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Topic Title: Volt Drop
Topic Summary: Calculation doesnt match reality
Created On: 25 June 2013 11:11 PM
Status: Post and Reply
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 25 June 2013 11:11 PM
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primo

Posts: 426
Joined: 10 January 2008

What am I missing here?

1mm2 cable. 44 mV/A/m.
50w load at 24v (2.1A)
10m length of cable

(44 x 2.1 x 10)/1000 = 0.92V

When I set this up and measured it i was getting about 13V at the lamp.
 25 June 2013 11:31 PM
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SKElectrical

Posts: 892
Joined: 01 February 2009

Are you sure you had your meter set to DC? Try turning meter off and measure again. Or try new batteries. Also check the cable is sound and connected soundly and also check the transformer without load (if possible).
The maths is right so I think you'll find the answer yourself.
 25 June 2013 11:45 PM
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primo

Posts: 426
Joined: 10 January 2008

I was using a Fluke T5 which is auto sensing for AC/DC but the output from the transformer is actually AC anyway.
I'm trying to get away with using a 24v transformer to supply 12v halogens as the installed cable is only 1mm and run is approx 10m with nowhere local to the fittigs to hide a 12v transformer.
Maths says lamp should blow, test bench shows a 50w halogen glowing at about the correct brightness and correct (ish) voltage at the lamp?!!
 26 June 2013 12:06 AM
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alancapon

Posts: 5698
Joined: 27 December 2005

Are you using a real transformer or an electronic one? A lot of the electronic ones give an ac output, but at a high frequency which you would need a true RMS meter to measure correctly. You also need to be aware that the figures in the green book are for 50Hz and will probably be different at the higher frequency.

Regards,

Alan.
 26 June 2013 01:08 AM
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crossy

Posts: 77
Joined: 14 February 2006

Dumb question, did you measure the output of the transformer under load?

It could be dropping off to 14V (undersized, faulty etc) or if it's electronic it may have gone in to current limit mode.

You also need to measure the actual current being drawn, lamps are non-linear resistances.
 26 June 2013 06:17 AM
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primo

Posts: 426
Joined: 10 January 2008

Originally posted by: alancapon

Are you using a real transformer or an electronic one? A lot of the electronic ones give an ac output, but at a high frequency which you would need a true RMS meter to measure correctly. You also need to be aware that the figures in the green book are for 50Hz and will probably be different at the higher frequency.



Regards,



Alan.


It's a real transformer! Ill get the 117 out later and set it up again and measure with that.

One (potentially) vital bit of information that I left out is that it is all being fed from a Lutron Grafik Eye and I was going to lower the top end trim if the output at the lamp was too high.
 26 June 2013 06:20 AM
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primo

Posts: 426
Joined: 10 January 2008

Originally posted by: crossy

Dumb question, did you measure the output of the transformer under load?



It could be dropping off to 14V (undersized, faulty etc) or if it's electronic it may have gone in to current limit mode.



You also need to measure the actual current being drawn, lamps are non-linear resistances.


Yes, that was measured under load. Ill check the current draw later and also try and measure the output with no load if possible as SK suggested earlier.
 26 June 2013 07:49 AM
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zeeper

Posts: 1378
Joined: 11 July 2008

When I set this up and measured it i was getting about 13V at the lamp


I'm trying to get away with using a 24v transformer to supply 12v halogens


It's a real transformer!


So if you have connected the 12V lamps in series with each other(eg The line conductor goes to lamp A the neutral conductor goes to Lamp B and then you link the lamps together with another conductor). The volt drop across each lamp would be 12 Volts. very close to what you measured. So the measured output of the transformer would be 24V but across each light you would measure 12V

The above is abit like the old xmas lights where if one lamp fails nothing works.

But the above would not comply with BS7671.

134.1.1
 26 June 2013 08:28 AM
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broadgage

Posts: 1244
Joined: 07 August 2007

Was it an electronic transformer ?
If so, these dont normaly produce the full output voltage if unloaded.
Also the output is AC at a very high frequency, too high for most test meters to measure accuratly.
Also, most electronic transformers are only suitable for limited lead lengths on the secondary.

Also double check the lamp wattage. 50 watts at 24 volts implies two lamps each 12 volts, 25 watts in series.
25 watts is not a standard wattage for 12 volt halogen lamps, which may suggest a mis understanding?

How bright were the lamps ? When dealing with the not easily measured high frequency output of an electronic transformer, observing the brightness of lamps is often more accurate than a test meter.

If it was an electronic transformer, then I would obtain a 24 volt, or better 25 volt, copper iron transformer of suitable wattage to supply a series pair of 12 volt lamps.

For series operation, a pair of 12 volt lamps must be the same wattage, and should preferably be of the same brand.
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