Trailer Stop & Turn Converter Schematic 2

Trailer Stop & Turn Signal Converter

If you ever wondered why your trailer Stop/Turn signal lamps are so dim you are not alone. The answer is simply that the typical Stop & Turn Signal Converter/adapter drops a whopping 2.5V! This is VERY significant on a 12V electrical system. This DIY project provides the information you need build a simple, inexpensive converter that reduces this drop to a mere 0.5V @ 2A, and even has sufficient guts to power a 7.5A load. While this documents the most common 4 way (4 wire) system, it may be easily incorporated into more complex systems.

Why is a converter required?

Most automobiles manufactured today have separate Stop and Turn signals — the stop signals are Red, while the Turn signals are Amber. Unfortunately, most trailers do not include the luxury of amber Turn signals, but use the Stop signal also as a Turn signal. The converter takes both input signals and generates one output signal for the trailer. The logic for accomplishing this is simple, but tricky—check out the following truth table. Curiously, when the Stop and Turn signals are simultaneously applied, the trailer Turn signal is “inverted” so that when the vehicle Turn signal is On, the trailer Turn signal is Off, and vise versa.

Truth Table

Trailer Stop & Turn Converter Truth Table

Fortunately, it can simply be performed the following circuit:

Trailer Stop & Turn Converter Schematic 2

How it works

Both left and right sides of the circuit are identical. The 12V power from both the Stop and Turn signals are OR connected by high current schottky diodes (D1 & D2) and fed to the drain of a P-Channel MOSFET (Q1). To turn Q1 on requires that either the Stop or Turn signal to be off. Zener diodes D3 & D4 (applied as rectifiers) make a negative OR connection so that whichever lamp is not powered applies essentially zero volts to R2 that in subsequently applies a negative voltage to the gate of Q1 thus turning it on. When both Stop and Turn signals are simultaneously powered (high), there is no negative gate drive signal available and Q1 turns off via R1.

Overvoltage protection

Note that the rather unconventional application of zener diodes as low voltage rectifiers. The purpose of this is to provide transient voltage protection for the Schottky rectifiers.

D9 is a zener that protects the MOSFET gate from overvoltage transients — its low cost is essentially an insurance policy in protecting against potential MOSFET failure. Automotive electrical systems can be very “dirty” when it comes to generating transients — e.g. what happens when you have your foot on the brake when you start the engine? The starter current is not the problem, but the termination of starter current can generate high transient voltages due to the series inductance of the starter wiring.

Failure modes

While I believe that the potential for circuit failure is low, it is important to recognize the symptoms. Otherwise, even a very experienced mechanic may have difficulty making an accurate diagnosis. The symptom for D1 or D2 failure is “crosstalk” between the vehicle Stop and Turn signal lights. In other words, when the brake pedal is pressed, both Turn signal and Stop signals light simultaneously; or when the Turn signal is activated, both Turn and Stop signals light simultaneously. This will occur with or without the trailer connected.

MOSFET failure is indicated by absence of the trailer Turn signal when the brake is pressed.

Feasibility prototype

I did not build the complete assembly — that will come later — what I tested was a feasibility prototype that I “haywired” together — see photo. This provided all the information needed to demonstrate feasibility and performance. At 2A, the MOSFET ran cool — no heatsink required.

7.5A Stretch version

If applied at 7.5A, the MOSFET power dissipation (Rdson = 0.07Ω) calculates to 4W. At this power level, a small heat sink is required. I recommend a heat sink with a thermal resistance of about 10°C/W. If the assembly is potted, flat pieces of aluminum may be used to spread the heat evenly throughout the assembly. The diodes may also require some heat sinking as the power dissipation calculates to 3.3W — the cathodes of each pair of Schottky rectifiers could be soldered to a common sheet of copper.

On second thought, while 7.5A may be possible, I do not think I would push it beyond about 4 to 6A.

stop & turn haywire

Test data

Trailer Stop & Turn Converter Data

Bill of Materials

Trailer Stop & Turn Converter BOM

Link for useful information regarding trailer wiring and connectors etc.

Why is the existing technology so poor?

The standard circuit that is available from numerous vendors including U-Hall is similar in topology except that it uses standard silicon rectifiers and PNP Darlington transistors. The Darlington transistor accounts for most of the voltage drop because Darlingtons saturate poorly. All components are assembled into a potted assembly. Reliability is poor. I did not know that mine had failed until I rented a trailer from U-Hall and found that the lights did not function. As a result, I was forced to purchase a new U-Hall converter unit that they expertly installed for me. At $17, they did not overcharge for the item. On the negative side, the convenient LED indicators that were molded into the trailer connector failed within a few months.

Typical specifications for existing technology

Stop/Turn signal current is limited to 2.1A (one standard lamp)
Tail/Clearance lights is limited by the connector and/or wiring to 7.5A.

U-Hall Converter

For the future

Relay topology Stop and Turn signal converter (lowest voltage drop of all)


Join the conversation!

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  • Brian

    Most aftermarket LED light companies recommend using 50W 6Ω resistors when replacing an incandescent with an LED to simulate the load of the bulb. When a turn signal bulb is burned out or removed the “flasher” or in my case (and most newer vehicles today) the body controlled module controls the signals. It begins to flash rapidly to let the driver know that there is an issue with a bulb out.

    The average bulb is around 27W @ 12-13VDC which is around a 2A draw while the LED uses considerably less, triggering the rapid flash. In order to simulate the amp draw of a bulb most people will use a 6Ω resistor to prevent the rapid flash, 50W as it’s having to dissipate 24W+ of heat.

  • Brian

    So I have just about got this laid out for a PCB design. I am trying to use this setup as a solution to swapping out my tail lights that have separate stop & turn incandescent bulbs to one that uses a combined stop & turn LED. The problem this creates is that the flasher will hyper flash without the turn bulb in place.

    I can use a 6ohm 50 watt resistor to take the place of the turn bulb which works fine for preventing hyper flash, however when the brake is applied and the turn signal is flashing it goes back to hyper flash. I believe if I use a 6ohm 50 watt resistor on both the stop and turn signal it will stop, but I would like to use only 1 resistor if possible. Would the use of diodes allow this?

    • Jim Keith

      On 2nd thought, you may need 2 resistors. However, I think that 100Ω, 5W resistors will work. Why did you select 6Ω?

    • Jim Keith

      I think that two diodes driving one resistor has merit –use 3A or 6A diodes. Works OK in my cranial simulator.

  • al

    I have a converter hooked up to my cycle, it has three wires, one for left/right signal brake/turn and tail light. I’d like to separate things so that my brake is separate from the turn circuit. but I want to keep the converter mainly so that my turn signals do not increase when I have the trailer on. I am using LED lights for all. what do you suggest I do?

  • Brian Hampton

    Does this require a load to operate? I have built this according to the schematic and I get 0 volts to the light when +12v is applied to either and both D1 and D2. I also have +12v on both sides of D3/D4.

    Thanks for your help.

    • Brian Hampton

      Ah! Thanks for this. I didn’t have the lamps installed. I put in some 50watt resistors to simulate the bulbs and it’s working like a champ! Now I just need to figure out how to put this all together on a board so that I can put it in an enclosure with plugs on each side.

    • Jim Keith

      To make it work, vehicle stop and turn lamps must be present–the non-illuminated lamp provides the negative (0V) turn-on voltage path for the gate of the MOSFET.

      For bench testing, just use 470Ω resistors instead of the lamps with the ends grounded to 0V.

      Hope all works out well for you.


    Good day to you!
    Those that want to build this circuit MUST make sure that red turn indicators are legal as here in RSA they are illegal. I had the same problem and eventually I fitted a fat wire from the battery to a relay box that I fitted to the rear of my truck Every circuit had its own relay with a fuse of 8 amps. At the battery I fitted a 30 amp for the cable (5 awg or 10 mm^2)This removed the load on the truck’s switches as we had to replace them occasionally and they were expensive as the switch contained electronics.

    • Jim Keith

      Never gave that issue a thought–this may be mainly a North America thing…

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