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I want to drive Brushed 12Vdc, 4A motor using two relays to achieve direction control. But what is the best way to connect freewheeling diode to it, I came up with this circuit. enter image description here

Is it the right way to do it, or is there a better way to freewheel the inductive current in such direction reversing setup. Also since it is a 4A motor, what should be the safe current rating that I should assume for the diode.

Operation: At a time only one relay will be activated. So initially If I activate relay 1, then current flows through motor from left to right. But when S1 is opened, and it is halfway in air then current should take this path,

Upward through D1, Left to right through motor, and to ground through S2.

And similarly when Directions are reversed.

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2 Answers 2

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You're halfway there. You also need a diode on each side going up to the positive rail.

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  • \$\begingroup\$ Could you explain why is that needed? \$\endgroup\$
    – Sajid
    Commented Oct 6, 2015 at 6:45
  • \$\begingroup\$ You have a path from the motor to the lower potential to attempt to dissipate the negative EMF, but you lack any way to also get rid of the positive EMF other than to arc across the switches. As with any circuit, you need both connections in order for it to be effective. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Oct 6, 2015 at 6:47
  • \$\begingroup\$ I thought our aim is to provide path for the current to go from 1 lead of motor to another. \$\endgroup\$
    – Sajid
    Commented Oct 6, 2015 at 6:50
  • \$\begingroup\$ Correct. But which path connects to the positive side? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Oct 6, 2015 at 6:50
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    \$\begingroup\$ Do you understand what happens when you connect the leads of a motor together? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Oct 7, 2015 at 6:50
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4 diodes like Ignacio said is correct.

Why not use a bridge rectifier, we did 20 years ago on a addon central locking system for cars which worked fine.

Now be careful about your diode ratings ,the surge current of the motor could be 60 amp or more, you should find out. The surge current of most diodes is for a mains half cycle. This time could be shorter than the motor time constant. You should find out. If there are any doubts then play safe with a 3 amp bridge.

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