TheBeastie Posted November 30, 2010 Share Posted November 30, 2010 My worklamp has stopped working and in trying to identify problem I find that if I put multimeter onto the integral supply wire for the bulb and also onto the metal surround (earth) it is all connected - the resistance is 0 ohms whereas I would have expected some resistance from the filament??? I assume this is why it isn't working but thought best to confirm this is not as it should be. Can only think something insulating must have corroded, rotted or fallen off but if anyone can confirm bulb is faulty it would be a big help! Thanks Quote Link to comment Share on other sites More sharing options...
crwoody Posted November 30, 2010 Share Posted November 30, 2010 Depends what kind of meter you're using, a 21 watt indicator bulb for example is only about 0.5 ohm, your meter may not have sufficient resolution to indicate this correctly, on the other hand of course, you could have a short in the lamp wiring. Try removing the bulb and check the resistance again, if it's still very low it suggests you have a short in the wiring, it's very unlikely that the bulb itself would have gone short circuit. Quote Link to comment Share on other sites More sharing options...
David Sparkes Posted November 30, 2010 Share Posted November 30, 2010 ... I find that if I put multimeter onto the integral supply wire for the bulb and also onto the metal surround (earth) it is all connected - the resistance is 0 ohms whereas I would have expected some resistance from the filament???... Your expectation is correct, are you sure you are getting a reading of 0 ohms? The resistance of a cold filament is going to be very low, I've just measured a 55W H1 bulb at 1 ohm, using a cheap digital multimeter (of therefore 'not to be relied upon' accuracy). If your reading of 0 ohms was correct the bulb would be short circuit, and therefore blow the fuse every time you switch the lamp on. Note that Ohms law is not a reliable guide when calculating the expected resistance, as the resistance when cold is very much less than the resistance when powered (and hot) which is the condition relating to the power and voltage we have values for (55W, 13 volts, say, which gives about 3 ohms). Quote Link to comment Share on other sites More sharing options...
TheBeastie Posted December 1, 2010 Author Share Posted December 1, 2010 Thanks. Will pit in a new bulb and see what it does. Hopefully nice cheap solution - always a first!!! Quote Link to comment Share on other sites More sharing options...
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