Buzzing Dimmer Switch

[tonygrewal]

I have 12 low voltage lights each containing a 20W bulb in my new lounge. Each light has its own transformer and the lights are controlled by a 400w dimmer. I bought the dimmer from GFE Electrical Wholesale Ltd in Slough. I have been told that the dimmer is suitable for low voltage lighting.

Whenever I turn the lights low, the dimmer makes a loud buzzing sound. When the lights are turned brighter, the buzzing sound reduces. Is there something that I can do to reduce the amount of buzzing that comes from the dimmer.

 

[Dragonrider]

'Cuse ignorance, didn't know low-wattage/12V where dimmable??

 

[breezer]

you should have electronic dimable transformers, if not oh dear, if they are try another brand of dimmer

 

[Lectrician]

Dimmers do buzz when turned low, some mor ethan others.

 

[breezer]

that is why i suggest another brand of dimmer

 

[Driller]

If your transformers are dimmable, what is the VA rating? If they are the usual 20-60VA then you've got 12x60VA = 720VA going through a 400W dimmer. You have to use the full rating of the transformer when you calculate the loads.

Also most dimmers need to be derated by 20% when used with LV transformers therefore the max you can put through it is 320W.

I know 'cos I've just spent a week on the phone to the manufacturers to sort out an identical problem.

The solution they've recommended is to swop out the 12 individual transformers for 3x105VA as 4x20W @ 12V is 96VA.

Which will give 315W through a 400W dimmer.

The only problem now is to see if I can keep the LV feed cables short enough to keep within the volts drop limits, without having to use something bigger than 1.5mm2.

I'll post back when I've got the stuff and installed it.

God I hate low voltage downlighters.

Good luck with yours

 

[ban-all-sheds]

If your transformers are dimmable, what is the VA rating? If they are the usual 20-60VA then you've got 12x60VA = 720VA going through a 400W dimmer.

In the very first line, Tony wrote "I have 12 low voltage lights each containing a 20W bulb". 12 x 20 = 240. Add on a bit for losses in the 12V power supplies and you're still at less than 300W.

You have to use the full rating of the transformer when you calculate the loads.

Can you explain why?

Which will give 315W through a 400W dimmer.

No - it'll still be 240W + a bit.

God I hate low voltage downlighters.

Use mains ones then...

 

[Driller]

B-A-S.

I thought that I could just use the wattage of the bulbs for calcs. But the manufacturers of the kit I used (Varilight) assure me that what I've stated in my first post is right.

The figures I quoted come from their customer support department.

They also don't recommend or guarrantee their equipment if you put more than 5 transformers on a dimmer.

Believe me I would use mains if I could but this is a customer bought install.

 

[ban-all-sheds]

B-A-S.

I thought that I could just use the wattage of the bulbs for calcs. But the manufacturers of the kit I used (Varilight) assure me that what I've stated in my first post is right.

The figures I quoted come from their customer support department.

Strange, and electrically bizarre - I wonder if they're just trying to cover themselves against somebody putting in bigger lamps on the basis that the transformers can take it?

Believe me I would use mains if I could but this is a customer bought install.

Ah.

 

[tonygrewal]

I will check over the weekend to see if the transfomers are electronic and dimmable. I'm pretty certain that they are.

I will not be able to swap the individual transformers with bigger sized transformers because the room is single storey and the wiring was done before the ceiling was put up.

If I've got 720VA going through a 400W dimmer, what does this mean? What should I therefore do?

 

[ban-all-sheds]

If I've got 720VA going through a 400W dimmer, what does this mean?

Well - no, you haven't. The total load of your lamps is 240W. I don't know what the power factor of the transformers is, but there's no way it will be as low as 0.33, so there is no way that the load on the primary side is 720VA. AFAIK, the PF of electronic transformers (actually they are switch-mode power supplies, not simple transformers) is pretty close to unity. It will probably worsen as it is dimmed, but then the current being drawn reduces as well.

What should I therefore do?

It would seem that, notwithstanding the above, you should follow the recommendations of the dimmer manufacturer, in case you ever need to make a warranty claim....

This article: http://www.powerpulse.net/powerpulse/archive/pdf/aa_022100d.pdf might be of interest.

 

[Damocles]

Well this still does not necessarily get us nearer a solution. We still do not know if there is anything wrong with the dimmer. As you turn a dimmer down, it has to carry a bigger and bigger load current in a shorter and shorter time. So anything inside which is loose and can hum at mains frequency will do so more as it is turned down.


I don't know exact;y what is inside an electronic transformer. But assuming it is really a switch mode power supply, does this mean it has a reservoir capacitor which then supplies current to the secondary? If so, slicing the applied mains voltage could cause some very wierd voltage effects on the reservoir capacitor. It would take a very big current when power was applied as it tried to get back to working voltage.

Also, I reckon the surge would be much worse if you have several over-rated supplies than one single one. If, say, each is limited to a maximum charging current of 1A, then having 5 would mean a load of 5A. Though for 1/5 the time. Much of the current needed to work the lights for a whole mains cycle would be compressed into a shorter time the more transformers are used. This just might be enough to blow up your dimmer.

 

[ban-all-sheds]

I don't know exact;y what is inside an electronic transformer. But assuming it is really a switch mode power supply, does this mean it has a reservoir capacitor which then supplies current to the secondary? If so, slicing the applied mains voltage could cause some very wierd voltage effects on the reservoir capacitor. It would take a very big current when power was applied as it tried to get back to working voltage..

They are SMPSs, and usually not very good ones. The problem you describe is exactly why they can buzz, or the lights can buzz, with leading edge dimers, and why both the supplies and the dimmers have to be designed with their characteristics in mind, why fuses are sometimes needed etc - again, that article explains quite a lot (but not what ELV stands for!) - I found it when I was looking to see what the typical PF of one of those supplies is.

The ideal solution I guess would be matched products from one manufacturer, i.e. a dimmer and supply designed for each other and guaranteed not to buzz or blow up. I've no idea if such products exist, but I've never seen them mentioned. There would seem to be a market for them, and I wonder if the lack of them illustrates the difficulty of making it work properly...

 

[plugwash]

an interesting possibility would be to dim the lights with a variable smpsu

 

[tonygrewal]

OK, I've now checked the transformers and each transformer is manufactured by Eminent Plus Lighting. I believe these are owned by the electrical wholesalers called GFE which is where I bought them from.

These transformers are labelled 'electronic transformers' and 'fully dimmable'. It states Primary 230V ~ 50 Hz 60VA. Secondary is 11.6V eff and Max 4.9A

The dimmer is manufactured by Hamilton. By the way, I've noticed that when the dimmer is buzzing, the fuse box is also buzzing.

Any further advice on how I can eliminate the load buzzing when the dimmer is on low?