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33V Shunt Regulator w/ external resistor help

I recently purchased some of the 33V, 3W shunt regulators along with some external 5ohm power resistors. I had hopes to use these to dissipate excess energy from my motor controller and keep my power supply from triggering over-voltage protection.

I attached my power resistor to one of the boards and placed it across my power supply (in parallel with the system I want to power). When the voltage reached the 33V shunt point, the regulator kicked in and began pulling a large amount of current compared to what the system regularly uses. It triggered the 5A OCP on my power supply which lowered the supply voltage to ~24V. The board also became quite warm. It appears this broke something, as now whenever I supply power (even far below the shunt point), the regulator draws current.

For context, my system is controlled to provide sinusoidal (1Hz) current to the motor. I am not sure the max current, but can find out if necessary. The voltage peaks last up to 500ms, I estimate that the system is moving around 70J of energy, and I’m supplying 29V.

My assumption was that if I populated an external resistor of sufficient power capability the board would be fine, but it seems like the resistor is not the limiting factor here. Is 500ms too long of a spike? Or is there some additional power restriction not related to the shunt resistors? I’d very much appreciate some help filling in any misunderstandings I have about the product, and determining whether it is possible to use this board for my use case!

Hi.

There is a FET on these boards that shares some of the dissipation load with the resistors. It sounds like that FET has failed in a short so the load resistors are always dissipating power now. Just using a 5 ohm resistor rated for more power won’t help, since the FET will still be getting the same share of the load. If you want to try again with a new board, you might increase the load resistance, which will reduce the FET’s dissipation load and help protect it. An external load of 7 ohms would still allow you to dissipate 70J of energy at 33V over 500ms without a voltage rise if the dissipation during the peaks is fairly steady. If it is an option, reducing the length of the peaks would also reduce the overall load. Unfortunately, your application seems to be at the edge of what the shunt regulator can handle, so I can’t give any guarantees it will work even with the 7 ohm load.

-Claire

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