| ??? 06/18/08 17:38 Modified: 06/18/08 17:40 Read: times |
#156010 - Most MOSFETs have those built-in. Responding to: ???'s previous message |
Jack Peacock said:
A solenoid is basically an inductor. The coil of wire stores energy in a magnetic field. When you cut power to the coil, the field collapses and discharges a relatively high voltage spike back through your circuit, right into the FET. To protect the FET you need a good fast breakdown Schottky diode to drain the discharging coil.
Some FETs are packaged with flyback diodes. Look at stepper motor circuits; it's the same problem. I use Freescale MC33886 half H-bridge drivers (rated at 5A) for both solenoids and stepper motors, no extra diodes required. I have to agree with what you say here. I'd say it's the majority of MOSFET's, though. However, not all MOSFET's will work in all applications. There's been no discussion of how the failed MOSFET was being driven, for example. Judging from the O/P's choice of a 1N4148, which is a small-signal diode, I'd guess he didn't consider the power in the circuit. A rectifier like one from the 1N400x series would be a better choice, though it may not even be necessary. Perhaps his MOSFET was simply too weak. If he wants to use a MOSFET, an IRLD120 is one that would probably work just fine and not require complex driver circuitry, as it's happy with logic level gate drive. RE |
