MaaTrrIxx wrote: ↑
Fri Jan 19, 2018 2:38 am
I thinks this will work, IRLB4132, am I in the right?
MaaTrrIxx wrote: ↑
Fri Jan 19, 2018 3:32 pm
David I have a question the IRLB4132 will it work?, and if I put a diode betwen the injector and the output of the mosfet, it will protect the transistor?
The spec's on the two FETs you listed are generally OK, except the VDS
(max voltage drain-to-source) is quite low. A new problem, that the device the FET is operating is often an injector, solenoid, etc, which are inductors, and they produce voltage spikes called flyback
. Your idea of using a diode is good, except that stopping the spike at the device would be a better approach, in order to protect the FET. So, rather than a diode in the wire between the device and the FET, place the power diode reversed across the device's power pins, as in the link above. A common part for this purpose in EFI is the 1N4004. This shunts the voltage spike and feeds it back into the device (shorts the spike), protecting the FET, and also reduces heat by not using the FET's built-in diode as much.
But . . .
Adding a shunt diode can slow the operation of the device as it feeds power back through itself due to the diode.
The most common solution is to use a Zener power diode, which only shunts the voltage above the Zener rating. A Zener rating for a 30V FET might be around 25V and at least 1W. This allows the diode to only shunt the dangerous voltage levels above the Zener rating, speeding device operation.
Given the added cost and complication of the Zener power diode solution, the alternatives are to use a 'protected' FET and live with more heat, or use a standard diode and deal with the slower device operation. These are your decisions. I hope that makes sense, and that it helps.
PS: Another solution is to use a gate driver, which drives the FET gate and protects Speeduino, such as the TC4424 used for Speeduino's ignition output. However, this is not included in the solutions above as it is even more expensive, but primarily because it is more complicated and difficult to hack into an existing