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Transistor Trouble Answered

Hi there,
I have been working on controlling a large toy car with a wireless receiver from a smaller toy car, the motors I need to control are rated for 9V but the receiver only outputs 3V, reversing polarity to reverse the motor. I made this schematic using transistors to handle the voltage, but when I built the circuit the transistors began to heat up and there was no output. I am not sure why this is happening, so any help would be greatly appreciated.
Thanks,
Daniel

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Josehf Murchison

2 years ago

Of course it won't work.

You built the circuit with two unconnected batteries like the first schematic in this pic.

You need to build the second circuit, if you need two 9 volt batteries connect them in parallel like the third circuit.

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daniel12354Josehf Murchison

Reply 2 years ago

thanks for your help, the two 9 volt batteries are actually the same battery, in this case I should use the second circuit right?

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Josehf Murchisondaniel12354

Reply 2 years ago

Right use the second circuit.

You can find other full bridge motor reversible drives here:

https://www.instructables.com/id/Arduino-Full-Bridg...

Some people call a full bridge an H bridge.

One other thing 2N3904 and 2N3906 are 625 mW transistors, so at 9 volts they can only handle 69 mA max.

Make sure your motor is only 35 mA or start up surges and running close to max will over heat the transistors.

How many mA is the motor free running?

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daniel12354Josehf Murchison

Reply 2 years ago

I'm not sure how many mA it takes but I do know the motor, it is a model 130 dc motor and has a white plastic back(I'm not sure if that means anything but ive seen some with blue plastic). Are there other transistors that would work better in this situation, maybe some that could handle the 9V better?

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Josehf Murchisondaniel12354

Reply 2 years ago

Yea you might want to use a 3 watt transistor with that motor, 2N3904 and 2N3906 is seriously under powered with no load you are at max power. at stall you need 3 watt.

I have a couple datasheets on model 130

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Josehf Murchisondaniel12354

Reply 2 years ago

As long as you can work with SMD they should be fine.

TIP31 and TIP32 will work if you like through hole.