Sunday, December 22, 2013

PWM Grip and Jacket Heater Controller

A variable output controller for grip heaters or heated clothing costs around $75. We decided to build our own.

We found this schematic at This is a "Pulse Width Modulation (PWM)" power controller. It is basically an oscillator which switches the power transistor on and off with the relation between on time and off time ("duty cycle") set by the potentiometer position.

In the schematic, the "DC Motor" represents the power load, which actually will be the heating element. Another modification is the potentiometer has a SPST switch which is used to control power to the circuit. In the case of the grip heaters, where the load is permanently connected, a relay is used to connect power to the load (called "+V MOTOR" in the diagram above.) This is done to prevent any possible drain on the battery when the grip heaters are not being used. Finally, the C1 capacitor was changed from 0.1 to 10.0 uF in order to slow the pulse rate down (from about 144 Hz to 1.4 Hz) so as to prevent possible disturbance to the motorcycle electronics.

(To be continued.) The grip heater controller was built and is in operation. Unfortunately, we forgot to take any photos. Hopefully, there will be construction photos of the jacket controller...

Heated Jacket Liner

This is a DIY heated jacket liner for use under our motorcycle jacket.

It was constructed by stitching a 20 foot length of heater wire from a defunct electric blanket to the inside of the red windbreaker. The green windbreaker was sewn inside the red one to protect the wires.
The white wire is the heating element. The black one is an 18AWG wire running from one end of the heating wire to the external connector at the bottom of the jacket.
Here is the routing of the wires.

The electric blanket wire is a twin conductor wire. Each conductor is made of a pair of 36 AWG strands spiral wound around what appears to be a nylon fiber core, and coated with a plastic insulation of some sort.

The external connector is a 5.5 x 2.1 mm DC power connector - similar to those used for laptop computer power supplies.

Each conductor of the heating wire has a room temperature resistance of approximately .24 ohms/foot. In the jacket, the two conductors are wired in parallel, giving a total resistance (with connections) of approximately 2.5 ohms. At 12 volts, the current starts at 4.8 amps, resulting in an initial power of 58 watts.
As the jacket heats up, the resistance increases slightly, dropping the power to around 47 watts. That is pretty low for a jacket; 70-90 watts is more typical. But as this was the first try, 47 is acceptable. It produces a noticeable amount of warmth.