Does my bathroom mirror-light work?

Hi,
I have a very basic understanding of voltage/current/resistance but have always wanted to know more. I did a year's physics about 10 years
ago and never anything since. I can work out how much it costs to power my TV for an hour though (which I was quite proud of)!
Anyway, now I've moved into a new flat and the bathroom mirror light doesn't work so I thought I'd do some digging. Bought a bulb and put it in, no joy. Bought a second as I though I may have broken the filament on the first but no joy either. So I borrowed an ammeter. Tested the resistance across the long filament bulb and it's about what you get from a bulb I know works (albeit an ordinary bulb). So I'm guessing the bulb is Ok.
Then I removed the little panel on the back of the bathroom mirror light and I can see where it's connected to the house supply. I can test the voltage there by touching each of the contacts with the two wires on the meter and I get 237V (in Ireland that's good). Then I put the bulb between the two metal contact points (carefully as I don't know if the light is on or off as there is only a pull-cord switch) and test the two ends of the bulb but I get nothing.
My question: Should I remove the bulb and put the ammeter in it's place testing for AMPS in A/C mode? Will this blow my Ammeter if there is a current? It has 2AMP and 10AMP fused connectors, which should I use?
My goal: I want to know if the light is broken. I want know which is the on/off position. Am I right in saying that if there's a resistance a cross the bulb the filament must be Ok?
Thanks so much in advance for any and all help,
Kind regards, Andoni.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@ie.fid-intl.com wrote:

Sounds reasonable...

No, you've already established that you've got power *to* the fitting, but not at the bulb contacts. Therefore there's definitely a fault within the light fitting. I should think it's the switch most likely; either that or a connection will have come adrift inside somewhere.

It is!

Probably knackered anyway; but the fitting would need to be disconnected from the mains - using all appropriate safety precautions - to investigate further.

I don't know if you could get a resistance under in faulty bulb under very unusual circumstances? I'd say the bulb's fine (and clearly the fitting isn't).
David
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Don't do this. There will be a large flash, a small explosion and various components will be racing to see which is the first to blow, and it may well not be the fuse, unfortunately.
If you're not getting 230V at the bulb contacts, even with the switch tried in both positions, you have either:
1. Broken fuse in there somewhere. 2. Dodgy wiring within the unit. 3. Broken switch.
These can (and should) all be tested with the whole lot entirely disconnected from the mains using a resistive scale on the mter.
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Neither. You musn't put an ammeter across a (potential) voltage source. Ther will be nothing to limit the current and you will, at best, blow the fuse in the meter.

If the resistance across the bulb is of the order of a few hundred ohms then the bulb is probably ok. If you have measured between the buld contacts in the lamp and got no volts in both switch positions the switch is probably faulty. Those small pill switches are not very reliable.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Neither. You musn't put an ammeter across a (potential) voltage source. Ther will be nothing to limit the current and you will, at best, blow the fuse in the meter.

If the resistance across the bulb is of the order of a few hundred ohms then the bulb is probably ok. If you have measured between the buld contacts in the lamp and got no volts in both switch positions the switch is probably faulty. Those small pill switches are not very reliable.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In alt.engineering.electrical snipped-for-privacy@ie.fid-intl.com wrote:
| I have a very basic understanding of voltage/current/resistance but | have always wanted to know more. I did a year's physics about 10 years | ago and never anything since. I can work out how much it costs to power | my TV for an hour though (which I was quite proud of)! | | Anyway, now I've moved into a new flat and the bathroom mirror light | doesn't work so I thought I'd do some digging. Bought a bulb and put it | in, no joy. Bought a second as I though I may have broken the filament | on the first but no joy either. So I borrowed an ammeter. Tested the | resistance across the long filament bulb and it's about what you get | from a bulb I know works (albeit an ordinary bulb). So I'm guessing the | bulb is Ok. | | Then I removed the little panel on the back of the bathroom mirror | light and I can see where it's connected to the house supply. I can | test the voltage there by touching each of the contacts with the two | wires on the meter and I get 237V (in Ireland that's good). Then I put | the bulb between the two metal contact points (carefully as I don't | know if the light is on or off as there is only a pull-cord switch) and | test the two ends of the bulb but I get nothing.
Is it possible for you to test the voltage WHILE you have the bulb in place?
I suspect the ground/neutral wire is broken on this circuit. What you are effectively measuring is residual voltage from the circuit being charged. I bet your meter is a digital type, which tends to have this issue. Testing with the bulb in place would then show 0 volts between the wires attached to the bulb.
| My question: | Should I remove the bulb and put the ammeter in it's place testing for | AMPS in A/C mode? Will this blow my Ammeter if there is a current? It | has 2AMP and 10AMP fused connectors, which should I use?
No. This won't tell you anything you don't already know.
| My goal: | I want to know if the light is broken.
The circuits is broken. Call an electrician.
| I want know which is the on/off position. | Am I right in saying that if there's a resistance a cross the bulb the | filament must be Ok?
You should see less than 10000 ohms across that bulb, but not zero.
Can you test the bulb in another location?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

From my experience with it is essential that the cover is in place as they have additional contact strips in the cover. I took one completely apart before discovering my error when replacing the lamp. No cover no light :-)
Peter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If only I'd posted this last week!! You are dead right. I had disabled the power and was in the process of taking the light appart after having removed it from the wall when I realised that it was sealed! Annoyed I looked for a hook or something I could push with a screwdriver and found two little tabs deep in little holes that I could easily push but seemed to do nothing! Then it hit me!!! Put it all together again and it workes fine!!
Thanks to you and all the others for your help, Andoni.
PS: I did learn somethings I'll use for next time along the way. :-) A.
Peter Andrews wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

<snip>
Buy a new light
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Its fine to ask questions in a newsgroup but it is not fair to expect the group to teach you basic principles. Your questions indicate you do not understand enough about electricity to avoid being killed so the only advice I would give you is GET A QUALIFIED PERSON to fix the problem.
--
John G

Wot's Your Real Problem?
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.