Unidirectional diodes

[matlee]

How do i calculate which diode would be suitable?

Im currently building a kit car and at the stage of wiring in a fog light which needs to be operated only when the dipped beam or main beam is (must not work when only the sidelights are on). Ive got the circuit bit worked, which involves a couple of unidirectional diodes, out but unsure of which strength diode i need.

[FlossyThePig]

If the dipped beam stays on when full beam is selected why not wire the fog lamps in that circuit?

[mrmr96]

Aren't these things normally done with relays rather than diodes? (I know they do different things, that's the point.)

[matlee]

The dipped beam goes off when the main beam comes on.
Heres a very rough sketch of what im planning on doing. My electrical knowledge is minimal so open to suggestions.

Green - headlight feed (side,dipped,main)
Blue circle - rear fog light switch
Black triangles - unidirectional diodes

[marshalla]

Relays must be the way to do it, surely ?

I think it only needs one.

Take a feed from the headlamp switch and use it as the input to the fog lamp switch. Route the current from the fog lamp switch through the coil side of a relay which switches the main foglamp power feed.

Your headlamp switch should also be using a relay to switch the power feed to the headlamps, of course.

Edited by marshalla on Tuesday 29th January 13:55

[ch427]

is that a new reg?
im sure my rear foglight is independent of the other lighting circuits.

[matlee]

ch427 said:

is that a new reg?
im sure my rear foglight is independent of the other lighting circuits.

The wording of the IVA is...

'The rear fog lamp(s) must only illuminate when dipped beam, main beam or front fog lamps are lit (See Note 3)'

You can have the fog light independent of other lighting circuits, you can wire it however you want, but the fog light must not be allowed to operate when only the sidelights are on.

[ch427]

what switches are you using for the lighting?

[matlee]

Standard mx5 stalk for all headlight functions (+tail lights) and retro fitted a rocker switch for the rear fog light

[SMGB]

The 1N4000 series are dirt cheap and readily available but a bit light current at 1A. bigger diodes get expensive. However you could drive a relay from the diades and then the lamp circuit from that.
As noted above you could also just use 2 relays, the diodes will take up less room.

[ch427]

it should be doable with a relay as suggested but you would need to test the output from the switch to the dipped beam circuit ie does it stay on when you select main beam. Depends on how it was wired in the mx5

[matlee]

Im trying to avoid a relay as all the wiring is complete so it would mean having to uncover quite a bit to get to the required wires. The area where im hoping to fit the diodes is currently uncovered.

Ive ordered some diodes which are rated at 3a going on the logic that the fog light has a 21w bulb and the engine gives out 12v so draws 1.75a....someone correct me if im wrong. I used the formula of watts = amp x volts...again, someone correct me if im wrong.

[SMGB]

matlee said:

Im trying to avoid a relay as all the wiring is complete so it would mean having to uncover quite a bit to get to the required wires. The area where im hoping to fit the diodes is currently uncovered.

Ive ordered some diodes which are rated at 3a going on the logic that the fog light has a 21w bulb and the engine gives out 12v so draws 1.75a....someone correct me if im wrong. I used the formula of watts = amp x volts...again, someone correct me if im wrong.

Yep ,thats right. The diodes will dissipate some power, at 2A and a i volt drop = 2W so you may need to pay some attention to mounting/heat sinking. Unless they are Schottys. All you need will be on the data sheet but post the part number if you need some help.

[matlee]

Amazingly the diodes turned up this morning, top marks to that ebay seller!

All installed and foglight working a treat with required headlight beam. Thanks for all the input chaps.

[spikeyhead]

As a general rule, if you want a component to run regularly at 2A, buy something rated to at least 4A for good reliability.