I'm trying to understand what happens at various points in a circuit during drive-coast operation (and also drive-brake, but haven't got there yet). Imagine Q1 and Q3 are on, and Q2 and Q4 are off, such that current flows left to right in the motor. If Q1 and Q3 are now shut off, point B would jump to 10V + Vf (forward voltage of diode) and A would jump down to -Vf. Current would flow from point B through D4 into positive side of the power supply. Current would also flow from ground, through D2, to point A.

Once some of the energy has dissipated point B would be just below 10V + Vf, and so current would no longer flow through D4, and similar situation with D, because the diode is no longer in the forward bias region. What happens after this? Does B just remain at that particular voltage? At this instance let's say the motor is still spinning freely (with only little deceleration due to friction), close to what is was when being driven. Would the back EMF cause A to increase and B to decrease? Basically I'm confused on what the voltages at points A and B are at various time points after Q1 and Q3 have been shut off.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ did you read these yet? electronics.stackexchange.com/search?q=h+bridge+back+emf if not why not? \$\endgroup\$ Commented Oct 2, 2018 at 20:55
  • \$\begingroup\$ I did not look through all of them, but had seen many of them before posting this. However, I'm still confused on how the back EMF that is produced due to the motor rotating (which induces a voltage drop from A to B close to 10V) interacts with the voltage spike when the motor is turned off (which induces a voltage rise from A to B, with A being -Vf and B being 10 + Vf). \$\endgroup\$
    – user198450
    Commented Oct 2, 2018 at 23:07
  • \$\begingroup\$ There are examples on web explaining all of this \$\endgroup\$ Commented Oct 3, 2018 at 0:19
  • \$\begingroup\$ Would you mind linking me to a specific example that answers this question? Honestly can't find an answer online \$\endgroup\$
    – user198450
    Commented Oct 4, 2018 at 3:48
  • \$\begingroup\$ Here's a clue google.com/… add key words \$\endgroup\$ Commented Oct 4, 2018 at 4:51

1 Answer 1


The motor has a back EMF proportional to its speed. With this arrangement it couldn't have gotten to a speed where the back EMF was higher than 10V - as the motor EMF approaches the supply voltage, the current falls to zero and no further acceleration is possible.

Once the bridge transistors are off, the EMF from the inductance of the windings generates an additional EMF as you describe, that allows the current to decay back into the supply until the current falls to zero. After that only the motor EMF remains, which drives no current since all the diodes are not forward biased and the transistors are off, so there is no net torque applied to the motor, and it'll just coast down under frictional and drive load. Both A and B will float somewhere between the supply rails, but remain with the same EMF between them.


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