Five Equals Four with a Diode OR: Something Didn't Add Up Here!

August 15, 2006

How this Gadget Freak Began

U-Haul rents trailers with 4 wires, called a 4Flat. My van has a combination tail light that needed a 'diode OR' circuit in order to properly run their lights. Would this be a good idea for another Gadget Freak article?

Five Equals Four with a Diode OR Original Gadget Freak Case File

Original Submitted Van Trailer Build Instructions

Trailer rental companies insist that tow vehicles provide operating lights – turn, tail and brake lights for every towed vehicle rented. Easy to do if the tow vehicle (our van) was wired that way. It was not. The standard connector on rental trailers is called a 4-Way Flat – one female and three male pins. The female is a ground return and the 3 male pins bring 12 VDC power to the tail lights and the right and left turn and stop lights. The combination turn and stop lights are where our troubles lie – our van, a 1996 Dodge Grand Caravan, has separate turn and stop lamp circuits. The solution was a diode OR circuit for each side’s lights, then either the van stop signal lit both trailer lamps or that side’s turn signal would flash only that side’s lamp. I used Schottky diodes for low voltage drop, rated 3 amps, which would handle 36 watts of trailer lights. Throw in a few quick splice connects for the rear tail light wires and the mate to the trailer’s 4-Way Flat and you’re set quicker than you can say “You haul!” Oh, yeah – be sure the vehicle and tow hitch can handle the trailer, cargo and tongue weight.

Bill of Materials

  • 2273-06203A Schottky Diode

  • 4534-1240Quick Splice connector, 22-18 gage

  • Champion Trailer # 4378 Car side Trunk Connector

The Gadget Freak Debate

Randy L Wrote: Did anyone such as the author test the circuit given in this latest design idea?

It is very simple (too simple), and it looks to me like only the turn signal function will work.
If the brake pedal is pushed, it will cause not only the trailer stop/turn lights to turn on, but also all four turn signals on the tow vehicle to turn on. This would be front/back and left/right.
If the brake pedal is pushed with the turn signal activate, the blinking turn signal will stop and again, all four tow vehicle lights will turn on solid, along with the trailer lights.
This would seem like a dangerous situation.
Adding two more diodes so that the brake light power would not backfeed into the two vehicle turn signals would improve the situation, but it still does not work properly since there will be no turn signal blinking on the trailer when the brakes are applied. I don't believe proper operation can be done with diodes alone.
Tony G: This Euro to US tail light problem has been coming up for years and the solution given is probably the first thing that comes to an engineers mind since it seems at first like an obvious solution. You will not find a commercial version


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