Why Do My Head Lights Go In Pairs ?

[over_the_hill]

Last week I got home in dusky conditions with the lights on and noticed one of the headlights had gone. No big problem as I always keep a few spare bulbs in stock and the next day I jacked her up, took wheel off for better access and changed the bulb. A few days later he other side packed up - presumably in sympathy.

I seem to recall the last time a bulb went (about 5 years ago) the same thing happened and the other side followed shortly afterwards.

Anyone else seen this or is there a reason why. I should add that I have standard lights using boggo H4's. (I am in the dark on this - very bad pun most definitely intended.)

[Barreti]

The reason I'm told they go in pairs is the sudden extra load put into the remaining bulb. That and the fact they are both probably the same age.

[taylormj4]

They are similar bulbs, they were installed at the same time and you always turn them on and off as a pair, so you would expect them to last about the same time really. Can't see why one blowing would cause the other to go. A blown bulb doesn't increase the load on the other as they are connected in parallel. Maybe you're thinking of lamps connected in series (such as christmas tree lamps) where a blown lamp increases the voltage across each of the others. At the point of blowing, the inductance in the failing lamp coil could cause a bit of a back emf but I don't think that would cause the other one to go and it would go straight away not some time later. Just very similar bulb manufacturing characteristics I would say.

[Ribol]

If you have two bulbs on one particular circuit (eg dip beam or main beam) and both are not lasting long this is generally caused by a poor connection somewhere on that circuit. The constant arcing/pulsing on that circuit takes the filaments out eventually.

If however one dip filament goes on one bulb and the main filament goes on the other one then this obviously doesn't apply and it is either bad luck or shid bulbs

[trashbat]

taylormj4 said:

They are similar bulbs, they were installed at the same time and you always turn them on and off as a pair, so you would expect them to last about the same time really. Can't see why one blowing would cause the other to go. A blown bulb doesn't increase the load on the other as they are connected in parallel. Maybe you're thinking of lamps connected in series (such as christmas tree lamps) where a blown lamp increases the voltage across each of the others. At the point of blowing, the inductance in the failing lamp coil could cause a bit of a back emf but I don't think that would cause the other one to go and it would go straight away not some time later. Just very similar bulb manufacturing characteristics I would say.

I've always thought it odd, but when you see cars with a bulb out it does appear that the other is brighter than normal. Obviously there's nothing to compare against, so is it just a trick of the mind or is there anything to it?

[Ribol]

trashbat said:

I've always thought it odd, but when you see cars with a bulb out it does appear that the other is brighter than normal. Obviously there's nothing to compare against, so is it just a trick of the mind or is there anything to it?

Unless the circuit isn't supplying enough current to do the job properly with two bulbs (very unlikely in a lighting circuit) then it isn't going to make the remaining light any brighter.