Motor investigation
Before committing too much time to the car, I wanted to check over the motors. The key things to look at were ensuring they rotate freely and to determine the correct motor voltage. Because the motors are sealed it's not possible to check much else such as brushes or bearings. I have taken these kind of motors apart before but I did not want to risk being in the situation of not being able to put them back together.
I started by disconnecting the RC controller board so the motors could be tested. The coil resistance was approx 4Ω so I knew I'd need to be careful about limiting the current. Looking at the board the capacitors were all rated at 10v so I suspected this was powered from two or three old style batteries e.g. 3v or 4.5v. So I started with my power supply set to 2v and 300mA. The motor turned and did not take a lot of current so I increased to 3v and took a reading.
I tried to measure the stall current of the motor but it went over 1A and I was not comfortable setting my current limit any higher as I did not want to burn out the motors. I did, however, measure the motor under load and it took 600mA. This will be important when I look at the supply as I'll need to have something that can provide these kinds of currents.
After re-assembling the car I ensured that both motors worked in both directions and the wheels turned. I'll likely open up the car chassis a little so that I can get to the wiring without having to keep unscrewing everything.
Driving the motor
I had thought about using an off the shelf H-Bridge module based on the L298. However looking at the RC board it divides easily into 2 halves for the radio and the power to the motors. I guessed that the latter is two, 6 transistor based H-Bridges.
RC Board Reverse Engineering
To reverse engineer the board, took a photo and blew up the print. Normally I would use any chip on the board to determine where the power supply lines are located but the numbers drew a blank, the name on the top is a toy manufacturer. So I followed the electrolytic capacitors instead, also the screws are typically in the ground plane. The transistors were easily identifiable as C3279 (NPN), A1015 (PNP) and A1300 (PNP) the resistors were mostly 1K and 100R. I had big pads for the motors.
Simulation
The circuit seems to have promise but I was a little concerned that I did not fully understand the circuit, so I opened up Circuits.io to simulate it.
With one input high and the other low the circuit worked as I wanted it to. The motor turned in each direction.
If I took both inputs high then it seemed not to switch off, staying with which ever direction had previously been selected.
If I turned both low then my worst fears were confirmed in that the circuit did not properly turn off the transistors resulting in the current limit being hit and the supply voltage dropping.
I wondered if there was anything specific about the combination of my transistors that might cause it to work differently to the simulation. My final experiment with the simulated circuit was to drop the supply voltage down to 1.5v. This worked better but the current draw for both inputs low was still quite large at 350mA.
Simulated circuit - https://circuits.io/circuits/4643484-h-bridge
Testing
I removed the RC chip and wired up a spare motor to the H-Bridge. I set the voltage to 3v and the current limit to 100mA. It behaved exactly the same as the simulation, when the motor was running it was fine, when the inputs were all low then the current limiter kicked in.
My conclusion is that the motors perhaps never stopped, the controller could have kept them running all of the time. I'd like to be able to stop my motors so it's back to my initial plan of using an off the shelf module, I've got a L298 with a big heat sink and a smaller TB6612FNG module. Both of these will run from 3-5v and logic levels at similar levels. The motors and gearboxes seem to be fine so otherwise the car is good to use.
Reference
https://www.sparkfun.com/datasheets/Robotics/TB6612FNG.pdf
https://www.sparkfun.com/datasheets/Robotics/L298_H_Bridge.pdf
http://www.micromo.com/technical-library/dc-motor-tutorials/motor-calculations
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