok?

I've also got some 2 core 6amp flat cable which fits a bit better but I think the earthed option is probably safer.

I'm definitely ok to solder - I've built guitar pedals and a Fender valve amp in the past. I have quite a bit of experience in this area but don't have any formal electricians' training so I'm in the process of learning the various rules and regulations.

I don't have any PAT testing equipment but I have a decent multimeter. I could test the current going through the cable...

 

[Avo Mk8]

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.