While electric motors "consume" energy during acceleration and continuous movement they tend to generate energy during fast deceleration or electric braking. With typical driver circuits this energy coming from the motor is fed back into supply rail. Depending on amount of energy supply voltage might increase up-to critical levels for driver electronics and other connected electronic circuits.
This is a common problem with applications requiring highly dynamics movements or where the motor is moved by external force. As long as the amount of energy is rather small adding capacitors between positive and negative supply and adding suppressor diodes with suitable voltage ratings will help and be sufficient. Beyond that dedicated brake chopper circuits with power resistors are used in order to convert the surplus energy into heat.
In contrast this brake chopper circuit uses a MOSFET in linear mode for dissipating energy into heat. For optimum heat transfer the MOSFET in SMT package is soldered onto a single layer Insulated Metal Substrate (IMS) pcb. The aluminium substrate maybe mounted on a suitable heat sink or enclosure depending on application.
The circuit has been designed in order to protect a +24V supply rail. With component values as specified the MOSFET will switch on at around +27V supply voltage. A standard 12V linear voltage regulator (78L12, IC1) is used for generating supply voltage for the OP (LM358, IC2) and via R5 / R6 resistor voltage divider reference voltage (approx. 6V) for the negative / inverting input of the OP. The non-inverting input of the OP receives scaled-down supply voltage via R2 / R4. As soon as the voltage at the positive input of the OP rises beyond voltage at the negative input the output voltage of the OP will get positive and MOSFET starts conducting.
For different supply voltage limits values of R2 or R4 may be adjusted. Please keep in mind maximum voltage ratings of components - e.g. 35V for the 78L05 etc.
A prototype circuit has been hand soldered and briefly tested using a laboratory power supply as input. With current limit set to 500mA the brake circuit limits supply voltage as soon as laboratory voltage supply setting is increased beyond around +27V. Laboratory power supply switches to +27V / 500mA constant current operation, then.
Using power MOSFETs in linear mode applications is no trivial task with latest generation MOSFETs due to "hot spotting" (see e.g.  for more details). This should be taken into account when considering selection of a different / newer MOSFET type.
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