Halogen desklight: badly designed or malfunctioning?

[Alex Coleman]

I have a desklamp which contains a transformer in the base and which has
a 12V 20W halogen capsule lamp (G8 fitting).

The base gets very hot and after an hour or two the underside of the
base is 53 Celsius (approx 125 deg F). The current consumption measured
at the mains plug is 26 W and this is at 230 V as I am in the UK.

This seems far too hot and is very noticeable when I put my hand there.

Is this simply a badly designed cheap desk-light? Or is it more likely
to be malfunctioning? Or, heaven help, is this all very normal?

 

[Clive Mitchell]

Is this simply a badly designed cheap desk-light? Or is it more likely
to be malfunctioning? Or, heaven help, is this all very normal?

The apparently higher power reading is iron and copper losses in the
transformer which generates the heat. Cheaply made transformers do get
quite hot, particularly if they are being used on the borderline of
their ability.

Most modern transformers in appliances like this have a thermal fuse
mounted adjacent to the primary winding. If the transformer gets too
hot this will trip and kill the power to the transformer. These fuses
are unfortunately one-shot and it's not advised to try and replace them.

Is it definitely a 20W lamp fitted?

 

[JohnR66]

I have a desklamp which contains a transformer in the base and which has
a 12V 20W halogen capsule lamp (G8 fitting).

The base gets very hot and after an hour or two the underside of the
base is 53 Celsius (approx 125 deg F). The current consumption measured
at the mains plug is 26 W and this is at 230 V as I am in the UK.

This seems far too hot and is very noticeable when I put my hand there.

Is this simply a badly designed cheap desk-light? Or is it more likely
to be malfunctioning? Or, heaven help, is this all very normal?

To keep costs down they probably selected a transformer size that can just
barely cover the power needed. 125 Deg is not exactly critical. You could
find a lower wattage bulb for the lamp.
John

 

[Alex Coleman]

The apparently higher power reading is iron and copper losses in
the transformer which generates the heat. Cheaply made
transformers do get quite hot, particularly if they are being used
on the borderline of their ability.

Most modern transformers in appliances like this have a thermal
fuse mounted adjacent to the primary winding. If the transformer
gets too hot this will trip and kill the power to the transformer.
These fuses are unfortunately one-shot and it's not advised to try
and replace them.

Is it definitely a 20W lamp fitted?

Yes it is a 20W bulb. I fitted it after the original one blew.

If I had a larger transformer then it seems it would run cooler but
is the amount of mains power lost as heat in the transformer going to
be the same as I have with my small transformer?

 

[Clive Mitchell]

Yes it is a 20W bulb. I fitted it after the original one blew.

If I had a larger transformer then it seems it would run cooler but is
the amount of mains power lost as heat in the transformer going to be
the same as I have with my small transformer?

There's a point you have to ask if it's financially worthwhile modifying
a cheap light fitting. If the fitting is not so cheap and if the
construction is suited to it (minimum bare conducting metal) then you
could fit an in-line electronic transformer with relatively low power
losses.