Shunt Regulators

Most motor controllers encounter scenarios (typically when a motor is slowing down) where energy from a motor is returned to the power supply. That power supply is commonly a battery that can tolerate the current fed into it, and the reverse current can be a welcome “regenerative braking” feature that extends the total life of the battery. For applications that do not involve batteries, however, that reverse current can be a problem when the power supply does not deal with the current well. In some cases, the voltage can build up to a high value that the power supply can tolerate but which might be damaging to the motor controller or other electronics supplied by the power rail. In other scenarios, power supplies have “over-voltage protection” features that detect the output voltage going up and shut down the power supply.

The simplest solution to the problem is often a transient voltage suppressor, or TVS, which is a big zener diode optimized for handling big current spikes. Unfortunately, TVS diodes typically do not have a tight enough tolerance for use with power supplies with overvoltage protection. For example, a 12V power supply might have 5% tolerance, meaning the output voltage could be as high as 12.6V, so the protection device must not kick in below 12.6V. If the over-voltage protection is triggered by a 15% deviation, any voltage spikes must be kept below 13.8V. Most basic TVSes do not have tight enough tolerances to ensure operation in that window.

Pololu’s shunt regulators are designed for such scenarios, with fixed values available with ±3% accuracy for some common voltages, and adjustable versions available to accommodate any voltage from approximately 5V to 35V. To use the shunt regulator, install it across (in parallel with) your power supply. Under normal conditions, the supply voltage should be below the set point of the shunt regulator, and the regulator should only draw its quiescent current of less than 0.5 mA. However, any voltage spikes above the set point will cause the shunt regulator to draw substantially more current, dissipating energy and limiting the extent of the voltage change.