How many mr16s on one switch?

[loveandmercy]

Hi

A simple question I think for a more knowledgeable person than me...

I've got 17 (yes seventeen - I didn't put them in) mr16 spotlights in my bedroom, controlled by 3 separate switches (8,7 and 2 lights on 2 way switching between 3 dimmers and 3 on/off switches).

I have replaced all the 50w bulbs with 20w bulbs, and now want to consolidate down to having all lights on a single dimmer and on/off switch connected for 2 way switching. The wiring changes in the loft are simple for me to achieve this, but...

Is there a limit on the number of mr16s I can have being controlled by a single siwtch?

I already know I'll need a 500w dimmer to build in the 25% latency I need - but apart from that, will I be OK?

I was just wondering if there were any resistance considerations for that many bulbs/transformers, or anything else that would hold me back - I hope not!!!

Thank you!!!

 

[Steve]

The dimmer needs to be compatible with the transformers. Dimmers dont "resist" current flow, they "chop" it up. If you picture standard mains is 50 hertz sine wave ~. This means the voltage crosses 0 volts 100 times a second, and the dimmer sees this and chops the waveform up, according to how you set it. Some dimmers chop off the last part of the wave, and some dimmers chop off the first part. Now, transformers dont like this, as they rely on the 50Hz waveform to work! So you need dimmable transformers, but ones compatible with the dimmer's method of dimming - trailing edge or leading edge.

Also, most dimmers require de-rating to work with halogen and trasnformer loads.

Have you considered LED lamps? Even if you replaced half with LED you would achieve huge savings, but you would need to make sure you dont dim them!

 

[RF Lighting]

Nope. As long as the the total wattage of the lamps does not exceed 75% of the rating of the switch, you should be fine.

Just make sure the dimmer is compatible with the type of transformers you have. It should say somewhere on the instructions that it is.

Don't put in LEDs. You'll just end up walking into stuff that you couldn't see.

 

[loveandmercy]

Hi Steve and RF Lighting

Thanks very much for your replies!

Unfortunately my wife's desire to be able to dim in the bedroom prevents me using LEDs [I have used them elsewhere where her influence is not as pervasive ]

I'll make sure I get a compatible dimmer - thanks for the great advice. I knew it was an easy question for a better man than me

 

[electronicsuk]

Don't put in LEDs. You'll just end up walking into stuff that you couldn't see.

I've said it before here and I'll keep saying it; if you're prepared to spend ~£20 a lamp, you can more or less comfortably use them in all your downlights with very little compromise in terms of light output. They also tend to have a much wider beam angle than typical reflector lamps, which no doubt helps.

 

[Steve]

Don't put in LEDs. You'll just end up walking into stuff that you couldn't see.

Reminded me of my ex. In a moment of passion I launched myself onto the bed, aiming to land on top of her (we were both fully clothed), and just as I decided to do this she lifted her knees up and kneed me in the teeth. Certainly didnt see that coming!

 

[ban-all-sheds]

just as I decided to do this she lifted her knees up and kneed me in the teeth

Could have been worse.

 

[loveandmercy]

OK, so I made the call to buy my dimmer switch today...but...

The guy I spoke to, who I have dealt with before and is (I believe) knowledgeable, told me that the issue with having 17 lights on one dimmer is "capacitance".

And moreover, that to compensate for that I need to look at a dimmer with a total max wattage greater than or equal to the sum of the max wattages for the transformers (i.e. not the bulbs).

As there are 17 transformers at 60w max per transformer, this would mean that I need a dimmer with max wattage 1020w or greater, and even then it is not clear if I would have overcome my capacitance issues.

I'm going to call him back tomorrow, having checked my transformers out this evening. If anyone's interested, they are TAM-ET60 transformers (tamlite) - these are electronic transformers apparently. I am not sure if that means they are leading edge or trailing edge - or neither.

Does anyone have any views on the above - I could go for a 1500w dimmer and see if that overcomes the capacitance issues??

 

[Steve]

these are electronic transformers apparently. I am not sure if that means they are leading edge or trailing edge - or neither.

Could be either.

And I'm not sure on capacitance. I believe thats a phenomenon on 2-way circuits where there are live and dead wires run sidebyside for long distance. It rears its head when energy saving lamps are used, and a flickering can be seen when the switches are "off". This is because of a slight charge leakage between the two wires.

A dimmer rated at over 1000w will be hard to find for a standard switchplate. 1000 watts is the largest i've seen.

 

[electronicsuk]

these are electronic transformers apparently. I am not sure if that means they are leading edge or trailing edge - or neither.

Could be either.

And I'm not sure on capacitance. I believe thats a phenomenon on 2-way circuits where there are live and dead wires run sidebyside for long distance. It rears its head when energy saving lamps are used, and a flickering can be seen when the switches are "off". This is because of a slight charge leakage between the two wires.

That's capacitive coupling. I think what we're actually talking about here are reactive loads.

This isn't the first time I have heard someone mention using the sum of all transformer maximum power ratings when choosing a dimmer, but it sounds as if it's probably tosh. As long as the transformers are designed to be dimmed, the dimmer is compatible with the transformers, and you stick to any manufacturer's recommendations regarding derating for reactive loads, it will be fine.

 

[ban-all-sheds]

Does the dimmer actually see a reactive load?

ELV "transformers" are actually SMPSUs, so the input gets rectified and chopped into a high-frequency waveform before it gets anywhere near a transformer.

 

[DCC]

Don't put in LEDs. You'll just end up walking into stuff that you couldn't see.

Reminded me of my ex. In a moment of passion I launched myself onto the bed, aiming to land on top of her (we were both fully clothed), and just as I decided to do this she lifted her knees up and kneed me in the teeth. Certainly didnt see that coming!

Just think yourself lucky you aren't the OP - 17*50W downlighters in a bedroom - can't see my other half coming out into that glare under any circumstances!

 

[RF Lighting]

Probably about the right lighting level for filming in HD

 

[electronicsuk]

Does the dimmer actually see a reactive load?

Indeed it does, because a SMPSU is constantly switching on/off many times a second at a frequency far higher that that of the mains supply, which introduces harmonics into the supply.

 

[loveandmercy]

Hi everyone

A quick update if you are still interested in my "project".

I decided to get the wiring right first, before solving the dimmer switch problem.

So, with a bit of time off at Christmas, I've successfully got all 17 lights working of a single switch - switched for 2 way switching but with straight on/off switches at both switches (no dimmer).

That works great, so I then experimented with an existing dimmer, that I wasn't afraid to test with and a 400w max load. I got the "phasing effect" when I dimmed at all with this switch - i.e. lights pulsing brighter, then less so, then brighter again and so on.

I wonder if this is the "harmonics" that electronicsuk was discussing.

Anyway, no harm done there (except perhaps to my "disposable" dimmer), so back to an on/off switch - and now I just need a dimmer that can handle the 17 bulbs/transformers.

I have 2 candidates - a 1000w dimmer or a 1500 dimmer. The 1000w is more than half the price of the 1500w dimmer so I'm tempted to go with that. The only niggling thought in my mind is that if there is any truth in the "sum of all the transformer max loads" theory, then 17 x 60 = 1020w, which is marginally over 1000w.

I'm probably going to pay the £35 for the 1000w dimmer and give it a go - any views to the contrary please advise!!!

BTW, here are my two candidate dimmers:

http://www.1stopelectrics.com/product/hc91//

http://www.1stopelectrics.com/produ...500w_va_iq_dimmer_dimension_screwless_chrome/