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Post subject: How to power 74volt lamp
Posted: Wed Aug 21, 2013 7:55 pm

Joined: Sat Jan 11, 2003 1:01 am
Posts: 200
Location: Chattanooga, TN
Been some time since I've submitted one of my strange requests; here is one of two recents:

I want to build a couple of lights to light up an area. The lamps I have access to are locomotive headlamps, and the engines are outputting 74volts. Is there a "cheap" (redneck engineering) way of powering these from simple AC?

I cannot afford to buy a commercial-type converter, so I'm trying to see if there is a way around it. (I know not the "correct" way, but you and I know that sometimes you gotta do what you gotta do..haha)

thanks for any help.

Tim

Post subject: Re: How to power 74volt lamp
Posted: Wed Aug 21, 2013 8:32 pm

Joined: Wed Nov 24, 2004 1:01 am
Posts: 2285
Location: ASHTABULA,OHIO
These were powered from a 72 volt battery bank which was being float charged in the high 70s voltage. Cheapest way out- run three in series from 240VAC mains (they must be all the same wattage to do this and all be switched on and off as a unit!)). Another way is to power them with three 120/24 VAC transformers with secondarys wired in series to give you 72 VAC. this would have the advantage of mixing any bulb wattage on the load. The big drawback is with increasing load comes increasing tranny cost.\$100 in trannys would cover a 500 VA load. Beyond that it may start getting too expensive. If you need separate circuits for these lights (individual switching etc.) You can run each leg in series with a 120 volt bulb to drop ~45 volts across it, leaving 75 volts for the loco bulb. This method would need the 120 volt bulb wattage determined both by ohms law and empirically to get the two to correctly match. Well thats all I can come up with for the cheap route unless E-Bay would have some 72 volt float chargers for a steal. Good luck.

Post subject: Re: How to power 74volt lamp
Posted: Wed Aug 21, 2013 8:35 pm

Joined: Wed Nov 24, 2004 1:01 am
Posts: 2285
Location: ASHTABULA,OHIO
SCRATCH

Post subject: Re: How to power 74volt lamp
Posted: Thu Aug 22, 2013 5:04 am

Joined: Wed Apr 10, 2013 5:03 am
Posts: 12
Just curious, what wattage are locomotive headlamps?

Post subject: Re: How to power 74volt lamp
Posted: Thu Aug 22, 2013 9:26 am

Joined: Mon Dec 06, 2004 1:01 am
Posts: 294
Location: Tennessee
Are these incandescent bulbs? What's the wattage?
You might try a 240> 120 transformer. Wire the primary to 120 instead of the rated 240, that'll give you 60 volts on the secondary.
Maybe that'll be close enough to do what you want.

Post subject: Re: How to power 74volt lamp
Posted: Wed Aug 28, 2013 8:23 am

Joined: Wed Nov 24, 2004 1:01 am
Posts: 2285
Location: ASHTABULA,OHIO
Can't believe this didn't occur to me in prior post. A 10 amp auto transformer, commonly called Variac or PowerStat would suit you perfectly. New ones are expensive but in the last 6 years I have purchased them twice on E-Bay for under \$25 a pop.

Post subject: Re: How to power 74volt lamp
Posted: Thu Aug 29, 2013 4:06 am

Joined: Fri Feb 08, 2002 1:01 am
Posts: 4284
Location: USA / North Carolina / Fayetteville
yes they are incandescent lamps sealed beam, and if i remember right they are 75 volt, 350 watt PAR56 sealed beam
they have about 500 hrs of life....halogen type last about 4,000 hrs. i hope you got a halogen type

there is also some regular light bulb type lamps around the stairs ,
these bulbs fit in a regular Edison socket. they are 75 volt, 40 watt E26 base
makes a great gag to stick one into a normal 120 socket.. ( can you say FLASHBULB )...

if you do use a variac make sure it can handle 5 amps or more.

Post subject: Re: How to power 74volt lamp
Posted: Sun Sep 01, 2013 6:09 am

Joined: Thu Oct 26, 2006 7:11 pm
Posts: 1316
Location: Maryland
A diode in series will drop the voltage nominally 50 percent. Use one at each lamp. They may be too dim but it's worth a try. Polarity is unimportant but current and voltage rating must be adequate.
Len

_________________
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

Post subject: Re: How to power 74volt lamp
Posted: Thu Sep 26, 2013 9:53 am

Joined: Thu May 24, 2007 3:28 am
Posts: 17
Hi
Run them from a light dimmer switch. These are low cost and adjust the average power to the lamp which is all that matters. Make sure you don't turn it up too far, use a mechanical stop on the knob or possibly a fixed resistor or pre-set in series with the control potentiometer. Don't mess with the insides with power connected as ALL of the circuit is live and potentially lethal. You will have to find the correct setting by the lamp brightness rather than measuring the voltage. As the dimmer outputs a distorted waveform a normal meter will not read correctly. If you have a "True RMS" meter this will give an accurate reading and you can check the voltage with it.

Post subject: Re: How to power 74volt lamp
Posted: Thu Sep 26, 2013 11:43 pm

Joined: Sat Jan 11, 2003 1:01 am
Posts: 200
Location: Chattanooga, TN
Very interesting, all...thanks again for the tips. However, my go-to guy who runs that department told me to hold off a bit, and he'd get me fixed up with something (I think he was afraid at how "rigged" my contraption might look..haha).
So I'll hold off for a bit and see what happens. But, I'm checking back here frequently to see what else develops...

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