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Fuse in 12v halogen lamp
- Thread starter martik777
- Start date
- Joined
- Feb 7, 2013
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- 2,485
A fuse is a device that opens up in the case of a shorted circuit. It will work as usual, however the protection that was
provided by the fuse is gone. I find it interesting that the fuse failed with no apparent short. The safe way would be
to replace the fuse of course. What amperage fuse was in the lamp? A 2 ampere fuse would be on the ragged edge
of opening up if a voltage spike came along so would surmise the fuse would need to be heavier, like maybe 3 amperes or so.
provided by the fuse is gone. I find it interesting that the fuse failed with no apparent short. The safe way would be
to replace the fuse of course. What amperage fuse was in the lamp? A 2 ampere fuse would be on the ragged edge
of opening up if a voltage spike came along so would surmise the fuse would need to be heavier, like maybe 3 amperes or so.
- Joined
- Apr 30, 2015
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- 11,250
There's really no good reason to fuse an incandescent light. There's a very remote chance the fixture or bulb could short and possibly cause a fire but on a 12 volt circuit I can't see it happening very often
There's more fire hazard just from the heat of the bulb
-M
There's more fire hazard just from the heat of the bulb
-M
- Joined
- Jun 12, 2014
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- 4,806
Not something you commonly see, sometimes they have a thermal disconnect switch. In addition to shorts, the purpose of the fuse may be to limit the wattage of the bulb you use to prevent overheating in a fixture or wiring. In some of my tube tester's they use bulbs for fuses, there is a marked difference in draw between NOS bulbs and newer Chinese bulbs.
- Joined
- Feb 1, 2015
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- 9,574
mksj may have hit the nail on the head.
Halogen lamps are notorious for the heat generated. If the fixture was designed properly, it should be able to safely dissipate the heat but replace the OEM bulb with a higher wattage bulb and all bets are off. Adding a fuse that would fail with a higher wattage bulb would be a good safeguard against those of us who would ignore the printed warning to not use a higher wattage bulb.
At least it would keep the lawyers happy.
Halogen lamps are notorious for the heat generated. If the fixture was designed properly, it should be able to safely dissipate the heat but replace the OEM bulb with a higher wattage bulb and all bets are off. Adding a fuse that would fail with a higher wattage bulb would be a good safeguard against those of us who would ignore the printed warning to not use a higher wattage bulb.
At least it would keep the lawyers happy.
- Joined
- Feb 1, 2015
- Messages
- 9,574
A fuse which is operating close to its rating may fail of its own accord due to aging or the fuse may simply have been defective from the start.
LED lamps which are intended to operate from 12 volts d.c may have a problem with a 12 volt halogen power supply. Low voltage halogen power supplies are usuaqlly just a transformer providing a.c. voltage. First, the LED current draw will be significantly less and the power supply normally compensates for higher current by providing a slightly higher no load voltage. Add to this that the peak voltage will be 1.414 times the rms voltage so the voltage that the LED experiences could be in the mid twenty voltage range. This voltage may exceed the maximum reverse voltage rating which may destroy the LED's as they typically have a maximum 5 volt reverse voltage rating per LED.
Secondly, the operating range of a white LED is between 3.0 and 3.5 volts. 12 volt LED's usually have three LED's in series for 1`0.5 volt plus a current limiting resistor to drop the voltage for 12 to 10.5, more or less. If the rms voltage of the transformer is 15 volts, the LED current would be about three times what it would be at 12 volts. The LED's will burn brighter but with a significantly reduced life.
Lastly, since LED's have a fairly rapid turn on and off, you may experience a faint 60 hz flicker.
LED lamps which are intended to operate from 12 volts d.c may have a problem with a 12 volt halogen power supply. Low voltage halogen power supplies are usuaqlly just a transformer providing a.c. voltage. First, the LED current draw will be significantly less and the power supply normally compensates for higher current by providing a slightly higher no load voltage. Add to this that the peak voltage will be 1.414 times the rms voltage so the voltage that the LED experiences could be in the mid twenty voltage range. This voltage may exceed the maximum reverse voltage rating which may destroy the LED's as they typically have a maximum 5 volt reverse voltage rating per LED.
Secondly, the operating range of a white LED is between 3.0 and 3.5 volts. 12 volt LED's usually have three LED's in series for 1`0.5 volt plus a current limiting resistor to drop the voltage for 12 to 10.5, more or less. If the rms voltage of the transformer is 15 volts, the LED current would be about three times what it would be at 12 volts. The LED's will burn brighter but with a significantly reduced life.
Lastly, since LED's have a fairly rapid turn on and off, you may experience a faint 60 hz flicker.
Thanks for the detailed explanation RJ. I have quite a few of the 1W bead led's and appropriate drivers that convert 120v to the constant current supply needed for 1-3 leds. I think, I'll just replace the 12v transformer with the driver and fabricate a heat sink/led holder where the old halogen bulb was located. I've made a few of these in the shop and they have lasted for 10+ years so far.