Circuit Blowing Bulbs - Overpowered?

[andybuk]

Hi,
I'm in the process of repairing a DJ light system with a knackered transformer. It was causing the internal fuses to blow, so i've replaced the transformer, but it's now taken to blowing the bulbs within it. I'm guessing this is due to too high a voltage, or too much current flowing, but I just need to understand exactly what the ratings I have mean.

The transformer is identical to the old one in rating. 240V input to 2 x 24V secondary outputs, in series to create 48V. I've already noticed that this is actually producing 52V with no load attached. The transformer is 300VA, used to power two 150W 24V bulbs, also in series.

When a transformer is rated at 300VA, does that mean, when it's supplying 48V on the secondary, that it will supply 6.25A, or that this is the maximum it can supply? Also, how sensitive are halogen bulbs to a couple of volts difference to the rating? If it would work, I could get hold of higher rated bulbs, but I'm not sure whether to go for the same voltage rating, but a higher wattage, or to go for the same wattage with a higher voltage?

Hope that makes sense

Cheers

 

[dpc]

Have you measured the incoming voltage on the 240 V side of the transformer (carefully)?

Are there taps on the transformer for adjusting the turns ratio?

 

[andybuk]

There aren't any taps on the transformer, will check the voltage asap

 

[andybuk]

226V in
49V out

guessing the output changed due to fluctuations in the mains input.

 

[itsmoked]

Remember the life of an incandescent bulb goes down to the 4th or 5th power of the voltage. That means running a 24V bulb @ 25V cuts it's life like in half, at 26V cuts it to something like 1/6th, you get the idea.

The current has no control. The bulbs will draw what they want. The higher the load the lower the voltage though.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[andybuk]

ok, cheers. i'll try getting a couple of new bulbs and see what happens. fingers crossed it was just a dodgy one

 

[itsmoked]

You might try the long life bulbs. They are usually made long life by utilizing the above mentioned fact. They make 130V bulbs and label them 120V this extends the life by 3 or 4 times.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[xfmrman]

A transformer rated at 48V 300VA at nominal input will have a nominal output at 6.25A. The no load voltage you mention of 52V is about right as 300VA transformers typically have regulation (Vnoload/Vfull load) of 8 to 10%.
As the load is increased from 0 to 6.25A the output voltage will decrease to the nominal voltage.

 

[MacGyverS2000]

Keith, those numbers can't be correct (or at least not correct for anything other than DC, and even then they seem overly harsh). If that held for AC, minor power fluctuations in the grid would shorten the lifetime of bulbs to a few days about once every week.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Skogsgurra]

I think that Keith is close to the truth.

http://www.logwell.com/tech/shdwe/indicator_lamps.html

Other factors like vibration and frequent on/off also have something to say. And some bulbs are inferior quality.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Thanks Gunnar.

I was wrong. The life of an incandescent filament is approximately inversely proportional to V[sup]16[/sup].

Far worse than I remembered.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nbucska]

From "wikipedia": (I inserted ^ to indicate exponent)


Incandescent lamps are very sensitive to changes in the supply voltage. These characteristics are of great practical and economic importance. For a supply voltage V,

Light output is approximately proportional to V^ 3.4
Power consumption is approximately proportional to V ^1.6
Lifetime is approximately inversely proportional to V ^16
Color temperature is approximately proportional to V ^0.42

So lifetime changes with sizteenth power !!!


----------------------------
Please read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[itsmoked]

Yep! One of the few places you will ever see x[sup]more than 3[/sup]

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Hang on -

A 240/48V transformer operating at full rated power with a resistive load produces 49V output with 226V input? Sounds like this isn't a 240/48 transformer but a 220/48V transformer. Output voltage should be close to nominal when running at nameplate conditions, i.e. rated voltage delivering rated current. This one isn't.


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Sometimes I only open my mouth to swap feet...

 

[kenvlach]

Isn't there a voltage problem using 52V for "two 150W 24V bulbs, also in series"? If a bulb is rated to draw 6.25A @ 24V, won't it approximately draw 13.5 A when given 52V (or 12.5A @ 48V)?
I'm no electrician, but for the initially given bulbs it seems necessary to add loads, so a proper supply would be 13A minimum @ 24V.

 

[itsmoked]

ken; You missed the "in series" as I first did.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com