400w dimmer switch

[wicketless]

Plse help,
I'm looking to convert 2 bathroom lights into 8 x 50watt lv downlighters, (2 rows of 4 lights) being controlled by a 2g 2w dimmer. As I understand it, I need a 400w dimmer switch, so will it be ok to purchase item no 93514 (2g 2w dimmer 2X40-250w) from screwfix at £7.99, or would I need a 400w dimmer which starts at around £25.00.
Also...should a transformer be used for each downlighter i.e 8, or 1 for each set a four (i.e 2).

Chers in advance

 

[ban-all-sheds]

The lower the ratio of lights:transformers the better.

Each gang of a 250W dimmer will be OK with 200W of load, provided it's designed to work with transformers, and provided the transformers are OK to be dimmed. But why 2-way? Where else do you want to control the bathroom lights from?

Don't forget to clear away any insulation in the ceiling void, and fit fire hoods and use heatproof cable or sleeving.

Make sure that the light fittings are suitable for use in bathrooms.

Remember that the dimmer switch, assuming it's not a pull-cord type, must be in Zone 3 or beyond.

 

[wicketless]

Cheers for that quick response but,
would the dimmers cope with 250watts per bank? as I'm thinking of installing 10 instead of 8. Also there were 2 switch plates, as the bathroom was originally toilet&bathroom with separate switches, I was hoping to run both switch cables to a single 2gang switch (is this ok), which will be outside the bathroom. Finally, the lights are item 14643 from screwfix, which aren't double insulated, should I be looking at alternatives?
If so all suggestions welcome...

cheers,
wicketless

 

[breezer]

when you use low voltage lights its the transformer you realy have to worry about.

each lamp may be 50w on the secondary side of the transformer, but you will be "dimming" the primary side, that's why you can use a lower rate dimmer.

but do get either a set with dimable transformer (suitable for bathroom use) or a separate dimable transformer (s)

 

[ban-all-sheds]

Err - I hate to argue with the great Breezer, but unless you've invented a form of perpetual motion machine, any transformer that's putting out 50W on the secondary is going to be drawing at least as much (assuming 100% efficency) on the primary....

Which I always thought was the reason, along with the harder-work reactive load, for the ROT of downrating dimmers by 20-25% for ELV lighting.

 

[breezer]

i was just going to bed as well

its to do with the ratio of coil size (number of windings /size of wire in windings)

put it this way.

take an arc welder, i have to admit i do not know exactly what one uses, but would you agree its a low voltage (it has to be other wise when you "strike" it would strike fom several inches) high current (say 55 - 160 amps agreed?)

er...yes

just like this one

from here

now you can see in the "blurb" it can supply up to 160 amps yes?

yesss

well if it supplies 160a (it must because it does exactly what it says on the tin )

how come it only plugs into a 13A socket?

it is as i said to do with ratio of the transfromer and its windings.

(i do not wish to appear to be patronising, and my sincere apologies if it looks that way, but its nearly one in the morming and i am very tired (busy day Friday)

 

[ban-all-sheds]

Well - leaving aside the issue of welder output figures being even dodgier than "peak music power" ratings of Japanese stereos in the '70s, it is capable of providing whatever current it does, but at what output voltage?

It's not 240V. Probably has a strike voltage of around 50, which will drop in sustained use.

But at the end of the day it is at heart just a big transformer, and you cannot get more power out than you put in, or you'd be creating energy from nothing.

The absolute best you could achieve if there were no losses whatsoever in the transformer is the same out as you put in, so your 12V 50W bulb would represent a 240V 50W load at the transformer primary winding.

In practice you can't get 100% efficiency - you only have to feel a lighting transformer to realise that some of what you put in is being given off as heat before it gets to the bulb. I've no idea of the typical efficiency of lighting transformer, but say, for argument's sake, it's 90%, and ignoring the power factor that means that for a 50W bulb the load on the dimmer would be 55.6W.

 

[AdamW]

He's right y'know. A 50W light would be drawing about 4amps at 12V. At the 230V side of the transformer you would only be drawing about 0.2amps. It is always going to be 50W.

Transformers are the most efficient piece of electrical equipment in the domestic environment, you can usually assume they are 100% efficient unless there is going to be a heat problem (anyone who keeps a Sky box in a cupboard will know what I mean!). Typically they are around 95%. Thus if you are going to draw no more than 200W then a 250W dimmer will cover it easily.

 

[Big_Spark]

Breezer, I know that Ban and I don't always agree on certain things, but he and Adam are 100% correct on this.

Most welders that are of the type you pictured have an output voltage in the 12 to 36V region, when you weld you create a dead short, effectively, this causes a huge inrush current across the weld surface which vaporises the metals due to the massive increase in heat.

The reason the transformer does not simply expire is because it is a huge inductive load.

Example.

The output of a Welder is 160A, but its voltage on the output side is only 18V, to calculate the power on the secondary is simple.

160 x 18 = 2880W

Now if you draw 2880W from the 230V side the load drawn is only 12.5A, therefore within the 13A HRC capabilities of the BS1361 fuse in the plug.

 

[breezer]

i know that, try reading my post again, i was trying to make the point about ratio, and size of windings on a transformer, of course i know it plugs in to a 13 a socket

 

[Big_Spark]

Breezer, I have read your post, several times to ensure that I was reading right before I replied, and unfortunately there is only one way to take it if you do not know otherwise.

The implication from both your posts in here is that the power drawn on the 230V side of a transformer is LESS than the power drawn from the secondary side, which is NOT the case.

I accept you may not have meant this, but it needed explaining so that others without our knowledge did not read your posts, make the wrong conclusion and buy something that will not do what they need it to do.