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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




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Simon47

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Esteemed
Arms
1. Is it safe to have the -ve from the DC output connected to the metal shaft?
It's not -ve. If it's a transformer then it's AC which doesn't have -ve and +ve - well yes it does, but they swap round 50 times a second.
It is fairly common to use that arrangement. I have a lamp somewhere with no wires between the transformer base and the adjustable lampholder - the arms are formed from two parallel bars, each of whcih forms one part of the circuit.
2. Should the thing be earthed. If so then should I attach the earth from the mains to the metal shaft?
Depends. It could be double insulated - is there a DI symbol (square in a square) on it ? Any chance of a phot of the internals, and especially any labelling on the unit itself and the transformer.
3. Why might it be running hot?
Could be many reasons. It could be that it was designed like that. The transformer could be faulty (internal short between turns). Does it get hot if there is no bulb fitted ?
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?
Probably not - but it may or may not work as intended, and it may create a lot of flicker.
If the LED bulb is designed to run from AC or DC or either polarity then it is likely to work. If it's designed for DC only and one polarity - then it may work the same on high and low, or only work on high.
5. Should the mains lead be replaced with sheathed 2 core?
Possibly, possibly not. The existing flex may, or may not, have double or reinforced insulation - if it does then it's OK, if it doesn't then it should be replaced.
And though it pains me to say it because those plugs are good quality, the likes of which it's hard to find these days - it'll have unsleeved L&N pins so it should be replaced.
 
It's not -ve. If it's a transformer then it's AC which doesn't have -ve and +ve - well yes it does, but they swap round 50 times a second.
It is fairly common to use that arrangement. I have a lamp somewhere with no wires between the transformer base and the adjustable lampholder - the arms are formed from two parallel bars, each of whcih forms one part of the circuit.

Depends. It could be double insulated - is there a DI symbol (square in a square) on it ? Any chance of a phot of the internals, and especially any labelling on the unit itself and the transformer.

Could be many reasons. It could be that it was designed like that. The transformer could be faulty (internal short between turns). Does it get hot if there is no bulb fitted ?

Probably not - but it may or may not work as intended, and it may create a lot of flicker.
If the LED bulb is designed to run from AC or DC or either polarity then it is likely to work. If it's designed for DC only and one polarity - then it may work the same on high and low, or only work on high.

Possibly, possibly not. The existing flex may, or may not, have double or reinforced insulation - if it does then it's OK, if it doesn't then it should be replaced.
And though it pains me to say it because those plugs are good quality, the likes of which it's hard to find these days - it'll have unsleeved L&N pins so it should be replaced.
Thanks so much for this. Gosh I must be tied - of course it wouldn't be DC as there is no rectifying circuit there. The transformer does look a bit old. The 3 position rotary HI-OFF-LOW selector switch feeds into the mains side of the transformer, adjusting the number of windings so I guess the output voltage varies accordingly. I have a decent multimeter and some decent insulated gloves so I guess it would be ok to measure the output voltage for high and low without the load..... what do you think?

I'll get you the photos and other info tomorrow if that's ok.

Cheers again

Tom
 
It's not -ve. If it's a transformer then it's AC which doesn't have -ve and +ve - well yes it does, but they swap round 50 times a second.
It is fairly common to use that arrangement. I have a lamp somewhere with no wires between the transformer base and the adjustable lampholder - the arms are formed from two parallel bars, each of whcih forms one part of the circuit.

Depends. It could be double insulated - is there a DI symbol (square in a square) on it ? Any chance of a phot of the internals, and especially any labelling on the unit itself and the transformer.

Could be many reasons. It could be that it was designed like that. The transformer could be faulty (internal short between turns). Does it get hot if there is no bulb fitted ?

Probably not - but it may or may not work as intended, and it may create a lot of flicker.
If the LED bulb is designed to run from AC or DC or either polarity then it is likely to work. If it's designed for DC only and one polarity - then it may work the same on high and low, or only work on high.

Possibly, possibly not. The existing flex may, or may not, have double or reinforced insulation - if it does then it's OK, if it doesn't then it should be replaced.
And though it pains me to say it because those plugs are good quality, the likes of which it's hard to find these days - it'll have unsleeved L&N pins so it should be replaced.
Hi again,

Just tested the voltages and we've got 15VAC on the HIGH setting and 10VAC on the LOW setting. Could this be an internal short on the transformer? The lamp casing does feel a little hot. Would a 12V rated LED bulb work okay at 15V. I am assuming that it will solve the problem of it getting too hot...?

There is no double insulation symbol on the labelling. It is a Lloyd lamp NA101

Cheers again.
 

Strima

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Esteemed
Arms
The transformer is probably starting to fail and TBH it would probably be easier and quicker to remove all the existing electrical components and replace with DC LED and a suitable driver in the body.
 
The transformer is probably starting to fail and TBH it would probably be easier and quicker to remove all the existing electrical components and replace with DC LED and a suitable driver in the body.
Thanks for that. Any idea how long the transformer might last for? And if it fails, is that dangerous - i.e. fire - or will it just blow the bulb and trip a circuit breaker?
 

Simon47

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Esteemed
Arms
The 3 position rotary HI-OFF-LOW selector switch feeds into the mains side of the transformer, adjusting the number of windings so I guess the output voltage varies accordingly.
Ah, I'd read it as the switch was between one transformer wire and the bulb. I've seen setups where there's a diode in series in the low position so the bulb gets half-wave rectified DC on low, and the full AC on high.
As Strima says, probably easier to strip out the internals and convert it to a modern LED.

If the transformer is getting hot with no bulb installed, then it's likely there's shorted turns internally where the insulation has broken down. Over time, the heat will help the insulation degrade further and cause further faults. Eventually it could get very hot - remember that a 3A fuse will carry well over 3A without blowing, so probably at least 1kW of dissipated heat in the transformer before a fuse blows. it all depends on how it fails, it may fail with a short significant enough to just blow the fuse without going through the "melted blob" phase. Or there may be an internal thermal cutout - but you probably don't want to rely on that since there might not be, or it might not work, ...
 

Lucien Nunes

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Mentor
Esteemed
Arms
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.
 
Ah, I'd read it as the switch was between one transformer wire and the bulb. I've seen setups where there's a diode in series in the low position so the bulb gets half-wave rectified DC on low, and the full AC on high.
As Strima says, probably easier to strip out the internals and convert it to a modern LED.

If the transformer is getting hot with no bulb installed, then it's likely there's shorted turns internally where the insulation has broken down. Over time, the heat will help the insulation degrade further and cause further faults. Eventually it could get very hot - remember that a 3A fuse will carry well over 3A without blowing, so probably at least 1kW of dissipated heat in the transformer before a fuse blows. it all depends on how it fails, it may fail with a short significant enough to just blow the fuse without going through the "melted blob" phase. Or there may be an internal thermal cutout - but you probably don't want to rely on that since there might not be, or it might not work, ...
Thanks again for the v helpful info. Actually the transformer feels quite cool - not overheating at all. Its the filament bulb and surrounding casing that feels a bit hotter than it should be and on the low setting it is not overly hot at all.
 

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