Dummy loads

[Rob]

I've been working as a chippy for the last few days buliding loads of cupboards etc. Being a Lampie at heart, I persuaded the client to have a couple of low voltage halogen spots under one of the shelves to light a bead head.

 

I got two 20W cheap fitings from Screwfix as well as a dimmable transformer and a dimmer switch as I did't want them to be too blinding over the poor inhabitants pillow. Having put all this in (see my post here), I turned it all on and the lamps flickered when dimmed. On closer inspection, the dimmer switch has a minimum load of 60W. Hence my problem.

 

I can't really add a dummy load (There's nowhere else I could justify another lamp, apart from the fact I don't fancy paying £4.50 P&P to order another one ) So I was wondering if there is anyother way of 'using up' my last 20W, with some sort of dummy loading circuit that I can chuck somewhere out of the way - like inside the dimmer knockout box.

 

Any suggestions??

[Dave]

Rob,

 

Firstly, do check that the LV transformer is OK with a 40W load - they normally have a minimum load rating just like dimmers. Also is the dimmer suitable for connection to a transformer? Not all are.

 

A quick bit of maths says that if you put a 2k7 resistor across 240V it will dissipate 21.3W.

 

So you could try that, but you will need a substantial resistor that is rated for both the voltage and the power. It will get hot!!! I would advise using something that is significantly over-rated so the temperature rise is kept to a minimum. Note that many power resistors can only achieve their quoted rating when mounted on a big heatsink.

 

I have never done this before and I'm not sure that it is a good idea. I would rather get another lamp or a different dimmer than risk a potential fire.

 

A possible complication I can think of is that lighting loads are non-linear and will have a lower resistance when they are not fully on (as the filiament isn't as hot). Hence a 2k7 resistor might not take enough current to keep the dimmer happy at lower settings. The only way to find this out is by experimentation.

 

Finally, note that the transformer won't be 100% efficient, so it will actually take slightly more power from the dimmer than required by the lights connected to it. You could, of course, put the resistor on the secondary side, instead of the primary, and I'm sure you're capable of calculating the appropriate value.

 

HTH,

 

Dave.

 

P.S. all advice given without liability!!

[gareth]

I've moved this topic from General Chat to Lighting, as it seems to pose questions that are electrics-specific ... G.

[robloxley]

Why not change the 20W lamps for 35W? After all, you've got a dimmer...