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SO I have 2 transistors hat control 1 motor each. They are the 2n3906 pnp transistors. Basically I have them connected to 6 volts for the motors and the bases to the outputs of a voltage comparator. What happens when I turn on the motors with the transistors is that the trannys overheat. Like burn your skin heat. This happens only after about  a few seconds of running.

I have a 9 volt battery for the voltage comparator circuit and a diode in parallel with the motor. The white line of the diode is facing the transistor.
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Any chance we could see a schematic?

Likely you are missing some resistors to limit current flow in the base, or are just running the motors at too high a current. What is the motor rating (volts & amps)?

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Got any current limiting resistors in there? Like between the comparator and the base, and between the motor and the transistor to limit the current to <100mA?
http://www.fairchildsemi.com/ds/2N/2N3906.pdf
http://www.kpsec.freeuk.com/trancirc.htm


* PNP_motor_driver.jpg (8.53 KB, 312x228 - viewed 22 times.)
« Last Edit: March 02, 2011, 12:51:40 pm by CrossRoads » Logged

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The 2N3906 is rated for an absolute maximum of 200mA and .65 W (in free air).   If you exceed this is will fry / get damaged.  Power is product of current through the transistor and voltage from collector to emitter.
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Got any current limiting resistors in there? Like between the comparator and the base, and between the motor and the transistor to limit the current to <100mA?
http://www.fairchildsemi.com/ds/2N/2N3906.pdf
http://www.kpsec.freeuk.com/trancirc.htm

that is exactly how I have it hooked up on my circuit.
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So out of the collector you have a current limit resistor and then your motor?

Remember, a motor is an inductor and basically looks like a DC short circuit.

To keep within the transistors limits, you need the impedance of the motor and the current limit resistor to equal about
(Vsource - Vce)/100mA, or (6-0.4)/.1 = 56 ohms

What is the motor you are using?
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So out of the collector you have a current limit resistor and then your motor?

Remember, a motor is an inductor and basically looks like a DC short circuit.

To keep within the transistors limits, you need the impedance of the motor and the current limit resistor to equal about
(Vsource - Vce)/100mA, or (6-0.4)/.1 = 56 ohms

What is the motor you are using?


ok I don't  have a resistor on the collector side but on the base side, it's a 1k ohm.
I don't have the motor with me but its a small cheap one , heres the picture:

http://image.made-in-china.com/2f1j00eCqaQlvdfUcw/Small-DC-Electric-Motors-OT-FA-130-.jpg
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That picture is no help.
The motor works by itself off the 6V supply? Try it, see how much current it draws.
If you could get a resistance reading across the motor terminals that would help also.

1K base resistor.
So you have about 5mA going into the base. The transistor has a gain of 80-100. If the load can absorb it, the transistor will try to let 400-500mA flow.
Try a 5K resistor, that should help,  by cutting down the amount current being allowed to flow.
The voltage drop across the transistor is 0.4V, so ~5.5V going into the motor.
You need a current limit resistor, or a different transistor.  A MOSFET will work much better for you here. Can turn it on full and the motor will become the limiting factor, not the transistor.
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There is a chance that reducing the base current will make the transistor get hotter. This is because if the transistor goes into the active region, the voltage on it will increase. True, the overall current will reduce, but with the increase in voltage, the power dissipation in the transistor will increase. Power dissipation in the transistor will be maximized when the voltage drop on the transistor c-e is equal to the voltage on the load. In this case that would be 3v and 3v.

Assuming the comparator output is at a steady state (low, in this case) to turn the motor on, then a voltage reading should be done on the motor. If the motor is getting over 5.5v (with a 6v supply), then the transistor is adequately saturated, and likely the excess heat is simply from the motor using too much current. In this case, a larger transistor is called for.

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2N3906's are not suitable for driving a motor unless its a tiny tiny one.  A medium power rated transistor is probably called for, rated at 1 to 2A and a gain of 100 or more.  These tend to come in a TO220 package or similar to which you can bolt a small heatsink if necessary.

Alternatively a power MOSFET with logic-level drive would do the job nicely.

And don't forget that protection diode.
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If the Transistor is overheating, may be by wrong connection it has been damaged. you can go for a new one or replacement!
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