Low voltage lamp

My recent new Colchester lathe, came with the original low voltage lamp. Which is 50 Volt / 40 Watt, The lamp is very dim. I assume the transformer in the low voltage lamp unit will be 415 Volt to 50 Volt . As the primary voltage is now 240 Volt the secondary voltage will be approx 25 Volt. Would a 24 Volt lamp be suitable ?

Seems logical

[ noting your ‘approx ‘ ]

MichaelG.

Edited By Michael Gilligan on 14/02/2023 20:07:19

My maths is unreliable but I make

(50/415)*240 = 28.8V

If I'm right a 24V bulb will burn hot and bright, and not last long. The extra heat may not be good for the lamp-holder either.

How many watts is the bulb?

A 24V 21W brake lamp draws 1.14A, so a resistor to drop the 28.8V to 24V would be 4.8/1.14 = 4.2 ohms rated for at least 5.5W. Two 2.2ohm 5W resistors in series would do the job.

One thing to watch is the current the transformer can supply. If the original 50V bulb was 100W, then the transformer can do 2A. Four amps would be needed to get 100W of light out of a 24V bulb. So rather than using a filament, it might be wise to go for a LED and drive it with an ebay adjustable regulator module. I'd probably disconnect the transformer and fit a LED lamp and power supply that suited it.

Dave

Edited By SillyOldDuffer on 14/02/2023 20:33:52

I would assume a 40 watt 24 volt lamp would give off the same illumination as a 40 watt 50 volt lamp. The current flow would be approx double, as these are AC voltages, ohms law is only approx.

Have a look at the transformer and see if it is tapped for 240v on the input side.

SOD's calculation is correct (also 50V / SQR3 gives correct result). However if you buy a "24V" vehicle bulb it will actually be intended to run at closer to 28V, the on charge voltage of a 12 call lead acid battery. So all in all a 18-20W "24V" lamp will be OK if a little over-run. Putting a resistor in series with a filament lamp is not ideal as the lamp tends to be a constant current load. Better to put a parallel pair (anode to cathode) of silicon dodes in series with the lamp. They will drop about 0.6V, four in series / parallel will drop 1.2V. 1N4002 (or iN4003/4/5/6/7) diodes are fine and good for over an Amp (each is only carrying current half the time). Or even better put a LED lamp in it.

Robert.

I use 25v in mine for the same reason

Good point. Should have thought of that because I know '12V' vehicle bulbs and electronics run at a nominal 13.8V, which is the on charge voltage of a 6 cell lead acid battery.

Better write my age down quickly before I forget it again...

Dave

One can expect 28.4v on a normal 24v system so there will be No over running. A 24v x24w indicator bulb should be fine.Alternator charging voltage on a 12v system is 14.25v Noel

Edited By noel shelley on 14/02/2023 22:29:31

Wouldn't a transformer for your low voltgage lamp use only one phase of a three phase supply? It would therefore be 240v to 50v

Mine still uses B22 Machinery (rough service) bulbs

Most 3 phase has NO neutral so NO 240v. It runs across2 phases so 415v. Noel.

Physically checking all the existing components before proceeding with any changes would seem to be the best idea.

MichaelG.
.

[corrected typo]

Edited By Michael Gilligan on 15/02/2023 00:33:25