Rating downlight transformers....

rocker · Feb 22, 2010

Here's a silly question. I've never put in a transformer-type light before, so bear with me please....

I=Watts/Volts, yea? But, when I am using a 35w halogen and a 240-12v transformer for each downlighter, do I work out the amperage as 35/12, or 35/240?? Or something else? 2.9 amps per light seems ridiculous, but is it? I've gotta put nine of the damn things in!

Well confused, any help would be appreciated.

Notsosmart · Feb 22, 2010

In a transformer to work out the current of the circuit you divide the power by the input voltage on the transformer: Power in = Power out (ignoring losses etc) & visa versa

Therefore 35W X 9 = 315W
315W/230 = 1.37A

rocker · Feb 22, 2010

Good man. Your screen name betrays you.

baldsparkies · Feb 22, 2010

Notsosmart is spot on.
Remember its the load at the primary windings that your working out not secondary. Ohms law dictates that the lower the voltage through a constant resistance the higher the current. The thing to remember is that you will need a larger csa cable on the 12v side than on the mains/supply side. I have often seen cases where inadequate cable sizing at 12v has caused the transformers to fail after short service life, also ventilation is another factor.

So as Kirchoves first rule states.
The resitivity of a conductor is proportional to its cross sectional area. And inversley proportional to its length provided the temperature remains ambient.

Why did I just say that ??? Now my brain hurts.

.

rocker · Feb 22, 2010

Yea that makes sense, of course we would have a higher current on the 12v part of the circuit as we're 'dividing by less'. These are plug and play jobbies and the cable CSA after the tranny doesn't seem that big, probably about 1-1.5 mil, but I guess at this stage we are only concerned about the single bulb's current, right? So at as near as dammit 3A, this bit of cable should be more than enough.

More complicated than it looks, isn't it.....

Guest123 · Feb 22, 2010

VD is the killer on 12V downlighters, pre-assembled ones are ok though. I never fit the infernal things myself, no matter how careful you are on installation either the lamps or trannies are always popping.

baldsparkies · Feb 22, 2010

Lenny said:
VD is the killer on 12V downlighters, pre-assembled ones are ok though. I never fit the infernal things myself, no matter how careful you are on installation either the lamps or trannies are always popping.

Ain't that the truth.
Have I just found another GU10 fan Lenny

Guest123 · Feb 22, 2010

GU10 trumps MR16 anyday......Just not worth the hassle. With the developemet of CFL's and LED, SELV downlighter days are limited I feel.

baldsparkies · Feb 22, 2010

Rocker, if you haven't bought the trannies yet, I would suggest the GU10 route. Lenny has pointed out some very valid reasons for this and I completely agree with him. Trannies are a pain and prone to failure.

Notsosmart · Feb 23, 2010

Have to agree with you both; (damn there I go again with obsequious adulation)

I'm waiting for LED's to become the norm so we don't have to pay the current ridiculous prices; perhaps we should start a downing street petition.

bugsy · Feb 23, 2010

In 3 years time you will be able to get LED dimmable wallpaper , No Kidding

Notsosmart · Feb 23, 2010

Harry Potter type newspapers? er I mean, ,,,I only went so the kids could see the film(s)... all six...I need help.

Guest123 · Feb 23, 2010

Notsosmart said:
(damn there I go again with obsequious adulation)

I didn't need to google that one.

rocker · Feb 23, 2010

I prefer 250v eg GU10s, to be honest have never fitted anything with a transformer on it as has always seemed to be more trouble than it's worth (are there ANY benefits at all?). Problem is, this customer is being pretty cheap, said that he will provide the downlighters as he can get them much cheaper than I quoted, and when I said I wouldn't install them if they weren't fire rated, he gave me a price and it was pretty much 'do it for that or I will find someone who will'. I told him if I put cheap ones in he may have problems in the future, and his response was 'they make all these things in the same factories and then just put expensive brand names on'.

So eff him, I'll put the cheapest fire rated ones I can find in, make sure he signs a watertight contract and then after 3 months (if he didn't buy the warranty, which he won't) it'll be 'yes mr ............. I will come round and fix them, for a nominal fee of course'.

I know I'm being a jerk, but it serves him right for being a miser (we're talking a difference of £20 to have top notch ones in but he wasn't interested, and by the look of his gaff he ain't short of a few quid).