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Topic # 133582 27-Oct-2013 10:35
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I have three zones which are constantly blowing halogen (12v and 240v) and 240v light bulbs. No idea of the problem – I just keep replacing bulbs!. But I have a new problem which is blowing 12v transformers.  

On the loom, power goes to a 12 v transformer which feeds 3 halogen bulbs. This works fine. The same wire then goes off to another transformer which in turn feeds three more halogens. Somewhere in the same switch the power feeds 2 x 100w light bulbs.  

In the past few weeks I have gone through 3 transformers connected to the same three halogens. (it’s one of these transformers that runs three halogens off it)  

First transformer blew. I had the choice of 2 live and two neutrals – I probably made the wrong choice. So I understand that fail.  

Next transformer I just replaced, turned the lights on and it blew.  

Next transformer I checked the bulbs. On one I found the wiring tail had melted the insulation and the two bare wires were now touching. So I replaced the tail. Before wiring the transformer I wired in a 240 light fitting and turned the power on. Everyting worked just fine.   So I took the light fitting off and wired in the transformer. From there I wired in the first halogen. Turned the lights on and everything worked. Wired in the second halogen, lights on all worked. Wired in the third halogen, turned on and all worked.  

Blow me down, half an hour later the three lights had stopped working and sure enough the transformer has blown again.  

So I have pulled the lights out and here is a photo of a tail. It’s melted and presumably shorted the wires again.  

I’ve run a volt meter over the wires and its 243v   Any ideas where the problem might lie.


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  Reply # 922383 27-Oct-2013 10:53
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A few questions

What wattage are the Bulbs
What VA(wattage) rating is the transformer
What cable are you using
What are wattage rating of the fittings.

Reasons:
Incandescent and Halogen bulbs generate a LOT of heat and they need a way of dissipating the heat
The wattage of the lamps will tell us how much power is needed
The VA/wattage of the transformer tells us if you have a big enough transformer
The cable looks like it may not cope with the heat generated by the lamp, also the size of the cable is important , cable has resistance which means it can get hot too through self heating.
The quality of the connections is important too, they must be capable of copping with the heat and the current
The voltage rating of a fitting is important because it will tell us how much voltage the are capable of before breaking down electrically, where as the wattage rating will tell us if the holder is capable of dissipating the heat the lamp generates without overheating (melting the fitting and wires)




Here is the problem I see it with low voltage lighting.
A 240v 120W lamp will take 0.5 Amps
A 12v 120 W lamp will take 10A , this current needs GOOD connections otherwise they will self heat and can cause a fire. Even a 1 Ohm connection will generate 10 Watts of heat. If you have 3 lamps in parallel the current at the transformer would be 30Amps.


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  Reply # 922386 27-Oct-2013 11:01
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Definitely check the lamp wattages. It is common to have 3x 35w lamps connected to a 105w transformer. But replace those lamps with 50w ones and the transformer is now badly overloaded. Also the fittings might not be able to handle the extra heat produced by the higher wattage lamps.

 
 
 
 




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  Reply # 922395 27-Oct-2013 11:33
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Thnask folks
Attached a schematic. Bulbs  are 50 w but transforemer is 150.




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  Reply # 922401 27-Oct-2013 11:57
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sir1963: A few questions

What wattage are the Bulbs
What VA(wattage) rating is the transformer
What cable are you using
What are wattage rating of the fittings.

Reasons:
Incandescent and Halogen bulbs generate a LOT of heat and they need a way of dissipating the heat
The wattage of the lamps will tell us how much power is needed
The VA/wattage of the transformer tells us if you have a big enough transformer
The cable looks like it may not cope with the heat generated by the lamp, also the size of the cable is important , cable has resistance which means it can get hot too through self heating.
The quality of the connections is important too, they must be capable of copping with the heat and the current
The voltage rating of a fitting is important because it will tell us how much voltage the are capable of before breaking down electrically, where as the wattage rating will tell us if the holder is capable of dissipating the heat the lamp generates without overheating (melting the fitting and wires)




Bulb wattage. Halogens = 50 each (6 of these: 2 sets of three) . Light bulb 100w each (2 of these - single fittings)

Transformer rating to 150w

Cable. Electrical cable standard lighting stuff the electrician put in. The cable from the transformer to the bulb is the stuff that came with the light fitting. 

Not sure the wattage rating of the fittings - they are standard fittings from a lighting shop.

I don't have any batts near the light fittings and the transformer is away from the lights and clear of batts as well.

I wonder if I should replace all the tail connector at the halogen bulb end. Fittings are around 6 years old. I doubt earthquakes will have had an effect but maybe they are just getting a bit old.

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  Reply # 922425 27-Oct-2013 13:03
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Given the cost of these parts you might save money by getting an electrician in to look at it for you.




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  Reply # 922430 27-Oct-2013 13:17
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Are you SURE they're 50w and NOT 50 degree lamps.

I'd strongly suggest getting someone to look at this, worst case here is a house fire.




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  Reply # 922461 27-Oct-2013 14:32
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minimoke:
sir1963: A few questions

What wattage are the Bulbs
What VA(wattage) rating is the transformer
What cable are you using
What are wattage rating of the fittings.

Reasons:
Incandescent and Halogen bulbs generate a LOT of heat and they need a way of dissipating the heat
The wattage of the lamps will tell us how much power is needed
The VA/wattage of the transformer tells us if you have a big enough transformer
The cable looks like it may not cope with the heat generated by the lamp, also the size of the cable is important , cable has resistance which means it can get hot too through self heating.
The quality of the connections is important too, they must be capable of copping with the heat and the current
The voltage rating of a fitting is important because it will tell us how much voltage the are capable of before breaking down electrically, where as the wattage rating will tell us if the holder is capable of dissipating the heat the lamp generates without overheating (melting the fitting and wires)




Bulb wattage. Halogens = 50 each (6 of these: 2 sets of three) . Light bulb 100w each (2 of these - single fittings)

Transformer rating to 150w

Cable. Electrical cable standard lighting stuff the electrician put in. The cable from the transformer to the bulb is the stuff that came with the light fitting. 

Not sure the wattage rating of the fittings - they are standard fittings from a lighting shop.

I don't have any batts near the light fittings and the transformer is away from the lights and clear of batts as well.

I wonder if I should replace all the tail connector at the halogen bulb end. Fittings are around 6 years old. I doubt earthquakes will have had an effect but maybe they are just getting a bit old.


150VA transformer...yeah.
VA stands for Volt x Amps.

So, if the transformers are 230v rated and you have 240v then you are marginal
Also you need to consider the lamps will not be exactly 50W
Also, the phase angle (power factor ) makes a difference

So, if you have the worst case you could be drawing maybe 160-170W, this may also cause the transformer core to saturate making the situation worse.

I would be looking at a 200W transformer for a start, the transformer will run cooler too.

The light fittings should have a maximum wattage written on them, but 50W sounds reasonable for a halogen.

Me, I went with the LED lighting option to save power.




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  Reply # 922486 27-Oct-2013 15:24
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I'e got more numbers than I can poke a stick at.

The lamps are Superlux 12v Halogen reflector lamp MR16. 50W Dichroic 60.

The Transformer is dimmable with resistive and inductive dimmers and self monitoring.

Inductive load compatible 1.0kv surge protection
Pri:240V~ 50/60Hz 0.62A
Sec: 11.4V ~ 50-150W/VA
Cos(forward slash through it) = 0.98
ta:45 degree C tc 75 degree C.

I've already had an electrician look at the other lighting problems (lights blowing) bnd he came up with nothing. Our lighting guy is running out of ideas. I'm trying to refine the options then I can find someone who can fix.

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  Reply # 922523 27-Oct-2013 16:02
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Some suggestions, change all the tales going to the lamps.
If they are starting to fail like the picture example, you will get poor lamp life (as the connection to the lamp pins will be poor causing the over heating and then melting of wires).
Get tails that are a decent length to keep transformer away from light fitting.
Get decent lamps not superlux or a no name brand( think cheap Chinese) .Phillips or Sylvania are good. Consider trying a 35 or 40 watt alternative to the 50 watt lamps you are using -they generate less heat but should still be bright enough. Or better use LED -but they are somewhat more expensive at a comparable light output to the halogen.
Make sure the transformer and light fitting is clear of insulation.

My two cents,cheers.

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  Reply # 922543 27-Oct-2013 17:07
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I don't know much about this stuff, but are you meant to run transformers at 100% of their rated capacity?




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  Reply # 922554 27-Oct-2013 17:22
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Most of the common electronic transformers have a wide operating range.For an example a electronic 60va unit will happily run a lamp wattage of between 20 and 50 watts.



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  Reply # 922560 27-Oct-2013 17:42
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Thanks everyone. I'm digesting all this information.

What I'm also not understanding is why does the transformer or my circuit board not trip. If a tail is failing  and gets hot enough to melt the insulation I would have thought the fuse at the circuit board would have gone - isn't this what they are supposed to do? And how is it a transformer lets the wire get so hot. I'd have thought it would trip and then cool down and restart rather than just totally blowing.

I've got a dozen or so of these things sitting in my roof possibly cooking away!

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  Reply # 922585 27-Oct-2013 18:19
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As long as the circuit is not overloaded or is shorted out it won't trip out with a faulty lamp connection on the secondary side of a transformer.
Halogen transformers will turn off and off on overloaded, but as long as the faulty connection is not overloading or shorting out the transformer it will happily keep on running.

I would drop out each lamp and check the lamp end of the tail. No need to climb in roof space. If it looks burnt or discoloured replace it. Disconnect each lamp from the tail look at the pins. Do the pins look discoloured or burnt? Compare a new lamp to see the colour of the pins. Does one pin look darker than the other? If you have doubt replace the tail AND the lamp as damaged lamp pins will stuff the new tail in no time.

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  Reply # 922621 27-Oct-2013 19:32
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minimoke: Thanks everyone. I'm digesting all this information.

What I'm also not understanding is why does the transformer or my circuit board not trip. If a tail is failing  and gets hot enough to melt the insulation I would have thought the fuse at the circuit board would have gone - isn't this what they are supposed to do? And how is it a transformer lets the wire get so hot. I'd have thought it would trip and then cool down and restart rather than just totally blowing.

I've got a dozen or so of these things sitting in my roof possibly cooking away!


Good thought, but wrong....sort of.

If you have a 5A fuse you would need to draw over 3000W for hours before the fuse blew
If you have a 10A fuse you would need to draw over 6000W for hours before the fuse blew.

So, if one of your light fittings shorted out here is a high chance that the resistance of the copper winding in the transformer would limit
the power drawn to less than this, however this is more than enough to cause a fire.

This is one of the reasons I hate low voltage lighting.

Personally I believe if you are going to use low voltage lighting its worth  having a decent SMPS and running ALL the lights off it




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  Reply # 922630 27-Oct-2013 19:56
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sir1963:
minimoke: Thanks everyone. I'm digesting all this information.

What I'm also not understanding is why does the transformer or my circuit board not trip. If a tail is failing  and gets hot enough to melt the insulation I would have thought the fuse at the circuit board would have gone - isn't this what they are supposed to do? And how is it a transformer lets the wire get so hot. I'd have thought it would trip and then cool down and restart rather than just totally blowing.

I've got a dozen or so of these things sitting in my roof possibly cooking away!


Good thought, but wrong....sort of.

If you have a 5A fuse you would need to draw over 3000W for hours before the fuse blew
If you have a 10A fuse you would need to draw over 6000W for hours before the fuse blew.

So, if one of your light fittings shorted out here is a high chance that the resistance of the copper winding in the transformer would limit
the power drawn to less than this, however this is more than enough to cause a fire.

This is one of the reasons I hate low voltage lighting.

Personally I believe if you are going to use low voltage lighting its worth  having a decent SMPS and running ALL the lights off it

Ahh - that makes sense at the circuit breaker end.

But the transformer has all these flash supposed settings so I wonder why these don't work.

On the transformer it says the LED glows:
- "ON" for normal operation.
- OFF - its out of order check mains power (nothing about checking tails / bulbs or that it is rooted!!)
- Slow flashing: Short circuit condition (but two melted wires touching doesn't trip it!)
- Rapid flashing: Transformer overload (its in the roof - who sees this before it blows!)

a 100w bulb handing from a wire in the middle of the ceiling seems a whole lot simpler!

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