Disco 1 Headlight relay?

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Indeed you are correct Brian47 I is indeed voltage divided by resistance. and the filament of a H4
lightbulb cold is liike .03 or .04 ohms cold ballpark. and some 3.5- 4 ohms hot (operating temperature) At no point did I ever imply it would use anymore current as measured in amps than 3.7 @ 55 watts or 4.3 or so at 60 watts. What I said was that both bulbs blowing suggests a hard short somewhere. I shouldn't have to explain this to a man with your obvious experience But a
"Hard Short" is when resistance suddenly drops to near zero.
Obviously when resistance suddenly drops, current as expressed in amps suddenly increases to the maximum
availability supported by the fuse, wiring or ancillary wiring til something burns free and breaks the circuit. or
there is no more supply left or the 1500 amp wrench you are using as a fuse finally breaks free, the rifle bullet
was 250 amps, right?
I am pretty gobsmacked myself MadHatMan
 
ironie said:
Indeed you are correct Brian47 I is indeed voltage divided by resistance. and the filament of a H4
lightbulb cold is liike .03 or .04 ohms cold ballpark. and some 3.5- 4 ohms hot (operating temperature) At no point did I ever imply it would use anymore current as measured in amps than 3.7 @ 55 watts or 4.3 or so at 60 watts. What I said was that both bulbs blowing suggests a hard short somewhere. I shouldn't have to explain this to a man with your obvious experience But a
"Hard Short" is when resistance suddenly drops to near zero.
Obviously when resistance suddenly drops, current as expressed in amps suddenly increases to the maximum
availability supported by the fuse, wiring or ancillary wiring til something burns free and breaks the circuit. or
there is no more supply left or the 1500 amp wrench you are using as a fuse finally breaks free, the rifle bullet
was 250 amps, right?
I am pretty gobsmacked myself MadHatMan
Click to expand...
shirley. both bulbs blowing can only occur if they act like a fuse - ie too much current drawn through them, and that is unlikely as they are connected to battery+ and ground respectively? ie the secondary side (after the bulb) would have to draw a large current and as it is connected to ground already, that is unlikely. If someone has fitted a large current device between the earth side of the bulbs and ground, that would be rather unusual, would it not? A short on the battery side to ground would not cause the bulbs to blow.
 
Obviously 30 years of cleaning bad grounds (earths?) for those who actually speak english, I am just a colonial, and Ah can't talk proper. separating melted together wiring harness's and repairing, cleaning corrosion and greasing battery terminals hasn't prepared me for the intricacies of 20 year old Land Rover maintenance. Who's wires after years don't rub against each other, Who's insulation doesn't crack, who's top quality switches never melt, who's wiring harness's are wonders of technical engineering and never corrode .... sigh. woe is me
 
ironie said:
Indeed you are correct Brian47 I is indeed voltage divided by resistance. and the filament of a H4
lightbulb cold is liike .03 or .04 ohms cold ballpark. and some 3.5- 4 ohms hot (operating temperature) At no point did I ever imply it would use anymore current as measured in amps than 3.7 @ 55 watts or 4.3 or so at 60 watts. What I said was that both bulbs blowing suggests a hard short somewhere. I shouldn't have to explain this to a man with your obvious experience But a
"Hard Short" is when resistance suddenly drops to near zero.
Obviously when resistance suddenly drops, current as expressed in amps suddenly increases to the maximum
availability supported by the fuse, wiring or ancillary wiring til something burns free and breaks the circuit. or
there is no more supply left or the 1500 amp wrench you are using as a fuse finally breaks free, the rifle bullet
was 250 amps, right?
I am pretty gobsmacked myself MadHatMan
Click to expand...

There is no such thing as a "Hard short", a short or short circuit is a short circuit.

The nearest thing to your explanation of such a condition would be connecting the load directly across the supply and that we've decided would result in a 55w bulb drawing 3.7 amps.

If there might be another circuit trying to draw current through a bulb, then that would be a series circuit and the current drawn would then depend on the value of resistance of R1 + R2.

A short circuit before the load will cause the current to rise and the voltage to drop but such a condition will not cause the filament of a bulb to burn out. The only conditions which could cause a bulb to fail in a vehicle are vibration causing a mechanical failure or age which can cause the filament to fail, usually at switch on when there might be a very short duration current surge as the filament reaches its working temperature.
Forcing the current to rise beyond the safe limit on a bulb causing it to blow is impossible in a vehicle where the voltage is limited to the battery voltage of nominally 12 volts.
 
brian47 said:
There is no such thing as a "Hard short", a short or short circuit is a short circuit.

The nearest thing to your explanation of such a condition would be connecting the load directly across the supply and that we've decided would result in a 55w bulb drawing 3.7 amps.

If there might be another circuit trying to draw current through a bulb, then that would be a series circuit and the current drawn would then depend on the value of resistance of R1 + R2.

A short circuit before the load will cause the current to rise and the voltage to drop but such a condition will not cause the filament of a bulb to burn out. The only conditions which could cause a bulb to fail in a vehicle are vibration causing a mechanical failure or age which can cause the filament to fail, usually at switch on when there might be a very short duration current surge as the filament reaches its working temperature.
Forcing the current to rise beyond the safe limit on a bulb causing it to blow is impossible in a vehicle where the voltage is limited to the battery voltage of nominally 12 volts.
Click to expand...
nothing could take more current thru a bulb, than connecting one side of it to ground, Shirley?
 
the wonders of speaking English, if only I could just learn. After all the Black Country has given me so many fine examples. a hard short would be a positive feed touching earth anywhere in the system, a sharp bit of rusted out battery tray, headlightbox positive and negative wires getting chummy in the harness, the connector finally melting down completely and welding positive and negative together , what have you, minor short circuits would be parasitical drains that might just run ones battery down. you are right there is no offical term of hard short but most other mechanics and engineers know exactly what I am referring to.
Pedanticism is the last port of call in a senseless discussion.
 
Last edited:
MadHatMan getting personal is the last port of call in a senseless discussion.
Click to expand...
Indeed it is. So we have covered all the bases and can bring this ****epost to an end now?

BTW, being obtuse and pedantic is "getting personal" in my book, it is just the sneaky passive aggressive way.
 
I am just here to discuss the landy's. and talk with like minded engineers. If I wanted to fight, I would be on IRC.
Where I have wasted a vast chunk of my life I am never gonna get back.
 
got my papers in a foundry, built dumptrucks, with big ass 12' foot snowplows, subway cars, worked in various garages factories and machine shops all my life. Welding, fabrication, various facets of building trades, mostly plumbing and heating, but a fair bit of general contracting. even did a skint as a network engineer and sysadmin.
 
ironie said:
got my papers in a foundry, built dumptrucks, with big ass 12' foot snowplows, subway cars, worked in various garages factories and machine shops all my life. Welding, fabrication, various facets of building trades, mostly plumbing and heating, but a fair bit of general contracting. even did a skint as a network engineer and sysadmin.
Click to expand...
ah! so not a "proper" Engineer then ;)
 
ironie said:
I have shamed many a "Proper Engineer" and respected a few. I suppose it is all in how it is defined.
But at the end of the day, I make work what engineers design, sometimes with their help, sometimes in spite of it
Click to expand...
in the same way a nurse can shame a doctor....... still cant call themselves "Doctor" tho.

One of my niggles :eek:
 
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