Electrical – Arduino hacked rc car, H bridge, logic voltage overwhelmed

arduinodc motorh-bridgemotortransistors

I have hacked my old rc car like in this tutorial here

I have measured manually the h bridge logic voltage (the voltage that the transceiver send to the h bridge) , which is 3.7volt

  • in the tutorial, the guy is sending 5v (instead of 3.7) from the output pin of the arduino to the hbridge, isn't that dangerous for the h bridge?

  • would it be possible to send pwm to control the motor speed, even if the h bridge wasn't constructed for this? (the original car only have 1 speed) (I'm afraid of breaking my h-bridge or my motor if I send pwm…)

Best Answer

That tutorial shows a Tyco R/C vehicle, which uses an RX2C decoder IC with outputs that can source or sink up to 6mA to drive H-bridge transistors. Typically the signals are coupled to the transistors through current-limiting resistors. Raising the drive voltage to 5V would just increase the Base current a bit and shouldn't hurt the transistors.

Here is a partial circuit traced from a Tyco 'Canned Heat' r/c car, showing the RX2C and H-bridge motor driver. Each decoder output is coupled via a 1k resistor, which would limit Base current to less than 4mA at 5V.

enter image description here

To verify that the higher voltage won't affect any components on your board you should trace out the circuit and and understand what each part does. Provided that they are all still within spec at the higher voltage it should be OK.

If you want to apply PWM then each output transistor in the H-bridge should have a reverse-biased Schottky diode from Collector to Emitter, to soak up back-emf spikes and recirculate current through the motor. If your your circuit uses FETs then you won't need diodes because they are built in to the transistors.

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