原创 [博客大赛]Servo Motor Repair DIY

Electronic engineers may love to play with small mechanical 'toys'. DC Servo Motor is one of these.

Most of the time, if we do not overload the servo, the lifespan of the motor can be very long, in spite of the brushed design. However, some DC servos are out of order not because of the motor is burnt (or the brushes are ruined), but due to some components on the PCB.

I have bought a brand-new DC analog servo from somewhere, it costs HKD $90.
I write some codes on my STM32 platform to test it. The program is simple, which makes it to run back and forth in small steps.

After testing it (not continuously) for some time which was less than an hour, the servo motor suddenly became out of order... As usual, what wass going to be next would be a screwdriver with the servo motor! Haha

Fig.3 The casing is opened

 

Then the motor was powered on and PWM signals were given to the servo motor.

I traced the signals from the motor. The signals inputted to the two Dual NMOS and PMOS ic, which formed a H-bridge, did not show any waveform, given the message that the later part of the circuit did not receive the control signal.

 

After that, I started tracing from beginning... I found that there are one of the two small SOT-23 transistors had some problems. The base terminal (pin 1) of the PNP transistor was LOW (turned on) and pin 3 (emitter) was 5V (supply voltage), but pin 2 (collector) was 0V... So the transistor was not functioning!

 

A soldering iron and utilities were prepared to replace the transistor

 

Fig.4 Prepare a solder iron to replace the transistor

 

I replaced the original transistor with a SS8550, keeping the other components unchanged. If it did not work, I would have tested other components.

 

Fig.5 Before and after transistor replacement

 

To my surprise, the servo motor worked normally again!! Yeah!