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.
Fortunately, it can simply be performed the following circuit:
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.
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.
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.
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.
Bill of Materials
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.
For the future
Relay topology Stop and Turn signal converter (lowest voltage drop of all)