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Joined: Nov 2004
Posts: 138
C
Member
ok...you have a 120V 60W lightbulb...it draws .5 Amps...right? now if you have low voltage lighting.....say you have a 10:1 stepdown transformer(120:12) driving a 20W lightbulb, would it be drawing 1.7 Amps?

Joined: Aug 2001
Posts: 7,520
P
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Yes, that will be the current on the secondary (12-volt) side of the transformer.

In a theoretically perfect transformer, the primary current would then be 10% of that, or 0.17A. (Primary voltage is 10 times secondary voltage, therefore for the same power secondary current must be 10 times the primary current).

In practice, the primary current will be a little higher due to losses in the transformer, and due to the circuit not being purely resistive.

Joined: Nov 2004
Posts: 138
C
Member
thanks, pauluk...so is low voltage lighting more economical? do you get the same amount of lumens per watt?...for instance do you get the same amount of light from a 12V 30Watt bulb as you get from a 120V 30Watt bulb?

also..the circuit not being purely resistive...is the transformer inductive?

[This message has been edited by Clydesdale (edited 01-16-2005).]

[This message has been edited by Clydesdale (edited 01-16-2005).]

Joined: Dec 2004
Posts: 329
I
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While low voltage lighting is safer I wouldnt say it it going to be more efficient.
While the lumins per watt would be about the same(assuming comparing the same type of bulbs ie incansecent, etc.) there is always going to be some power loss in the stepdown Xfrmr, the smaller ones are particularly lossy, as well as voltage drop in the cables.
Remember in order to make the same light output(ie power) the low voltage lighting will draw more current thus requiring heavier cable to combat voltage drop. So overall efficiency will sffer there.

Joined: Nov 2004
Posts: 138
C
Member
yes, i understand that there will be more current on the load side...but since there is 10 times less current on the line side of a 10:1 stepdown transformer, wouldn't your electric bill reflect that? doesn't the meter read what the line side of the transformer is drawing?

Joined: Aug 2001
Posts: 7,520
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If we assume lightbulbs of similar efficiency in terms of lumens per watt, there is nothing to be gained by stepping down to low voltage. The primary xfmr current in the above example works out to 0.17A (ignoring xfmr losses), but if you had a 20W 120V bulb connected directly to the 120V line that too would draw 0.17A, so the meter would still register 20 watts of power being consumed.

As Ian pointed out, you would likely be running less efficiently due to the xfmr and copper losses.

I remember I actually had a hard time explaining a situation like this to somebody a couple of years ago. They had a hallway with two pendant lights, each containing a compact fluorescent, about 18 watts each.

They had bought two low-voltage spot assemblies, each with three 12V bulbs rated at about 50W each.

They were convinced that because they were low voltage it was going to save energy, when in fact they were planning on replacing 36W of fluoro lights with 300W of spots.

I fitted them as that's what they wanted there, but I'm still not sure I convinced them that the low-voltage lights were actually going to cost more to run.

Joined: Nov 2004
Posts: 138
C
Member
ok..i totally see it now, because if you divide 60W by 120V you get 1/2A ...but if you divide 60W by 12V you get 5A, on the line side of that it's still 1/2A...totally understand! thanks!pluss the inefficiency of the Xfrmr will burn up more $$$$$!!..ok ...next question, why then do all these companies make their low voltage lights so much more fancy/expensive and have more of a selection?..only for safety? (not to be undermined of course)
easy to market to people who think they are saving power?
[This message has been edited by Clydesdale (edited 01-17-2005).]

[This message has been edited by Clydesdale (edited 01-17-2005).]


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