Dougal
Well-Known Member
Don't see any screws on mine. How on earth do they come off for bulb replacement or repair etc?
How Do These Come Off?!
- Thread starter Dougal
- Start date
pvb
Well-Known Member
Twist and pull - they have a bayonet-type construction.
johnalison
Well-Known Member
Also, you can post photos here by clicking on the Img box in Photobucket which copies it and then pasting it into your post. Just be sure to make sure it is not oversize or you will get complaints.
Dougal
Well-Known Member
Thanks both
pvb
Well-Known Member
Here's a pic from Hella's catalogue - the yellow arrows indicate the bayonet action.
Dougal
Well-Known Member
Here's a pic from Hella's catalogue - the yellow arrows indicate the bayonet action.
Brilliant thanks. Got some strange sh** going on with my mast lighting... There's a three core cable going up - a common ground and two supplies. At the mast base, my ohm meter shows continuity between ANY two of the three! Can't be right!
VicS
Well-Known Member
That'll be right.
Common negative. and two positives, one for each bulb
You will see continuity between all three if both bulbs are good.
but the resistance between the two positives will be equal to the sum of the resistances between the negative and each positive. ( double if both bulbs are the same power)
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Dougal
Well-Known Member
Ohhhh my head hurts.... Why would I get continuity between the two positives?
neil1967
Well-Known Member
Because they are connected together via the two bulb filaments.
VicS
Well-Known Member
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Dougal
Well-Known Member
I think i'm getting the picture, but electrics was never really 'my thing'.
Another question then please guys... With all wires DISCONNECTED from supply and ground, how do I determine which of the three mast wires is the common earth? I'm guessing the lamp fittings will be designed so that it matters?
I really appreciate your time and patience;-)
Another question then please guys... With all wires DISCONNECTED from supply and ground, how do I determine which of the three mast wires is the common earth? I'm guessing the lamp fittings will be designed so that it matters?
I really appreciate your time and patience;-)
VicS
Well-Known Member
With bulbs in place
Measre the resistances between all the wires
3 readings ... got it.......
one reading will be higher than the other two
The high reading is between the two positives .... so the other wire is the common negative
Dougal
Well-Known Member
Good god Vic.... Do you ever sleep or do anything else? ;-) Seriously, thank you! I thought that's what I'd understood from your earlier post, but my relationship with 12v electrics is about the same as that with horses, so it's always as well to TREBLE check;-)
William_H
Well-Known Member
Filament bulbs have a peculiar resitance when cold. Always a lot lower than when hot. So a 24 watt bulb draws 2 amps on 12v so 6 ohms.(hot resistance) However when you measure it is may measure something more like 3 ohms. (cold resistance) Hopefully you can on a 200 ohm scale on Digital multimeter read the low ressitance and hence discern the +ve wires for the lamps. good luck olewill
lw395
Well-Known Member
The cold resistance of the bulbs may be very low.
I have a 12V 20W halogen here, it's showing <1ohm.
So can be hard to detect with a meter, allowing for long wires and a bit of contact dirt.
It's normal to use mains 3 core cable. Most people use the green/yellow earth as 'common', but some might use black or blue.
If you pick two wires and put 12V on them, and both bulbs light, a bit less bright than normal, the other wire is the common.
Assuming filament bulbs not LED....
I have a 12V 20W halogen here, it's showing <1ohm.
So can be hard to detect with a meter, allowing for long wires and a bit of contact dirt.
It's normal to use mains 3 core cable. Most people use the green/yellow earth as 'common', but some might use black or blue.
If you pick two wires and put 12V on them, and both bulbs light, a bit less bright than normal, the other wire is the common.
Assuming filament bulbs not LED....
