Powering a WW2 Searchlight

Option 1: build a power supply unit

The thing that converts <...110VAC...> to 24VAC is a transformer with a 4.5:1 turns ratio (approximately).

The thing that converts 24VAC to 24VDC is a bridge rectifier.

Sizing them for the purpose of selection depends upon the power consumed by <...the bulb...>, which will have a rating in watts [W]. The forum has nothing to go on here, though it is likely to be, oh, say well north of 50W if it is tungsten filament based, which is likely because of its provenance. So the equipment the and wiring from it to the <...lamp...> is going to be on the chunky side.

The rest is circuit protection for both the nearby personnel and the equipment components and wiring, which is electrical design. It would be tempting to engage a local electronics hobbyist to help here.

Then comes the shopping list, followed by assembly, probably with the <...soldering iron...>, and testing.

Option 2: buy a power supply unit

An alternative to building one would be to search for a secondhand power supply unit of suitable rating and use that. There are a number of internet auction sites (names withheld) that can help.

Option 3: batteries and a commercial car battery charger

A third alternative would be to use two 12VDC lead-acid batteries, wired in series to operate the <...lamp...> and to charge them from the <...110V ac...> mains separately when the <...lamp...> is not in use. A suitable car battery charger for this purpose can be had over-the-counter and ready to use for £20-30GBP locally. Lead-acid batteries can be had from around £50GBP each new locally.

The choice depends upon how one values one's time.

A search online for "24VDC Power Supply" will offer some options. If you can figure out the watts needed will help. Here's a random link to a 24V, 120W max external supply (brick) that runs off 120VAC. I'm sure you can find something appropriate for your need if you do a little searching. You can just cut off the plug and hardwire to your light, or install a jack for it to plug into (so you can detach it for storage).

https://www.trcelectronics.com/View/Mean-Well/GSM120A24-R7B.shtml

No need for DC if this is only just a light bulb.

It will work fine on 24Volts AC.

A 120/24 volt transformer and dimmer switch should work just fine.

You must know the wattage to calculate the KVA of the transformer required.

There should be a wattage somewhere on the light,maybe on the bulb itself.

I tried to edit this post,but the site erroneously said I was out of time.

Anyway,presuming 150 watts,incandescent type for the bulb,then 150 VA transformer would be minimal required,but I would use at least a 200 VA transformer for durability.

A standard wall light dimmer would work just fine as long as it is rated high enough for the wattage.

Good Luck!

https://www.replacementlightbulbs.com/lampsdiscontinuedoutofstock.html

https://en.wikipedia.org/wiki/Mazda_(light_bulb)

Light bulb types...

https://www.superbrightleds.com/blog/home-lighting-101-guide-understanding-light-bulb-shapes-sizes-codes/2315/

If the lamp is as indicated on the bulb and I have no reason to doubt the marking then it is only for 10.8Volt and to run it on 24volt would mean they are over-driving it enormously. Now the over-driving will shorten the life of the bulb enormously for I had a spotlight that used a 100W 6V lamp running on 12V but at those values it had a lifespan of 5 hours with forced cooling.

At 375W and 10.8Volts then the current draw is 35 amps when up to temperature. Are you sure there is no ballasting resistor when it is used on 24V?

Let us think for a moment that to supply 35amps from a 12Volt battery will mean a large battery for if you use one of the online calculators to supply 35A for 1hour continuous would require a 166 ampere hour battery. This is not cranking amps but real ampere hours see ; https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

To run the light off 12 Volt AC you would need to limit the voltage to 10.8V and have a minimum of 375VA transformer. Using a dimmer on the primary of a transformer to lower the secondary voltage requires a symmetrical dimmer capable of 3.5Amps. Symmetrical dimmers must be used on transformers else the asymmetrical introduces a dc bias which creates extra heat and can destroy the transformer.

You can obtain 10.8v transformers but as most people are only issued with two legs and two arms and the cost of an arm and a leg is prohibitive unless you want disabled parking. You could use a variable auto transformer to supply the transformer primary and wind the voltage up or down to obtain the required brightness.

The only "tricky" thing you might encounter with powering this incandescent lamp with a modern switching DC power supply is the load change that happens as the filament gets hot. The wattage rating of the bulb is usually rated once the lamp is at the higher resistance of full brightness, hot. So if the bulb is rated at 600W 24VDC the operating current would be 25 amperes. However, the surge current turning ON a cold filament in February on the Bering sea could be 100 amperes or more. A battery and/or shipboard generator will just easily pump this much current for most of the second it takes for the filament to get hot. Many modern DC supplies with clever short circuit output protection designs might turn the output OFF because the surge current exceeds the very fast short circuit trip threshold.

This is a very rare but plausible scenario I've proposed. In theory a simple cold filament resistance measurement and bulb wattage rating can solve this if one has a meter that can read resistances this low.

These bulbs had life expediencies measured in 10's of hours. I would not operate it at 24 volts, but closer to 20

In addition, sooner or later (probably sooner and later) the dimmer will be set wrong. I would put the effort into making the power supply not capable of delivering excessive voltage to the bulb. If you want a dimmer to go lower then that is ok. You can't go down to Home Depot or Lowes to get a replacement bulb so you should try to have a setup where the user can not damage the bulb due to a faulty setting.

I would go with a cheap battery pack of 24 volts and a cheap wall wart charger with a battery minder....the light is probably in the 500 watt range and any transformer is likely to be expensive....

https://www.ebay.com/itm/UPG-12V-10AH-Battery-for-Lashout-24-Volt-400-Watt-Scooter-2-Pack/132335820144?hash=item1ecfd45d70:g:w-AAAOSwSidb6z9n

This 500 watt light as an example would draw around 20 amps theoretically, but may draw more or less on start-up, and increase or decrease when warmed up...

500/24 volts = 20.83 amps

You might get 2 12V car batteries and a DC ammeter (they have clamp-on meters that work on DC) and see how much current it draws at full voltage. (You can't, of course, measure cold resistance since resistance increases many-fold with temperature.) That way you can determine how much power (24 x I) your transformer or power supply needs to handle.

Just a thought...

So,this is a 10.8 volt bulb,not 24volt.

You can still use a transformer and dimmer switch without going through all the trouble for a DC Supply.

This is a purely resistive load,and does not require DC voltage.

I personally would go with SE's suggestion and use a 120volt AC spot light and skip all of the headaches,esp when the bulb burns out,which it will if used frequently.

There are LED Diode replacements for just about any incandescent or halogen bulb.

Measure the diameter of your bulb base

It is probably a Mogul-type considering the amp draw of the bulb.

There are converters for just about any base type to another.

A 500 watt dimmer switch can be bought for around $10-12 at Amazon.

The K.I.S.S system is usually the best.

As SE says, wouldn't it be easier to change the light bulb? I have one similar to what SE shows; it was ex-fire department and already had a 110-volt bulb. 500 watts if I remember correctly, and mogul base.

The answer to that question depends upon the degree of authenticity called for in the final outcome for its new role based upon the provenance of its old one.

The original poster clearly wants light to come out of this thing, though the need to illuminate incoming piston-engine bombers at anti-aircraft gunfire altitude has clearly passed. If it is there simply to illuminate a corner of a studio flat/apartment as a decorative feature, for example, then a compromise may be called for in order not to blind others in the building when it is on. However, if it is to be used in close-to-accurate historical re-enactment events then a different set of parameters apply.

Maybe he wants it for his storm chaser vehicle...

Looks sorta WWII ish....in which case I would certainly go with an SMD type 100 watt LED, for rough duty....

Thanks for all the great replies. The main goal is to have it as an art piece in the living room for a house on an island in the Pacific Northwest. Dimly lit would be ideal for indoors but have the option to point it up in the night sky which is why I was looking to incorporate a dimmer.

Go big or go home....