A 12v lamp that became a 8.37k resistor. - UK Vintage Radio Repair and Restoration Discussion Forum

The Philpott · 12th Dec 2019, 1:03 pm

I should certainly get out more, but:

I removed a pair of 12v W5W push-in type automotive wedge lamps and replaced them, based on one of them having failed, and both of them being badly blackened/gettered.

Stuck a meter across both of them before binning, to find the failed lamp showing a stable resistance of 8.37kohm. Different type of meter, same result.
Freezer spray made no difference, however pointing a hairdryer at it gave a slight increase in resistance to 8.39kohm- which then returned to normal on cooling!

I did the maths for this and when wired to a 9v PP3, sure enough the lamp was passing 1 milliamp.

Needless to say i eventually broke it using excess voltage, but I assume the few milliwatts it was previously dissipating were emitted as heat. 'Fascinating Captain..'

Dave

broadgage · 13th Dec 2019, 1:45 am

Almost certainly, the evaporated filament material had formed a slightly conductive film between the lead in wires.
This only happens with lamps of very low voltage, a mains voltage lamp would either pass enough current to evaporate the conductive film, or would operate the fuse.

turretslug · 13th Dec 2019, 5:32 pm

Just occasionally, I've noticed something like this with "blown" low voltage bulbs- when I could be bothered to get the ohm-meter on them. I've also noticed a "many megohms" reading with a few blown mains-voltage ones- presumably a very thin film whose resistance was too high to heat up sufficiently to burn away.

The Philpott · 13th Dec 2019, 5:46 pm

..So fundamentally a metal oxide resistor had formed on the surface of the ceramic blob between the wires. I was surprised by how stable it was, i think the particles must have been very small indeed.
Dave

Leon Crampin · 13th Dec 2019, 6:02 pm

W5W lamps run too hot and usually end up with vaporised filament material on the inside of the bulb. Predictably, German lamps usually outlast Chinese in the this respect. An unfortunate consequence of this failure is overheating of the holder with resulting damage, as the heat is reflected inwards.

5W automotive lamps need a bigger envelope to be reliable - better vehicles use these for the tail lamps where there is sometimes no redundancy as is provided as the front by the headlamps. Auto makers are surprisingly ignorant as to the optimum running conditions for incandescent lamps. This explains the high failure rate of incandescent day running lamps, which are not continuously rated at 13.5V.

Leon.

G6Tanuki · 13th Dec 2019, 6:03 pm

Quote:

Originally Posted by The Philpott

..So fundamentally a metal oxide resistor had formed on the surface of the ceramic blob between the wires. I was surprised by how stable it was, i think the particles must have been very small indeed.
Dave

Yes, over time the metal atoms from the hot filament are gradually 'emitted' from the hot surface and condense on anything a bit cooler. The darkening/silvering of the inner surface of the glass envelope is the effect - that's the blackening you saw on your bulbs.

A form of it is also a cause of inter-electrode leakage in some audio-output valves like the EL/UL41 - the deposited partly-conductive film causing hum and/or excessive grid-voltage which can lead to thermal-runaway.

13th Dec 2019, 7:15 pm

In a lamp it would be more of a metal film resistor, not a lot of oxygen to make an oxide in there. Fine resistors those metal film ones.

12th Dec 2019, 1:03 pm	#1
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,103	A 12v lamp that became a 8.37k resistor. I should certainly get out more, but: I removed a pair of 12v W5W push-in type automotive wedge lamps and replaced them, based on one of them having failed, and both of them being badly blackened/gettered. Stuck a meter across both of them before binning, to find the failed lamp showing a stable resistance of 8.37kohm. Different type of meter, same result. Freezer spray made no difference, however pointing a hairdryer at it gave a slight increase in resistance to 8.39kohm- which then returned to normal on cooling! I did the maths for this and when wired to a 9v PP3, sure enough the lamp was passing 1 milliamp. Needless to say i eventually broke it using excess voltage, but I assume the few milliwatts it was previously dissipating were emitted as heat. 'Fascinating Captain..' Dave

13th Dec 2019, 1:45 am	#2
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,129	Re: A 12v lamp that became a 8.37k resistor. Almost certainly, the evaporated filament material had formed a slightly conductive film between the lead in wires. This only happens with lamps of very low voltage, a mains voltage lamp would either pass enough current to evaporate the conductive film, or would operate the fuse.

13th Dec 2019, 5:32 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,393	Re: A 12v lamp that became a 8.37k resistor. Just occasionally, I've noticed something like this with "blown" low voltage bulbs- when I could be bothered to get the ohm-meter on them. I've also noticed a "many megohms" reading with a few blown mains-voltage ones- presumably a very thin film whose resistance was too high to heat up sufficiently to burn away.

13th Dec 2019, 5:46 pm	#4
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,103	Re: A 12v lamp that became a 8.37k resistor. ..So fundamentally a metal oxide resistor had formed on the surface of the ceramic blob between the wires. I was surprised by how stable it was, i think the particles must have been very small indeed. Dave

13th Dec 2019, 6:02 pm	#5
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: A 12v lamp that became a 8.37k resistor. W5W lamps run too hot and usually end up with vaporised filament material on the inside of the bulb. Predictably, German lamps usually outlast Chinese in the this respect. An unfortunate consequence of this failure is overheating of the holder with resulting damage, as the heat is reflected inwards. 5W automotive lamps need a bigger envelope to be reliable - better vehicles use these for the tail lamps where there is sometimes no redundancy as is provided as the front by the headlamps. Auto makers are surprisingly ignorant as to the optimum running conditions for incandescent lamps. This explains the high failure rate of incandescent day running lamps, which are not continuously rated at 13.5V. Leon.

13th Dec 2019, 7:15 pm	#7
Guest Guest Posts: n/a	Re: A 12v lamp that became a 8.37k resistor. In a lamp it would be more of a metal film resistor, not a lot of oxygen to make an oxide in there. Fine resistors those metal film ones.