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I am running 2 DC motors with 3 SN754410's: each motor gets one half of each bridge. Everywhere I look I see that pulling both logic inputs low with the enable high should brake the motor by shorting its leads together. However, when I measure the resistance between the two motor power leads (motor disconnected) with the enable high and both logic inputs low, I get a resistance of 60 Ohms. The H-bridges are otherwise functional

Accordingly, I am attempting to implement an alternative brake. I have some of these (http://www.vishay.com/docs/91217/91217.pdf) 30A power transistors lying around. Could I use them to brake my motor by putting one from each motor lead to GND (see below for setup when I want to brake) and turning them on when I want to brake?

schematic

simulate this circuit – Schematic created using CircuitLab

I am not using a relay as they are too slow for my code: I get shorts through the bridges when the bridges turn on but my relays have not finished switching.

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    \$\begingroup\$ Schematics are better than words. Hit Ctrl-M in the editor. \$\endgroup\$ – Transistor Apr 9 '16 at 22:42
  • \$\begingroup\$ @transistor thanks, I didn't know about that. \$\endgroup\$ – dpdt Apr 10 '16 at 0:09
  • \$\begingroup\$ I would try resistive braking myself. \$\endgroup\$ – Tim Spriggs Apr 10 '16 at 0:20
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enter image description here

Figure 5 of the datasheet shows the standard half H-bridge configuration with protection diodes.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Motor circuit.

schematic

simulate this circuit

I would expect that you should be able to give maximum braking by holding closed one of the already closed switches and let the freewheel diodes look after the rest. This will initially maintain the current and rapidly cause dynamic braking.

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  • \$\begingroup\$ When I said 3 chips, I meant 3 chips in parallel for higher current. \$\endgroup\$ – dpdt Apr 10 '16 at 18:00
  • \$\begingroup\$ Also, I don't have the motor between the bridge output and GND, I have it between 2 bridge outputs. \$\endgroup\$ – dpdt Apr 10 '16 at 18:02
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    \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Apr 10 '16 at 21:26
  • \$\begingroup\$ that's the config, but I think you're missing something on the datasheet. I tried to move the motor when it should be braked according to that schematic, and it turns easily. When I try to move it with the relays on, it is much harder. \$\endgroup\$ – dpdt Apr 10 '16 at 22:56
  • \$\begingroup\$ Darlington output doesn't turn on until ~0.7V, so won't have braking action unless the motor is spinning fast enough to generate >=1.4V (Darlington + flyback diode turn-on voltage). Relay and MOSFET stay on down to almost zero volts, so they have much better low speed braking. \$\endgroup\$ – Bruce Abbott Apr 11 '16 at 0:56

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