bigtrak Circuit Diagram - H Bridge Motor Driver

The trail began with these four transistors on either side of the board. These are obviously power transistors used to drive the motor. There are four of them, which allow the motor control in either direction. No prizes for guessing that we are looking at an H bridge type configuration. The transistors are the following.

Q4 B772S
Q13 D882S
Q12 D882S
Q5 B772S

In a TO92 package, the leads will be emitter, collector, base, when the flat side is facing you. The D882S is an NPN transistor, whilst the B772S is a PNP type. These complementary transistors operate together as a pair. They are low frequency types designed for switching loads. Their absolute maximum collector rating is 3-amperes.

There appears to be two chips that generate the signals to drive the transistors. I decided to trace the circuit to see if I could figure out where to inject the signal to actuate the motors. The top chip appears to be some form of servo control chip driven by this larger chip.

As you can see, there are four rows of three pins on the PCB trace side. These pins will be emitter, collector, and base.

Reverse engineering the circuit, you can see that the transistors configure to form an H bridge motor driver.

Do not call a NASA engineer just jet, because as these things go, the principle of this circuit is quite simple and straightforward. Click on the image above to see an animation of how it works.

A signal through R33 makes Q8 conduct in the saturation region, this in turn makes Q4 and Q12 conduct as a switch would, forming a path for the current to flow through the motor. When the current flows through the motor in the direction ML+ to ML-, the shaft will turn clockwise and the toy vehicle rolls forward. When a signal through R30 makes Q10 conduct, it switches ON Q5 and Q13. The current then flows from ML- to ML+ and therefore the shaft of the motor turns anti-clockwise, and the toy vehicle reverses.

Here is an even more enlarged view of the printed circuit board so that you can trace the circuit for yourself. You can also read the resistor component values.

I am not sure of R02, and there might be mistakes... However, all I needed to know was where to inject the signal to actuate the motors, and the points are at R33, and R30 as shown above. It is certainly possible to replace the discrete H bridge driver with a chip. There are plenty of sellers on eBay selling these providing a good option. However, my mod is to keep the original circuitry in place. MOSFETs can of course replace bi-polar junction transistors, after some modifications, and that would make the circuit more robust and efficient.

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