What resistor do I need?

sean1141

Guys can anyone help me out here? I plan to wire the front fog lights to come on with my high beams. I did this on my old ford but the light for the front fogs was displayed on the dash every time I flicked on my main beams. I want to fit a resistor this time to stop it happening. Can anyone tell me what resister I need or how to go about working out what one I need. Cheers

biko

To start with, what car is it?

sean1141

04 skoda fabia.

dougie-lampkin

You want a diode, not a resistor. The diode will act as a valve, stopping power from travelling back to the dash. The diode needs to have a suitably rated forward current, or it will short out when you turn on the fog lights from the switch. 10 A per side would be a good idea. Maplins don't seem to carry anything above 3 A for a simple axial diode, but something like this on eBay would be fine.

You need to cut the live going from the lamps back to the car, and insert the diode here with the band facing the lamps. This masterpiece will explain better (the two lamps are your fog lamps):



The "wire from the high beams" should ideally be through a relay switched by the high beams, with power drawn straight from the battery through a 15 A fuse. Without a relay you're drawing 20 A through a circuit only meant for 10 A. It's probably fine, as the fuse will protect the existing wiring, but it's less than ideal.

Chimaera

I'm wondering why you'd do something like this tbh. It's illegal for foglights to operate when there's no fog. You're also making it impossible to use your fogs in actual fog since they're now linked to high-beams which are going to make driving in fog impossible.

If you want extra light when driving on high-beams, buy a set of auxilliary driving lamps which are designed for this purpose.

sean1141

Chimaera wrote: »

I'm wondering why you'd do something like this tbh. It's illegal for foglights to operate when there's no fog. You're also making it impossible to use your fogs in actual fog since they're now linked to high-beams which are going to make driving in fog impossible.

If you want extra light when driving on high-beams, buy a set of auxilliary driving lamps which are designed for this purpose.

They can be used as aux lamps with the high beams. Nothing illegal about that. They will switch off when you go back to dims.

No they can still be switched on from the light switch with the side lights and dipped lights. The only difference I'm making is they will switch on with the headlights every time.

sean1141

dougie-lampkin wrote: »

You want a diode, not a resistor. The diode will act as a valve, stopping power from travelling back to the dash. The diode needs to have a suitably rated forward current, or it will short out when you turn on the fog lights from the switch. 10 A per side would be a good idea. Maplins don't seem to carry anything above 3 A for a simple axial diode, but something like this on eBay would be fine.

You need to cut the live going from the lamps back to the car, and insert the diode here with the band facing the lamps. This masterpiece will explain better (the two lamps are your fog lamps):



The "wire from the high beams" should ideally be through a relay switched by the high beams, with power drawn straight from the battery through a 15 A fuse. Without a relay you're drawing 20 A through a circuit only meant for 10 A. It's probably fine, as the fuse will protect the existing wiring, but it's less than ideal.

Cheers. Wasn't sure if it was a resistor I needed.
I plan on fitting the relay around the steering and tapping into the feed from the headlights at the switch or fuse box and the fire going from the switch to the relay for the fog lights.

On the fabia the switch for the parking, dipped and front and back foglights is on the dash and the switch to flick between dipped and headlights is at the steering wheel.

Would one diode work so?

Something like this with the live fussed and the diode between the foglight switch and where I tap into the wire?

dougie-lampkin

Chimaera wrote: »

I'm wondering why you'd do something like this tbh. It's illegal for foglights to operate when there's no fog. You're also making it impossible to use your fogs in actual fog since they're now linked to high-beams which are going to make driving in fog impossible.

If you want extra light when driving on high-beams, buy a set of auxilliary driving lamps which are designed for this purpose.

If they come on with the high beams and are extinguished with the high beams, they're treated as driving lamps, and perfectly legal. Since they're switched through a relay, the high beams won't be back fed by switching the fogs on with their original switch.

sean1141 wrote: »

Cheers. Wasn't sure if it was a resistor I needed.
I plan on fitting the relay around the steering and tapping into the feed from the headlights at the switch or fuse box and the fire going from the switch to the relay for the fog lights.

If you can get at the switched, low power side of things, that's better again. I was thinking it was a multiplexed car, which would make it pretty hard. A cheap Maplins 3 A diode would do fine in this case. I assume your orange "existing fog lamp" wire is going to the low side of the fog lamp relay? If so, that's ideal.

You could replace your relay there with another similar diode going from yellow to green if you wanted, to stop the fogs back feeding the high beams. It saves having to find an earth and a live for a relay. This is also assuming that your fog and high wires at this stage are both before an existing relay.

sean1141

dougie-lampkin wrote: »

If you can get at the switched, low power side of things, that's better again. I was thinking it was a multiplexed car, which would make it pretty hard. A cheap Maplins 3 A diode would do fine in this case. I assume your orange "existing fog lamp" wire is going to the low side of the fog lamp relay? If so, that's ideal.

I'm guessing the 'existing fog lamp wire' is only the sensor or trip wire for the original fog lamp relay. Its a pretty light wire coming out of the switch anyway..

dougie-lampkin wrote: »

You could replace your relay there with another similar diode going from yellow to green if you wanted, to stop the fogs back feeding the high beams. It saves having to find an earth and a live for a relay. This is also assuming that your fog and high wires at this stage are both before an existing relay.

Its pretty easy to get a battery live and an earth under the dash so I don't mind using the relay in this case.

I just want to avoid the fog light symbol lighting up on the dash when the high beams are on!

sean1141

From looking at a few owner sites they are saying there is no relay for the fog lights

dougie-lampkin

If there's no relay you'll have to add one, or use a beefier diode. You can use 2 x 10 A diodes in your diagram in place of the single one. Put them in parallel and the load is halved across them. Ideally in this case you'd add a fog relay, or else the headlight dip switch will be taking a lot more current than it was meant to.