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, which you can use as a trigger for your HID relay. 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.
The WolstenTech Time-Delay Relay offers you an easier, cleaner solution to this problem, with the following features designed to make your HID retrofit on an H4 or 9003-equipped car easier:
This relay is rated for a maximum current of 20A peak, and 10A continuous. A typical HID ballast uses around 6A at start-up, and settles to around 3A after a few seconds, so two ballasts are well within the power limits of this relay.
Should you order one or two relays? That's something every customer will have to decide for him or herself. These relays can handle the power consumption of two 35W HID ballasts, so if you wish to use one relay for your whole headlight system, it will work just fine. Of course, if you have a motorcycle with only one ballast, only one relay is needed.
However, if you have two ballasts and desire maximum redundancy, you can use two relays, with two separate fuses, so that if one ballast or relay malfunctions, you still have one headlight functional. It is also possible to use one relay with two fuses, between the relay and the ballasts, to protect against ballast failure, though you will still lose both headlights if the relay fails (extremely unlikely, but it's important to be aware of all possibilities when planning a system for maximum reliability).
Please also see the Installation Instructions page for sample wiring diagrams showing how this relay can be installed in a typical car.
Example 1: Single TDR
Example 2: Dual TDRs
Example 3: Single TDR-P
Example 4: Dual TDR-Ps