Simple transistor motor driver does not work properly?

Question

The transistor gets hot to the touch and the motor will continue to run when I disconnect the transistors base from 9v+ (I am trying to make sure the circuit works before I hook it up to an arduino). The motor will continue continue unless I disconnect the power supply.

Is this how the circuit is supposed to work like this? I am using a 9v power supply, simple toy motor, 1uf cap, 1k resistor, and a PN2222a npn transistor.

I was following this instructables (http://www.instructables.com/id/Simple-Cheap-Motor-Driver-Board-for-Arduino/?ALLSTEPS). What am I missing with this circuit? Should I go through and do all the calculations for the circuit? Thanks!!!

In addition to the good answer(s), you may want to look at my related answer [here](http://electronics.stackexchange.com/a/103232/38335). It talks about different aspects of controlling motors with transistors. Although it uses MOSFETs instead of BJTs, it should still help... — bitsmack, Jun 10 '16 at 01:03
What you are missing with this circuit is a circuit diagram. Cartoons don't help at all. — Andy aka, Jun 10 '16 at 09:15

score 3 · Accepted Answer · answered Jun 10 '16 at 00:47

First, you need to learn how to read and draw schematics. Fritzings are terrible to work with.

With that said, your circuit was never going to work. The circuit is bad. The voltage delivered to the motors will be low.

Let me show you what you have

schematic

^{simulate this circuit – Schematic created using CircuitLab}

This will only work for a (about) 3 volt motor. Depending on how much current the motor draws it may also cause the transistor to get very hot. "Simple toy motor" is not remotely a motor specification which is useful.

What you need is

schematic

^{simulate this circuit}

Note that the resistor has been changed to a lower value. This may or may not be necessary, but you haven't stated the motor characteristics.

This also assumes your motors will operate well on about 9 volts. You can check this by simply connecting a motor across your battery. If it works and doesn't overheat, you know the motor is OK.

user66377 · Answer 2 · 2016-06-10T01:47:30.083

2 suggestions: 1st Put a 10k return resistor on the base to (ground or -9 volt) to keep voltage off of base when not in operation. Removing the resistor from the base could also let the base float continuing conduction. 2nd The suggested transistor is not handling the current necessary to run your motor. There-forth your transistor is getting hot. Possibly shorted. Replace with a Darlington NPN, TO-220 case style and a hfe of at least 1000 and able to handle current draw without over heating, may need a heat sink. TIP121 All motors have different current requirements.

Simple transistor motor driver does not work properly?

2 Answers2