I have a 17PM-K307-05VS unipolar stepper motor. I am building the driver for the stepper motor. The exact rated voltage and current are unknown, it seems the motor is old or discontinued, but the measured coil resistance is 6.6 ohms. For this, I am not using an IC, I am using the TIP120 darlington transistor as a switch. The picture will show the circuit of one coil of the stepper motor and the darlington connected.

enter image description here

My problem is when the arduino sends pulses to turn on or off the transistor, no current flows through the darlington when its on. Is this has to do with the low logic voltage? How can I resolve this problem? is there any ideas to improve my circuit or is there something wrong in the circuit?Also I noted when I turn on the circuit the base to emmitter current drops down to .324V, which is low. Any advice will be of great help!

  • 1
    \$\begingroup\$ use a smaller base resistor like 1K make sure that the port direction is set to output. \$\endgroup\$
    – Jasen
    Nov 3, 2018 at 6:56
  • \$\begingroup\$ Edits required: (1) "Also I noted when I turn on the circuit the base to emitter current drops down to 0.324 V ..." Current is measured in amperes, not volts. (2) What is the reading when you turn off the circuit? A tip: Turn of the grid when taking screen grabs. It improves legibility on more complex circuits. \$\endgroup\$
    – Transistor
    Nov 3, 2018 at 9:25
  • \$\begingroup\$ transistor- I meant "voltage from base to emitter" instead of current. The voltage from base to emitter measured was 0.324V, which is very low for the transistor to be active \$\endgroup\$
    – Crivera
    Nov 3, 2018 at 15:59

1 Answer 1


Here is a modified version of a TIP120 running from a buffer impulsed by Arduino. I want to credit the original design here

enter image description here

NOTE: the protection diodes around the Darlington transistor are not necessary because they are included in the chip.

15 to 5 voltage use a Voltage regulator setup . I do not recommend using the 5 V source from the Arduino unless you are using a 9 Meg supply to power Arduino and NOT depend solemnly in the the standard computer USB connection)

  • \$\begingroup\$ It's hard to know where to start with such a poor circuit design. 1) The diodes shown are indeed unnecessary since they provide NO protection. 2) The 4050 driver has no current limiting into the base of the TIP120. 3) You need diode protection for the TIP120 as the back EMF from the coils will exceed the Vce capability. \$\endgroup\$ Nov 3, 2018 at 16:07

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