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Topic Title: 12v halogen
Topic Summary: AC , DC
Created On: 21 February 2014 12:02 PM
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 21 February 2014 12:02 PM
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zeeper

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Joined: 11 July 2008

Are the lamps ok with DC and AC 12V supplies. I think they should be fine just wondering.
 21 February 2014 12:55 PM
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leckie

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I would think the lamps would last longer at 12v dc.

The peak values of 12v ac would be 12xsq root of 2 approx. 17v.
 21 February 2014 01:03 PM
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zeeper

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Ok good point, I was thinking along the lines that the current would be the same . with a slight flicker with AC. Didnt think about peak voltage.
 21 February 2014 01:55 PM
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broadgage

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AC or DC are fine with any incandescent lamp.
I am old enough to remember DC mains and I would assure you that incandescent lamps worked equally well on AC or DC.

The fact that the peak voltage is greater than the RMS voltage is a red herring. What matters is the heating effect of the current.
One definition os RMS current is "that AC current that has the same heating effect as a DC current".

12 volt halogen lamps are normally worked from AC mains via a transformer, but work just as well on DC from a battery.
Some lighting systems use span wires that are live at 12 volts use 12 volts DC and the lamp work just the same as on 12 volts AC.
Flicker on AC will be imperceptable due to the thick filament.

Flicker can be noticed on AC supplies with mains voltage, low wattage lamps such as 15 or 25 watt.
 21 February 2014 03:26 PM
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leckie

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So if Power = IsquaredxR for either AC or DC, and voltage peak voltage is about 17v in an ac circuit, do you get a peak power as well? if the restance is fixed and voltage peaks then so will the current wont it? So the heating effect of Isquared x R will also peak and I presume would lessen the life of the lamp.

But then I might be misunderstanding it
 21 February 2014 04:08 PM
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anastasis

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Originally posted by: leckie
But then I might be misunderstanding it

I'm afraid so.

Yes, at the instant the voltage and current peak, the power also peaks. But at the instant the voltage and current are zero, the power is also zero. The instantaneous power varies throughout the AC cycle (or half cycle to be precise).

There's enough thermal inertia in the filament to average it all this out, as the frequency of the AC is sufficiently high. The rating of the bulb is this average power.

The RMS calculation takes the square root of the average (mean) of the squared voltage or current. You use this in the equations and it all works out just fine.

But, as a contrived example, if you ran a 12V lamp from a 12V RMS AC supply at a frequency of 0.1Hz, the filament will get above average power for a considerable time which might be enough to blow it.

The other complication is that the resistance of the filament (like almost everything) changes with its temperature. Putting half the voltage on a bulb almost certainly doesn't mean it runs at half power and half brightness.
 21 February 2014 04:45 PM
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leckie

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Ah, so a12v lamp will have exactly the same life whether supplied from a 12V DC or a 12v AC supply.... and there was me thinking I had come up with a great way of extending the life of lamps!
 21 February 2014 08:09 PM
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anastasis

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The only thing to add is that small changes in voltage have a big change on bulb life. I know that LV electronic transformers put out a fraction less than 12V (incidentally at high frequency) but I don't know exactly what voltage the lights are designed for.

I believe magnetic (wound) transformers for LV lighting are designed with very good regulation (much better than a normal transformers) to keep the voltage within tight limits.
 21 February 2014 09:56 PM
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leckie

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I bet AC makes lamps fail quicker. I can feel it in my water!
 21 February 2014 10:25 PM
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westfield6

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Originally posted by: anastasis

I know that LV electronic transformers put out a fraction less than 12V (incidentally at high frequency) but I don't know exactly what voltage the lights are designed for.



Those things are NOT transformers, they are switch mode power supplies, and yes they put out an RMS voltage of around 11.6 volts but with good regulation. Transformers on the other hand have poor regulation and should be run at full load otherwise the output voltage rises causing premature lamp failure. If several lamps are on one transformer failed lamps should be replaced immediately.
Don't run 12v halogen lamps off a battery in a motorhome or boat as the voltage can rise to over 14v on charge.
 22 February 2014 06:35 AM
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leckie

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It's great really; a seemingly simple question from the OP, and with a bit of input from several people we have had pretty much chapter and verse on, excellent!
 22 February 2014 06:52 AM
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normcall

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westfield6 -
The voltage will only be higher in a caravan if you are charging the battery.
We don't have the luxury of that facility with our van as we only end up in the middle of fields with a couple of solar panels to charge the battery.
can assure you they don't charge overnight (when the light might be used) or in inclement weather.
We really ought to save up and go on one of these site with electricity, hot water, toilets and even washing machines, I believe.

Otherwise you are, of course.

-------------------------
Norman
 22 February 2014 07:17 AM
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ebee

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"We really ought to save up and go on one of these site with electricity, hot water, toilets and even washing machines, I believe. "

Takes the pleasure out of camping/caravanning though!

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 22 February 2014 08:55 AM
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broadgage

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Mind you you, I have seen GLS lamps on retail sale with "230 volts 50 cycles" stamped on the cartons !

I wonder what would happen if I used them on 60 cycles or on DC
 24 February 2014 12:45 PM
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broadgage

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Originally posted by: broadgage
Mind you you, I have seen GLS lamps on retail sale with "230 volts 50 cycles" stamped on the cartons !
I wonder what would happen if I used them on 60 cycles or on DC


And not only dodgy local shops, but RS COMPONENTS list a number of incandescent lamps as "230 volts AC" they really ought to know better !

And numerous types of flex and cable rated at 450/750 volts AC! I have used some on DC, will the part pee police chase me for this ?
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