nead a high speed transistor

Screech said:

As I sad before, I don't know how they work.
The pics that I found on the net don't work.

So I tryed the following and it works.

Click to expand...

Looks fine to me!, but I'm not an 'FET person' either 8)

Obviously it inverts as well though.

From what I've read and seen about FET's you often need a fairly high voltage to turn them fully on, quite often in an audio amplifier the drivers will be fed from a higher supply rail than the output FET's, in order to provide sufficient drive voltage.

The biggest problem with your FET driver is that MOSFET turn-on will be slow. Add a transistor and diode as shown below. It will speed up turn-on a lot. Drive the base of the common emitter stage (your original transistor) fairly hard for optimum turn-off speed. This will also allow you to make the collector resistor lower value, which will speed up turn-on even more. For the NPNs, use something with decent beta at high currents, but still a reasonably fast switch, like 2N2222 (PN2222).

I too relised the output is inverted 8) .
Still have days of testing left.

A big THANKYOU to everyone.
I would not have got this far without you lot

If you have a complimentary output which can pull current high or low, you would usually just hook the gate straight to the driving circuit.

More elaborate gate drivers are usually only needed if you run high speed, at tens of kHz at least. You're dead slow as these things go, pretty close to DC. So I don't understand why you're asking about high speed switches.

What is the circuit which drives the transistor?

Pins are gate, drain, source. Like any transistor, it can act as an analog element or a digital switch. The level of current the transistor allows to flow, whether bipolar or FET, is determined by the base current (bipolar) or gate voltage (FET). When the current your load can draw is less than what the transistor is driven to, the transistor is basically full-on and acts close to a shorted wire.

MOSFET ratings are often fairly bogus, they exceed the current capabilities of the package! As such, I don't even know of a circumstance where that number is a useful parameter.

IF i only have 1 injector connected, it will energise fast.
The more injectors connected, the longer the time to energise them.

whats the circuit thats drives the transistor?

Click to expand...

whats a circuit :?:

Ron, your right .
The fet seems to energize way,way too slow at the curent I'm using.
That fet has a mind of it's own too. Not going to use it anymore.

I'm going back to using bi-polar transistors.
Hammering the one transistor didn't seem to make a difference.
I am now useing two in parrallel.
3 injectors to each transistor.
WOW, What a difference!

Not sure if its the extra cableing, or just using more transistors, but it makes a huge difference in energising all the injectors quickly.

Injector No.2 and Injector No.6 are taking longer then the others to turn on.
Faulty I guess and getting too old.

Will try a mj10012 darlington soon.
Thats the one i found in my custom electornic ignition.

I'm a happy guy now

yes the FET will turn-on slow since the speed that a FET turn's on at (and turn's off at) is related to the charging curve of its GATE region.

You have a pull-down trasistor to pull the charge out of the gate capacitance very fast BUT you have a pull-up resistor to charge up the gate capacitance. It is that pull-up resistor that is really slowing you down.

You need a decent gate-drive circuit to really switch the FEt (and IGBT's) as fast as they can be switched.

Change the Top resistor for a NPN transistor and the bottom NPN for a PNP. Common the two bases together (to come from your control signal) and also common your two emitters together to make a PUSH-PULL trasistor arrangement. That is one of the best gate-drive cct around.

Aslo dont forget to put at least a 10R resistor between the Emitters of your PUSH-PULL and the FET's gate, this will limit the inrush current to the gate of the FET and minimise any oscillation due to stray inductance and the millar capacitor (I think it is called a pierce oscillator).


Please dont give up on FET's BJT's are nice but FET and IGBT's have their advantages (esp since they are voltage controlled)