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Lowering mains voltage

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msrtlt

New Member
Hello,

I live in the UK and like to use my Christmas lights from the US. I bought a step-down transformer. The bulbs are rated at 110v and are all connected in parallel. The step-down transformer outputs around 120v, or a bit less. The lights work fine - however, occasionally - definitely more often than normal, bulbs burn out. I think this is because of the extra 8 to 10 volts they receive from the transformer.

The question is - is it possible to "swallow" these extra 10v?

Would some sort of coil in series with the lights work?

Thanks.
 

chemelec

Well-Known Member
Hello,

I live in the UK and like to use my Christmas lights from the US. I bought a step-down transformer. The bulbs are rated at 110v and are all connected in parallel. The step-down transformer outputs around 120v, or a bit less. The lights work fine - however, occasionally - definitely more often than normal, bulbs burn out. I think this is because of the extra 8 to 10 volts they receive from the transformer.

The question is - is it possible to "swallow" these extra 10v?

Would some sort of coil in series with the lights work?

Thanks.
In North America, These bulbs are Actually rated between 110 to 125 Volts.
(Actual "Nominal Voltage" in North America is 117.5 Volts)
There is NO Problem with your transformer putting out 120 Volts.

Unfortunately these Christmas Bulbs are Made VERY CHEAP, and Probably come from China. They do Not last a good life.

If You had a "Variac", you could reduce the voltage and they would last longer.

But WHY would you want to use these lights anyway?
 

Willbe

New Member
Would some sort of coil in series with the lights work?
Yes, or a resistor, or a diode. The diode I²T rating has to be more than the lamp inrush current (10x to 15x the steady state lamp current for a few dozen mS). Do the diode; the lamps will last forever.
 
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msrtlt

New Member
Thank all!

Willbe's suggestion seems the most workable! I'd like to understand: why a diode will make bulbs last much longer?

If I have 25 7w bulbs, does that mean that the diode should be rated at 15A? [(7w/115v)*25*10 ]
 

crutschow

Well-Known Member
Most Helpful Member
The diode half-wave rectifies the current and reduces the power to the lamps. This dims the bulbs and increases their lifetime. But a half-wave rectifier generates DC current in the transformer which may cause it to saturate and overheat. Better to use a resistor or a bulb (much larger size) to reduce the voltage.

Another way is to use two strings in series to make 240 volts and don't use the transformer.
 

audioguru

Well-Known Member
Most Helpful Member
My electrical utility company gave away new LED Christmas tree lights in exchange for power-hungry incandescent ones. The LED ones are dim.
I cheated a little because I cut my old incandescent strings in pieces and got a new LED string free for each piece I turned in.
 

KMoffett

Well-Known Member
Put a dimmer/switch between you step-down transformer and the lights. Slower turn-on and lower voltage makes bulbs last longer. Plus the dimmer lamps are more pleasing than at full brightness (my opinion ;) ).

Ken
 

msrtlt

New Member
Chemelec,

To answer your question, I want to use these lights because they are beautiful. No LED lights are ever so warm and gorgeous. Also it is a very easy maintenance - if a bulb goes out I can just replace it. These lights are also environmentally friendly - I cannot count how many strings of LED lights I threw out due to inability to fix them - and believe me, I do not throw things out easily. I spent countless hours trying to fix those bloody LED lights.
In comparison, I had the old-fashioned lights now for 14 years - and never had to change a bulb until I moved to the UK... So now after trying to move to LED lights I am regressing - or rather advancing back to the future.
 

xanadunow

New Member
You have had so far all good answers that will work but I would stay away from those dimmers. These that are cheap, generate RFI heard in radios all around the neighbourhood, and the "clever" ones that incorporate "zero crossing" switching - are to expensive for this application. I'd use rather a small autotransformer that may have several taps to suit.. It could also be a salvaged transformer from an extinct apparatus that was originally made to run a device from 117/220 - you will use only the primary section of the transformet turning it into autotransformer (the primary winding will have the appropriate tap (sometimes more than one) on it. Autotransformers are much more efficient than transformers and any service point still fixing stereos would have one to offer free or almost free of charge. The switch for the lights needs to be wired in the primary section od such device as you would not like the autotransformer to run (consume power) if the switch for the lights was wired in series with these bulbs..
 

msrtlt

New Member
Thanks xanadunow - is it possible to see a diagram - I am not sure from the description how the whole thing is connected...
 

msrtlt

New Member
Sorry, xanadunow - no need for the diagram. I wasn't familiar with autotransformers - I looked it up and now I get it. I am not sure it would be easy to get one cheaply...

BTW how can you tell which dimmers are "zero crossing"?
 

audioguru

Well-Known Member
Most Helpful Member
Dimmers are made as cheaply as possible. I think they use an inductor filter instead of zero crossing.
 

Willbe

New Member
Thank all!

Willbe's suggestion seems the most workable! I'd like to understand: why a diode will make bulbs last much longer?

If I have 25 7w bulbs, does that mean that the diode should be rated at 15A? [(7w/115v)*25*10 ]
If they're in parallel, 7*25/120 = 1.5A steady state current. There are 3A diodes available for use without a heat sink.

Surge current = 15A to 22A for ~0.1 sec, so the I²T rating for the diode needs to be more than 22²(0.1) = 48 Amps²·seconds.

A 1N4004 has 1A steady state and 7 A²·s so you need a heftier diode than this. A 1N5404 is 16¢ from these guys
HOSFELT ELECTRONICS, INC.
but you need to find a datasheet for it.

You could also series the diodes or parallel them (with a current sharing arrangement).
 
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msrtlt

New Member
Thanks all again.

On a slightly different note: suppose I wanted to drop 10v (assuming 1.5A current) using a coil connected in series to the bulbs. Also suppose I wanted to make the coil myself. Assume I use sufficiently thick wire that will not introduce resistance.

Does anybody know how to calculate (VERY approximately) the number of winds the coil will need to have resistance of about 7 Ohm at 50Hz?

Addition: I think I managed to calculate that I will need induction of about 20 mH.
(impedance = 2 * pi * 50Hz * 20mH = 6.28 ohm)

The question now is: how many winds (assuming either air or some sort of steel core)?
 
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KMoffett

Well-Known Member
Another trick is to add a low voltage control ("filament") transformer to the secondary of your mains step-down transformer. The control transformer's secondary current rating must be the same (or greater than) as the main's secondary. The primaries are connected in parallel and the secondaries are connected in series. If the phasing of the secondaries are the same, the output voltages add. If the phasing of the secondaries opposite, the output voltages subtract. The latter is what you want.

Ken
 

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msrtlt

New Member
Another trick is to add a low voltage control ("filament") transformer to the secondary of your mains step-down transformer. The control transformer's secondary current rating must be the same (or greater than) as the main's secondary. The primaries are connected in parallel and the secondaries are connected in series. If the phasing of the secondaries are the same, the output voltages add. If the phasing of the secondaries opposite, the output voltages subtract. The latter is what you want.

Ken
This is a very interesting trick - the only problem is that most standard 240v:12v transformers are designed for low current (probably up to 1A) - but if I find a suitable one I'd definitely try this! I would think that if I wanted say, 2A - I would have to find a 240v:12v with secondary's rating of more than 2A.
Thanks!
 

KMoffett

Well-Known Member
I would think that if I wanted say, 2A - I would have to find a 240v:12v with secondary's rating of more than 2A.
Thanks!
No, if you need 2A, you can pick a transformer with secondary rating of 2A. More doesn't hurt, but is not necessary. With center-tapped transformers and/or multiple transformers with different output voltages, you can get lots of different output voltages. There are a lot of higher current control transformers in the surplus market, and on eBay.

Ken
 

Willbe

New Member
Does anybody know how to calculate (VERY approximately) the number of winds the coil will need to have resistance of about 7 Ohm at 50Hz?
QUOTE]
Yes, they do.

You could also use 70' of #30 AWG, wound on a 2w resistor of value >>7Ω.
 
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