What happens when a freewheel diode goes?

Is their sole purpose to allow the motor to freewheel or do they protect the FETs somehow? I've never payed any close attention to how they were oriented in relation to the FET drains. :?

I ask because my Orvac search was fruitless (mostly because I ran out of time to look for something suitable), and I'm considering just replacing the FETs and caps in the controller and seeing if the diodes could take 72V. Normally I'd find a spec sheet, but even with my best Googling I can't find anything. If not I'll just use one of my monstrous 150V 240A schottkys. I'd rather not though, since it's major overkill and just one is worth more than the whole controller, 4110s and caps included, LOL.

Just don't wanna damage anything else in the event they get destroyed.

The freewheel diode recycles the current from the magnetic field in the motor when the switch opens. In a brushed controller, when the freewheel diodes shorts, it shorts the motor, making it very hard to turn. If the freewheel diode blows open (rare), the voltage spikes immediately blow the FETs.

The primary purpose of these diodes is to conduct motor current when the FETs are off. If they are removed, the motor will generate massive reverse voltage spikes when the FET turns off. This drives the FET into "avalanche mode" and the FET lives or dies (usually dies) depending on how much magnetic energy is stored in the coils of the motor. Unfortunately, these diodes see the full battery voltage and need to be rated to block as much voltage as the FETs in the controller.

Marty

P.S. The body diode of a 4110 looks pretty husky. An extra FET could work as a substitute for the diodes. Gate and source would go to the motor lead, with the drain going to the battery's positive terminal. Expect the controller to get hotter though.

lawsonuw said:

P.S. The body diode of a 4110 looks pretty husky. An extra FET could work as a substitute for the diodes. Gate and source would go to the motor lead, with the drain going to the battery's positive terminal. Expect the controller to get hotter though.

Click to expand...

I considered it, but didn't want to because I thought I had just enough for the Golden Motor controller and the brushed one. Turns out I only have 13, though. :?

So I'll prolly just do that if the diodes blow. Worst case the whole thing blows up and I have another identical controller as backup. I won't be running the 4110s even close to their limit (only 72V 30A), and given that they undoubtedly have lower resistance than what's already in there, the controller will probably be cooler, and it never really got hot in the first place.

I got the controller open and the diode model numbers are "HBR20100". I don't recognize the logo, but it looks like "HF". Googling and a search through the DigiKey catalogue turned up nothing. :?

Hmm, given that all the numbering schemes of all the other schottky rectifiers are the same (amps then volts), and that the amperage is right for the controller (2 x 20A for a 30A controller), I'm pretty sure they're rated for 20A and 100V.

*crosses fingers*

Looks like I was right. Tests at 72V were successful!