Any Electricians, please?

Mike Healey

I have a 12v halogen ceiling fixture with 4 MR16 20v lamps. It has gone kaput.

With my trusty electiricians screwdriver I checked the various terminals and found that both terminals on the 12v side of the transformer are live. Is this normal? If not, does it mean that the transformer is shot?

Advice would be deeply appreciated

ansbaradigeidfran

I'm not an electrician per se, but I know a bit about electronic-y bits. Whilst I haven't mucked about with halogens, I think the transformer or wiring is shot.

If both terminals are at the same voltage (12V), then there will be no current flowing between the terminals, and the bulb won't light.

nwallace

Assuming it is the sort of Electrical screw driver where you earth the neon bulb, its possible that both leads out the transformer should light the neon but the Potential difference is still 12V. Can't see how though transformers are just big coils. Could be really bad and have 240V out both suggesting the coil has collapsed. 12V on the -ve woudl suggest a wires got loose and is touching the -ve output. Could be fixable. A multi meter woud lgive a better idea

STEFANOS4784

nwallace wrote:

Assuming it is the sort of Electrical screw driver where you earth the neon bulb, its possible that both leads out the transformer should light the neon but the Potential difference is still 12V. Can't see how though transformers are just big coils. Could be really bad and have 240V out both suggesting the coil has collapsed. 12V on the -ve woudl suggest a wires got loose and is touching the -ve output. Could be fixable. A multi meter woud lgive a better idea

Exactly what i was about to say

ansbaradigeidfran

nwallace wrote:

Assuming it is the sort of Electrical screw driver where you earth the neon bulb, its possible that both leads out the transformer should light the neon but the Potential difference is still 12V. Can't see how though transformers are just big coils. Could be really bad and have 240V out both suggesting the coil has collapsed. 12V on the -ve woudl suggest a wires got loose and is touching the -ve output. Could be fixable. A multi meter woud lgive a better idea

Ah, yes. I'd forgotten all about those electrical screwdrivers (despite owning one) and was assuming the OP had a multimeter. Sorry. :oops:

feel

Speaking from a position of total ignorance (and probably total boll*cks) i would suspect that since a transformer is an AC device that both terminals could show live. However, it depends upon whether or not it is an alternating 12v supply rather than the input 230ish alternating supply .You should be able to establish that with a very cheap (£6 or £7 from wilkinson's) multimeter. Be very careful though when fiddling about with the supply still connected. A proper sparky would do the testing with the supply isolated.

edit : on the other hand the transformer could have a rectifier built into it as well so that the bulbs are actually running on DC. I told you i was totally ignorant. A cycling website probably isn't the best place to ask

Mike Healey

Many thanks. I've got a multimeter, but haven't used it for so long (many years) I've forgotten how to use it. Ah well, RTM

oldwelshman

On the dvm, turn the dia to dc volts, then measure across the output of the transformer.
If you get 12v then the transformer is ok, if you get 0 its fooked
These lights are really crap. I have seen transformers go opencircuit and also seen the cables get extremely hot and burnt and damaged at the transformer terminal and also at the light fitting end.
You metioned you had a light on the output so sounds if the transformer is ok and probably a cable fault from there outwards.
I replaced some of these in my parents house for these reasons and just replaced my bathroom ones also. The lamps don't last long either.

Al_38

Disclaimer: If you aren't confident then be don't play with the mains. it isn't fun when you give yourself a big shock. Also where possible do not use both hands at once when working on mains (or anything with big voltage/ current), and always have some form of circuit protection in place.

The bulbs will work on DC, so somewhere there is a rectifier and converter circuit that will change the ac to dc.
If you use the multimeter to check the voltage of one rail compared to the other (should have two leads want one in the COM port and the other in one marked for V or V(dc) shouldn't need a huge voltage rating) Turn the dial to DC Volts (might be indicated with a dashed line above a solid line next to a V) in an appropriate range (slightly above what you think the voltages to be measured are). If you measure 0V across the rails when it is on then something is either a) shorting the rails together or b) both rails are somehow disconnected from everything.

If a) I would guess that every time you try to use the lights it trips the circuit breaker it is connected to. With the power off you can test this by switching the multimeter to ohms and testing the value between the rails if it is low then this is the case.
Visually inspect the rails (with power OFF to see if there is anything metallic touching both. otherwise you will need to get into the wiring for the lights and find the rectifier and converter circuits to find where the short is. By this point you would almost certainly be better having someone who knows what they are doing

if b) edit - if you said they are both at 12V this isn't possible

also incase I am massively up the wrong tree and somehow the lights take AC and convert it to DC in the casing, check to see if there is an AC voltage (will be a V with a sine wave next to it - squiggle sort of like this ~) across the rails. If there is then I would guess on the fault being with the bulb units somehow.

oldwelshman

As I said, check the terminal cables for heat damage/ Isolate the power to transforemer and remove the cables from the tramsformer to light and it may be very brittle due to heat damage, I have seen this before. Especially as you said you have voltage at transformer and all lights fail. These lights are connected daisy chained so if the cable is damaged at transformer they all fail.
Transforemers and light fittings generally go open not short circuit.

DavidBelcher

feel wrote:

Speaking from a position of total ignorance (and probably total boll*cks) i would suspect that since a transformer is an AC device that both terminals could show live.

Precisely what I was thinking - unless it's got a built in rectifier to deliver DC at the low-voltage end then yup, either terminal could give a p.d. of 12V.

David

feel

oldwelshman wrote:

. The lamps don't last long either.

If you touch these bulbs with your fingers they will fail often within 3 or 4 weeks but if you handle them with a cloth they will actually last quite a decent time (not quite sure why this is mind, but i know it's true as i carried out a rather expensive experiment by handling 3 out of the 6 in our ceiling and all 3 handled ones blew very quickly)

jairaj

I'm not an expert but I've seen halogen systems that run on AC and DC. So your system could well be running on AC which would explain both terminals showing 12v.

Halogen bulbs run at a higher temperature than normal bulbs, so the glass is different, I think its quartz based which is easily contaminated by sweat on your finger tips when handling the bulb. This can create microscopic holes that let air in which replaces the halogen gas and burns the element out quickly and is a likely cause for the quick failure.

I would suggest buying a replacement bulb and taking care not to touch it directly with hands when installing.

Mike Healey

Many thanks to all.

Will get out the multimeter and follow instructions. It was a cheap unit, (the light fitting, that is) so I don't hold out much hope, but, who knows.

peanut

Mike Healey wrote:

I have a 12v halogen ceiling fixture with 4 MR16 20v lamps. It has gone kaput.

With my trusty electiricians screwdriver I checked the various terminals and found that both terminals on the 12v side of the transformer are live. Is this normal? If not, does it mean that the transformer is shot?

Advice would be deeply appreciated

I take it you are testing the 12v output with a DC tester not an AC tester ? :roll:
oops missed the previous post sorry.
Are you certain that the output isn't 240v ??
The bulbs will have little terminals on the ends of the pins and will be bayonet if they are
http://www.thelightingsuperstore.co.uk/category.asp?catcode=147

sithebike

Quite often the lamps will fail due to them overheating.
Go into any electrical wholesaler: CEF, Senate, Edmundsons eyc and ask for Aluminium Reflector 12v Dichroic lamps.
The Aluminium reflector will reflect the heat away from the enclosed light fitting. The lamps and transformers will fail due to overheating
They should not cost you much more than £2.50ish, depending where you go off course.

oldwelshman

Peanut, if they were 240v there would be no need for the transformer
With respect to the posts about failing lamps, I know you must not touch them, I have a rubber plunger for fitting my lamps and never touch them and always clean them with alcahol before fitting (no not beer). The fact remains they often fail, the gu10 lamps are even worse.
I think I will go back to the old fashined fluorescant fittings
Anyway eventually only energy efficient lamps will be available, though they look bloody ugly most of them.

oldwelshman

By the way, as I mentioned I have seen cables burnt on these and this is due to the currently they take and not the heat from the lamps themselves.

Mike Healey

peanut wrote:

Mike Healey wrote:

I have a 12v halogen ceiling fixture with 4 MR16 20v lamps. It has gone kaput.

With my trusty electiricians screwdriver I checked the various terminals and found that both terminals on the 12v side of the transformer are live. Is this normal? If not, does it mean that the transformer is shot?

Advice would be deeply appreciated

I take it you are testing the 12v output with a DC tester not an AC tester ? :roll:
oops missed the previous post sorry.
Are you certain that the output isn't 240v ??
The bulbs will have little terminals on the ends of the pins and will be bayonet if they are
http://www.thelightingsuperstore.co.uk/category.asp?catcode=147

Definitely 12v - I hope so, or the 12v/20w lamps that came with it would have not been a good idea.

Anyway, thank you again to one and all. When I eventually find the multimeter (garage/office/bedroom - my place or garage/ bedroom/livingroom - her place), I will carry out your suggested courses of actions and probably decide to replace it. If the fault is in the transformer/recitfier/whatever, it's sealed and the whole unit was so cheap that it won't be worth trying to get replacement parts.

I think that I've enough info to sort it out one way or another, so please regard the thread as closed so you can go back to the more usual helmets/lanes/drivers' idiocies/RLJing/stupid articles or broadcasts threads with the gratitude (undying, hopefully) of a "make sure the electical supply is switched off" DIYer