Dual-Input H4/9007 HID Relay

Last Updated: 7/03/2009

Status: Superceded by new Time-Delay Relay


I am now selling a solid-state relay for use in HID retrofits, which is especially suited for H4, 9003, 9004, 9007, and similar applications, where the vehicle's stock headlights have only two bulbs (one per side), each with two filaments inside. This relay will allow you to connect your HID ballasts to your headlight system with minimal modifications, while getting full power from your battery to your ballasts. In addition, this relay has an adjustable delayed turn-off feature.

Why It's Needed
On any halogen-equipped car that is being converted to HID, it's considered best practice to use a relay to connect the HID ballasts to the car's battery (through a fuse of course), so that the factory fuses for the regular headlights aren't overloaded and blown because of the HID ballasts' high start-up power draw. On a car with four headlights (two per side), this is pretty easy: you can just use a standard automotive relay, available cheaply at any auto parts store, and connect it so that the low-beams are used to turn on the relay. This way, whenever you turn on your headlights, the HID ballasts will turn on, and power your HID low beams. When you switch your high beams on, your low beams (HID) will of course stay on, and your halogen high beams will also come on, or if you have bixenon projectors, your bixenon solenoids will activate, giving you HID high beams. This is because, in most 4-headlight cars, the low beams stay on when the high beams come on.

However, in 2-headlight cars (with one headlight on each side), things work differently: there is only one bulb per side, with two filaments inside, one each for low and high beams. Common numbers for these bulbs are H4, 9003, 9004, and 9007 . On cars with these lights, when the high beams are turned on, the low beam filaments are turned off. This means that connecting a relay to this system to power HID ballasts isn't so simple: there's no single wire that's powered when your headlights are on. If you connect your relay to your low beams, as on 4-headlight cars, your HIDs will turn off when you turn on your high beams! And since cars like this are usually retrofitted with bixenon projectors, and have no separate halogen high beams, this means they have no headlights at all in high-beam mode!

One simple solution to this, commonly found on the internet, is to connect two diodes to the relay's control pin, and connect each of these relays to the low and high beam wires. This works, but it's a bit messy, requires soldering, and just doesn't look very good. In addition, there's a small delay when switching between high and low beams, which will cause the relay to switch off very briefly, and this probably isn't very good for your ballasts.

HID Relay
HID Relays

The WolstenTech HID Relay
To make retrofitting HID lights into H4 and 9003-equipped cars easier, I've devised my own relay. It contains several important features.

  • Two input connections, for connecting to your low and high beam wires
  • Comes in two varieties, one for positive-switched headlight systems, and one for negative-switched systems
  • Has minimum turn-off delay of 1/2 second, so that ballasts never lose power during low/high switching
  • Has adjustable turn-off delay of up to 30 seconds, so that, if desired, headlights will stay on for some time after turning off
  • Is a solid-state transistor-based relay, with no moving parts, and higher reliability than standard relays

Specifications
This relay comes in a small plastic box with two mounting holes, and 5 standard spade connectors on the bottom. The electronics inside are encapsulated, or potted, in epoxy and are waterproof. There is a small adjustment knob on the back to adjust the turn-off delay, from 1/2 second to 30 seconds. The five connections are:

  • +12V from battery (an inline fuse is required)
  • Ground
  • Ballast - to be connected to ballast's Ground input
  • Beam 1 & 2 - to be connected to the two switched headlight wires

Variants: Positive and Negative Switched Models
This relay will come in two variants, one for positive-switched systems, and one for negative-switched systems. Most cars have positive-switched systems, where the positive voltage connected to the headlight bulbs is switched by the headlight switch, and the bulbs are connected to fixed grounds. However, some cars, such as some Toyotas, have negative-switched systems, where positive (battery) voltage is always connected to the bulbs, and the grounds are switched.

Please be sure to check your car and determine which system your car uses, and order the appropriate model relay.

4-headlight cars
This relay can also be used on 4-headlight cars if you wish, by simply leaving one of the Beam inputs disconnected and connecting the other to the low-beam wire. Why would you want to do this, since regular relays are generally much cheaper?

  • This relay is solid-state, and since it has no moving parts, is probably more reliable.
  • This relay has an adjustable turn-off delay, which some people may like to have.