Friday, November 2, 2018

H-bridge circuit of stepping motor

As simple in construction as a bipolar 12v stepper motor is as complicated control circuits can be. However of-the-shelf ICs or complete electronic modules for controlling stepper motors, with costs not exceeding a few dollars, are available. Generally, in order to reverse the flow of current through a winding, i.e. reversing polarity, a H-bridge circuit is required for each winding. In order to better understand the inner working of such a system such circuitry is also used in our example. The circuits are powered by a 12 VDC source but a 24 VDC power source can also be used. The power transistors are BD139 NPN with their respective PNP BD140 complements, supporting up to 1,5 A current.

In addition to the simple H-bridge circuit, a protection circuit is added in order to prevent short-circuits when both bits controlling the H-bridge are 1, thus preventing high values of current flowing through the power transistors. Transistors used in this section are BC639 NPN, together with 1 KOhm resistors.

H-bridge circuit of stepping motor

Q1, Q2 – BD140 power transistors;

• Q3, Q4 – BD139 power transistors;

• Q5, Q6 – BC639 transistors;

• All resistors have 1 KOhm ratings.

If the first data bit is 1 and the other 0, Q3 will conduct suppling ground to Q2 which will also conduct and the motor will turn in one direction. Conversely, if the sequence is 01, Q4 and Q1 will conduct, allowing for reverse polarity and thus reverse current flow through the winding. If there would be no protection circuitry, if the sequence would be 11, all four transistors, Q1-Q4, will conduct and creating a short circuit. Due to the high current they will burn out.

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