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I am designing a MOSFET based H-Bridge to handle high current. I would like to place a SHUNT resistor in the circuit in order to detect over current. The voltage across the shunt resistor is to be read by a PIC micro controller (so the + voltage side cannot exceed 5V). The only place in the circuit i can think to add the shunt resistor is after the H-Bridge, however this means placing a tiny load on the source side of a MOSFET (as shown in image). Will this be a problem when considering the load is only 0.0001 Ohms? and is there a better place to position the shunt resistor?

enter image description here

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  • \$\begingroup\$ Is this just for testing purposes? You could potentially use a current mirror... \$\endgroup\$ – user92289 Dec 2 '15 at 3:02
  • \$\begingroup\$ no this is for operational purposes. \$\endgroup\$ – user3095420 Dec 2 '15 at 3:08
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That is the location where it is commonly placed.

If your shunt is really only 0.1 mohm, you will need to ensure you use a 'Kelvin' connection to measure the voltage across it. You will have to connect GND of the micro controller at the bottom (GND) connection of the sense resistor, else the additional wiring resistance will swamp your readings.

You will also need to be careful about inductive spikes, and perhaps need a decoupling capacitor on the 24 V to make your whole system work.

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  • \$\begingroup\$ Yes the shunt I am looking to use in the Vishay Dale WSBS8518L1000JK. Any suggestion on what value decoupling i will need? \$\endgroup\$ – user3095420 Dec 2 '15 at 5:22
  • \$\begingroup\$ depends on the motor current (stalled ?), and the power supply quality. \$\endgroup\$ – jp314 Dec 2 '15 at 6:06
  • \$\begingroup\$ 50A stall current with a 24V supply coming from a battery. \$\endgroup\$ – user3095420 Dec 2 '15 at 15:04
  • \$\begingroup\$ With that high a current, 'just' decoupling isn't sufficient. You'll need to isolate your gate drives, watch for inductive spikes, shoot through currents. If a fault occurs, your PIC won't be fast enough to protect you. When you connect power to the system, you need to ensure the FETs are held off before you start driving the motor -- else capacitve coupling will turn them all on, and they'll burn up. High current, high power drive is tricky. \$\endgroup\$ – jp314 Dec 3 '15 at 3:52

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