Page 1 of 1

Rev-limiter operation

Posted: Fri Oct 17, 2008 8:02 pm
by hcp474
Ok, let me get this straight;

normally the trigger is low (off) and the SMBT... does not conduct, making the gate -ve via the ground link.
then when trigger'd the SMBT conducts and the makes the gate +ve which stops the flow through the MOSFET.

is that correct?

i am trying to modify this arrangement so that the MOSFET is off when the trigger is off, then when there is a +5v signal the mosfet is turned on. but am encountering problems (probably in my choice of Mosfet (STD30PF03L) and transistor (FMMT491TA)

any help would be great,

Cheers,
David

Posted: Sat Oct 18, 2008 4:19 am
by brentp
David,

The MJLJ rev-limiter output is as follows:

Below rev limit: high impedance (open circuit)
At or above rev limit: connected to ground

The SMBT2907A is there to invert the output so the P-Channel mosfet is on when the rev limit output is de-activated.

To have the mosfet on when the rev-limit output is enabled would involve eliminating the SMTB2907A and having the rev limit output drive the mosfet gate directly. Of course, you would need a pull-up resistor on the gate when the rev limit output is inactive.