low voltage tungsten lamps

hello ladies and gentlemen, I hope this isn't a silly question, but I have an old angle poise lamp on a lathe that keeps blowing fuses. I have checked the wiring and all is ok, even measuring 25v open circuit at the lamp holder, but as soon as I put a lamp in it blows the 2A MD fuse (this was the size fitted when I originally was called out to it)

it is a 25v 60W lamp, so basic ohms law should make it just over the 2A surely or is the 60W the amount of light it gives off? A 2A HRC fuse should handle this?

I've done some sums as an example on a 240v 60w lamp (75 ohm) and the current is 3.2A for that one and for a 50V 60W lamp(3.5 ohm) works out at 14.29A!!

surely this can't be right? Am I missing something?

any help would be appreciated

The voltage at the lamp is 25v a 60w lamp should pull just over 2A,  in fact it's 2.4 amps. This would be a constant draw for as long as the lamp is on. For 50v I think your calculations are far off, it should be around 1.5A. For 240v it should be around 0.25A.

thanks, yes should be but that's done with actual test meter values! I haven't got the 25v lamp to hand to check its resistance. I can't afford to keep putting fuses in at £5 a pop( literally) that's why I checked the values and did the sums

I know its 25v open circuit because I've had my meter on it so there's no short circuit, the neutral is the transformer 0v which is connected to earth

Is it 25v dc by any chance? The resistance should be 10.4 ohms for a 60w lamp at 25v.

What size is the transformer Kevin ,  may only be designed for a 40W lamp  @25V  .  Can you get a 40W X 25V lamp ?

Evans Electric said:

What size is the transformer Kevin ,  may only be designed for a 40W lamp  @25V  .  Can you get a 40W X 25V lamp ?

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It would make sense, 40w would give 1.6A and a resistance of 15.6 ohms.

The resistance of the lamp will vary wildly with temperature so it's probably not the best measure of potential current.

My first thought also was are you sure it should have a 60W lamp?

Lurch said:

The resistance of the lamp will vary wildly with temperature so it's probably not the best measure of potential current.

My first thought also was are you sure it should have a 60W lamp?

Click to expand...

@Lurch I think a 60w lamp has been fitted by mistake, and it should have been 40w. On turning up to any job we all assume that what is fitted should be replaced, which is where the problem lies.

thanks guys, just been doing some more sums and im more confused.....

I've measured some normal lamps I've got here

240v 100w lamp = 43 ohm

240v 40w lamp = 112 ohm

110v 400w TH lamp = 3.1 ohm( just for fun)

now unless ohms law has changed since I've left college, V=IxR , R=V/I, I=V/R and W=IxV

so-

240/43=5.58A =1339W

240/112=2.14A = 514W

110/3.1=35.4A=3894W

has anyone ever measured a lamp before?

these values shouldn't work, but they do!

as mentioned by lurch, resistance increases with temperature. so once hot, resistance will be far higher, current will be much lower

are you guys on about inrush etc, the initial surge of current? I don't suppose it takes too long to get up to temperature and a higher filament resistance?

kevin worrall said:

thanks guys, just been doing some more sums and im more confused.....

I've measured some normal lamps I've got here

240v 100w lamp = 43 ohm

240v 40w lamp = 112 ohm

110v 400w TH lamp = 3.1 ohm( just for fun)

now unless ohms law has changed since I've left college, V=IxR , R=V/I, I=V/R and W=IxV

so-

240/43=5.58A =1339W

240/112=2.14A = 514W

110/3.1=35.4A=3894W

has anyone ever measured a lamp before?

these values shouldn't work, but they do!

Click to expand...

Your calculations are way out.

Remember the triangle? Voltage = current x resistance, power = voltage x current.

If it’s a LoVo transformer and two arm angled holder the lamp base is three pin bayonet, I wish you the best of luck finding them.

http://alexnld.com/product/3w-12-24v-500mm-cnc-machine-led-lamp-aluminum-alloy-flexible-light-bar-working-tool-lamp/?gclid=CjwKEAjw_bHHBRD4qbKukMiVgU0SJADr08ZZCIVfKC19y9ybTOjzVYzyre_6DUFfI4h-awq9QHoWFhoCCEHw_wcB 

You could always go Led it's not that expensive.

Manator said:

Your calculations are way out.

Remember the triangle? Voltage = current x resistance, power = voltage x current.

Click to expand...

that's what I've got, even googled electrical calculations and put resistance valves into the online calculator and got the same results, hence the confusion

try the values yourself

kevin worrall said:

that's what I've got, even googled electrical calculations and put resistance valves into the online calculator and got the same results, hence the confusion

try the values yourself

Click to expand...

You can't use the resistance of a cold tungsten filament, 

How are you measuring this resistance BTW,?

Using a DC meter,?

kevin worrall said:

that's what I've got, even googled electrical calculations and put resistance valves into the online calculator and got the same results, hence the confusion

try the values yourself

Click to expand...

Hello Kevin, welcome to the forum. Some of your assumptions are a bit wrong I think. You do not know the resistance and you cannot calculate it as you do not know the operating temperature of the lamp or the precise construction of its filament. The values that you do know are the rated voltage (volts) and power consumed (watts). Put the voltage and power into this calculator and see what you get. http://www.ohmslawcalculator.com/ohms-law-calculator  Also remember strictly speaking resistance relates to DC voltages, with AC supplies you are looking at impedance which is a slightly different beast. However, power = voltage x current, is what you need to know. Forget resistance, (unless you are measuring it whilst the lamp is on (hot)). Remember some basic rules of physics, electricity always generates heat whilst traveling along a cable, and a traditional light bulb relies upon this heat to produce a glow that we see as emitted light. So lights bulbs whilst on are hot, because the filament has got very warm, this heat alters the resistance compared to the cold values.

Doc H.  

Tony  , now you mentioned them , I remember those 3 pin lamps .  As in 3 pegs sticking out to connect with same in the lampholder.  As you say , are they available now ?

Also remember ..."LampLocks".   It was a brass ring bit like a female brass bush which had a triangular grubscrew at an angle and a special screwdriver to tighten it . 

They were for locking 240V lamps into inspection lights to prevent stealing and taking home like all the wash basin plugs.

Didn't the old lathes etc have the LoVo  anglepoise inspection lamps ...415V / 25V  transformers and I seem to remember ...25V X 25W lamps.

Wasn't there another machine inspection light called the Revo ?

kevin worrall said:

that's what I've got, even googled electrical calculations and put resistance valves into the online calculator and got the same results, hence the confusion

Click to expand...

Are you actually reading any of the replies?

Evans Electric said:

Tony  , now you mentioned them , I remember those 3 pin lamps .  As in 3 pegs sticking out to connect with same in the lampholder.  As you say , are they available now ?

Also remember ..."LampLocks".   It was a brass ring bit like a female brass bush which had a triangular grubscrew at an angle and a special screwdriver to tighten it . 

They were for locking 240V lamps into inspection lights to prevent stealing and taking home like all the wash basin plugs.

Didn't the old lathes etc have the LoVo  anglepoise inspection lamps ...415V / 25V  transformers and I seem to remember ...25V X 25W lamps.

Wasn't there another machine inspection light called the Revo ?

Click to expand...

there were also some "fireglo" lamps that had 3 pins, they were used in those early log effect fires. I remember this because we had one and someone came up with the idea of using a different coloured lamp, trouble is it wouldn't fit because of those 3 pins on the bayonet.