I'm trying to get my head around the headlight wiring in my car, and I'm curious what purpose the two diodes in this diagram serve. Hoping one of the clever people on here can explain it for me:

enter image description here

Is there purpose evident from this diagram or are further related diagrams required?


  • \$\begingroup\$ Wow - thanks for editing my post and the very quick replies. Fantastic. I understand at a high level what the circuit is doing and that there's the two independent ways of activating the light relays - through the light switch or when flashing the high beams - but I guess what my real question is - specifically what are the diodes doing, to ask it this way - if you removed the diodes from the circuit and replaced with straight links what would go wrong? \$\endgroup\$ – madz Jul 16 '12 at 12:40
  • \$\begingroup\$ Appreciate the replies. What I'm struggling with when I look at this diagram is that if either switch is not switched to a position to activate high beam, it appears the terminal is open circuited - so would be isolated anyway wouldnt it (without the need for the diode which is now only connected on one end anyway)? And if the switch is closed then the connected diode will conduct as it is oriented almost the wrong way around to be isolating the switch? Or am I misunderstanding? I was wondering whether the diodes were perhaps more to do with controlling current flow when BOTH switches were on? \$\endgroup\$ – madz Jul 16 '12 at 21:58
  • \$\begingroup\$ Are they not just indicators? LEDs? \$\endgroup\$ – user97992 Jan 23 '16 at 3:44
  • \$\begingroup\$ @user97992 no, that car uses incandescent bulbs for indicators. See the combination meter at the bottom \$\endgroup\$ – Passerby Jan 23 '16 at 3:58

The diodes are there so that there can be two separate mechanisms for enabling the main headlight relay without interfering with each other. From the labels on the right, it looks like the two mechanisms are the normal headlight switch, and the momentary passing switch, which makes sense. You want each to be able to turn on the headlights independently.


You now say you are looking for more of a low level electrical explanation of what the diodes do instead of the high level conceptual explanation you originally asked for.

The diodes allow either switch to turn on the relay, but still remain independent. Presumably at least one of the switch lines is used elsewhere in a way that it shouldn't be signalled when the other switch is activated. Without the diodes, there would be only a single switch line, and that would be activated identically by either switch, making it impossible for other parts of the system to determine which user action caused the headlights to turn on.

For example, the general headlight switch may also turn on some running lights or maybe a indicator on the dash that the passing switch is not supposed to turn on. With the diodes in there, the general headlight switch line remains unasserted when the passing switch is activated. Each switch can activate the relay, but there is no connected between the switches.

  • \$\begingroup\$ Many thanks for adding more information Olin - reading your explanation I understand it and it makes sense. I'll add a further comment above with what I'm still struggling with as it's kind of common to all answers I've received - but basically the switches provide the isolation wouldn't they? \$\endgroup\$ – madz Jul 16 '12 at 21:50

The diodes on the circuit are working as a OR gate.

The relays are activated when at least one of the switches is on.


In Logic Theory... since the voltage V+ is direct connected to relay coil and switched on the "Low Side" or V- and the diodes are also switch by either source going to V- , we have a logic specification defined as " if either input goes lo,the output goes lo" By rules of Logic and conversion to positive logic, this actually an AND GATE function. ... Meaning that when both inputs are high the output is high in the OFF state.

If you may recall from logic theory, if you invert the inputs and the outputs, you transform between the AND vs OR gate.
The 3rd diode is a typical back EMF clamp diode on the coil. enter image description here So technically the diode OR logic with inverted logic active low inputs is an AND gate for turning off the light.

As Olin correctly points out the isolation of each driver is important for many reasons such as fault isolation. Wire OR or Open collector or Open Drain (Low Side Switches" would also work without diodes, but fails to permit ease of troubleshooting and sense with self-test or BITE (built in test equipment ) by design for reliability of isolating faults. The trend in Automotive Design for reliability is enhanced now with fault isolation on electro-mechanical parts and drivers, such that isolated driver states can be sensed easily for feedback.

  • \$\begingroup\$ I believe the switch to using diodes from before when no diodes were used with wired OR Low Side or Hi Side switches is because of fault isolation in Canbus controllers so that maintenance is easier for auto-techs. driver sensing is standard design feature that requires this isolation diode vs hard-wired OR Normally Open switches. the smart instrument panel of cars today can self test for major component failures. In the past it was just light bulbs. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 16 '12 at 17:04
  • \$\begingroup\$ Thank Tony this made for interesting reading - my car is 20 years old (Subaru) so although it may well be the case, in general it's more a bulb than CANBUS car :) \$\endgroup\$ – madz Jul 16 '12 at 22:00
  • \$\begingroup\$ Always a please to inspire engineers to ask intelligent questions... My wife is due for a Mercedes and has an old Passat 1993 with CANBUS and the instrument card is failing to communicate with the Mein Kontroller Kard under the hood and the Tach does not work nor the brake pad and ABS failure sensors. But it otherwise it works. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 16 '12 at 22:08

After pondering this some more, I have an alternative theory which might explain the Lighting Diode. As others have pointed out, the headlight relays will be energised when either the Lighting Switch is in position II (position I is parkers only) OR the dimmer/passing switch is pulled to momentary on (flash/pass). This is fairly straight forward and I haven't accepted any answers yet as I'm not convinced the diodes have any role in achieving this OR arrangement as they would effectively be disconnected from the circuit by the switches when not activated.

The dimmer/passing switch is a fairly substantial switch and you can see that despite the circuit having headlight relays, the full low or high beam bulb current still flows through this switch. The Lighting Switch however is just a small rotary switch on the end of the stalk and it only passes the relay coil current.

If both switches are turned on (so Lighting Switch to position II and you also flash the hi-beam), then without the lighting Diode I'm thinking it would be possible for the hi-beam bulb current to pass from HU to HF and to earth through the Lighting Switch. I guess this could happen if the contacts between HU and E got dirty or whatever and became higher resistance than the path through the lighting switch?

What do you guys think? Would this make any sense as an explanation? It doesn't explain the Diode R-fog but from the name I'm wondering if it serves a similar purpose to do with the fog lights that's not really indicated by this diagram?


the diodes are there to cut down on the amount of copper wire required to run everything independent - save on wire -save on weight -better fuel economy I think that is the simplest explanation.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.