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Topic Title: Faulty dimmer or faulty lamps?
Topic Summary: Problem GU10s
Created On: 09 May 2010 12:37 PM
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 09 May 2010 12:37 PM
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sparkler

Posts: 67
Joined: 16 September 2006

Hi, I keep having to replace a 400 watt dimmer switch which controls 8 recessed lights fitted with 50 watt GU10 lamps, every time a lamp blows ( once a month on average) the dimmer fails and has to be replaced. Any ideas on what could be the cause and how to remedy it would be welcome.
 09 May 2010 12:48 PM
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microy

Posts: 365
Joined: 25 October 2005

Sparkler

Read the instructions that came with the switch.

Mike
 09 May 2010 12:52 PM
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rocknroll

Posts: 8905
Joined: 03 October 2005

Its a common problem with dimmers, thyristors/triacs are susceptible to damage by surges when a lamp blows due to that good old plasma arc.

In real terms the thyristor;/triac must have 2.5 times greater than the current rating of the fuse to survive, the answer is either buy an expensive dimmer module that has an inline fuse, fit an inline fuse in the module or keep buying dimmer modules.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 09 May 2010 12:54 PM
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michaelbrett

Posts: 941
Joined: 28 December 2005

You may find that the dimmer is not compaitble with GU10 lamps. 8 off 50W lamps represents 400W of load. The dimmer may need to be rated at about 800W!! The lamps are 'reactive' and you will need a dimmer rated in excess of the load.

In the past when I have seen this type of failure, on speaking to a number of manufacturers, I have found that they will not 'support' the use of dimmer switches with GU10 lamps.

A further thought, you may need to ensure that the lamps are internally fused. Alledgedly, the cheap lamps from the supermarkets are not and consequently cause the dimmer to fail when the lamp fails.

Regards

Mike
 09 May 2010 12:58 PM
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sparkler

Posts: 67
Joined: 16 September 2006

I don't remember the instructions mentioning this potential problem so if you coul.d elaborate I would be grateful.
 09 May 2010 01:05 PM
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daveparry1

Posts: 6377
Joined: 04 July 2007

I learned recently that the internally fused lamps have a 'C' after GU10, a customer of mine got some off the internet somewhere, might be worth Googling GU10-C. I don't know if these always prevent the dimmers from damage when the lamps fail, I only fitted them a couple of months ago, haven't heard anything since!
 09 May 2010 01:12 PM
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alancapon

Posts: 5831
Joined: 27 December 2005

Even fitting lamps with fuses, it is possible that the dimmer may still fail. At the moment, the design accounts for 100% of the dimmer's rated maximum capacity. Assuming that it is built to a price (and most are in my opinion), there may not be enough spare capacity to operate the fuse before the triac fails.

Regards,

Alan.
 09 May 2010 01:29 PM
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daveparry1

Posts: 6377
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Yes, even when using fused lamps the dimmer needs to be rated at least 50% higher than the load when using Halogen lamps.
 09 May 2010 02:00 PM
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microy

Posts: 365
Joined: 25 October 2005

Sparkler

Some manufacturers recommend a maximum load ( usually about 70% ) of the switch rating.

Regards

Mike.

PS. Sorry I didn't include that in my original post.
 09 May 2010 02:08 PM
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sparkler

Posts: 67
Joined: 16 September 2006

Thanks everyone, sounds like a 600watt dimmer might do the trick.
 10 May 2010 06:47 AM
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normcall

Posts: 8175
Joined: 15 January 2005

Buy a 1000w to be safe, but I've had them expire after a year or so.
You could try 35w lamps instead.

-------------------------
Norman
 10 May 2010 09:26 AM
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broadgage

Posts: 1356
Joined: 07 August 2007

Halogen GU10 lamps are a resistive load like any other incandescent lamp, they have no appreciable inductance.

GU10s are more likely to kill dimmers than other lamp types because the ends of the filament are very close together, and therefore more likely to form a short circuit arc at the end of lamp life.

Most cheap dimmers are in my experience unreliable at more than about 50% of the stated load.

I would uprate the dimmer to 1,000 watts.
If the wiring arrangement permits, it might be worth protecting the dimmer with a 2 amp MCB or fuse, which is more likely to open before the dimmer is killed.
 10 May 2010 02:52 PM
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rocknroll

Posts: 8905
Joined: 03 October 2005

I can tell you from experience that this often has to be a problem you have to live with, a few years ago when the band I was with was gigging three to four days a weeks all over the place incl Europe and we had our own lighting rig I think RS used to love me because I was buying components a bag full at a time, I tried many options like up-rating the triac, SPD's and the suchlike but when a par went it still invariably took out the triac even though the equipment had a fuse that was supposed to blow.

I think you have to remember that the plasma arc that is created when a bulb goes is something like 40,000 to a million A^s thats a fair old whack for a discrete component, the amazing thing was it never used to affect the IC or the other components just the Triac.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
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