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Blown Transistor
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I have just reinstalled my hard cut limiter after diagnosing some problems with the my MJLT V3 (all Sorted Now) and i think i have blown the transistor Q1.
I lowered the rev limit to 2000 rpm to test and it all worked fine, i was demonstrating it to a friend and held the engine on the limiter for about 5 seconds, then the engine started to rev again, the advance is going to 0 degrees at limiter point but the engine does not cut.
So with my limited knowledge i am assuming that i have blown Q1.
Can any one tell me:
A) will the engine run propely if UN1 was damaged and
B) how long can you hold it on the limiter before damage happens..

Many thanks

Andy

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someone must know!!!

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Sorry for missing your post.

If Q1 is damaged it wouldn't forward the signal to the MOSFET, to activate/deactivate the hard limiter. I haven't considered this transistor as a particularly vulnerable component. Now, we do know if the Rev Limiter module does not have a proper ground between it and the MJLJ unit, damage can occur quickly to the MOSFET.

What you were doing it should be fine. However, Q1 acts as a logic signal inverter and therefore bears no electrical load: the load during rev limiting is brunt by the resistor array- this is what gets warm while the limiter is activated. The rev limiter can sustain several minutes of continuous high RPM rev limiting at room temperature. I have a temperature plot kicking around somewhere.

In it's current state, when you're trying to activate the rev limiter, do any of the components get hot?

During normal operation the MOSFET is "on", bypassing the resistor array. When the limiter is activated, the MOSFET is turned *off*, routing coil power through the array, attenuating the spark energy.

Have you checked all of the connections between the MJLJ main board and the rev limiter? Do you have the test equipment and comfort to poke around the rev limiter board and check voltages? If so, we can do some quick checks.


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Brent Picasso
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brentp wrote:
Sorry for missing your post.

If Q1 is damaged it wouldn't forward the signal to the MOSFET, to activate/deactivate the hard limiter. I haven't considered this transistor as a particularly vulnerable component. Now, we do know if the Rev Limiter module does not have a proper ground between it and the MJLJ unit, damage can occur quickly to the MOSFET.

What you were doing it should be fine. However, Q1 acts as a logic signal inverter and therefore bears no electrical load: the load during rev limiting is brunt by the resistor array- this is what gets warm while the limiter is activated. The rev limiter can sustain several minutes of continuous high RPM rev limiting at room temperature. I have a temperature plot kicking around somewhere.

In it's current state, when you're trying to activate the rev limiter, do any of the components get hot?

During normal operation the MOSFET is "on", bypassing the resistor array. When the limiter is activated, the MOSFET is turned *off*, routing coil power through the array, attenuating the spark energy.

Have you checked all of the connections between the MJLJ main board and the rev limiter? Do you have the test equipment and comfort to poke around the rev limiter board and check voltages? If so, we can do some quick checks.


Hi Brent thanks for the reply.

The rev limiter is in the main case and takes the ground from the power connector, so this should be ok.

I have the means to test voltages but not wave forms so i can do some basic stuff.

I have called into the local electronics shop and bought a 2n3906 so i will replace this first as it was only 27p.

From what you say, could be a dodgy ground connection? this would have taken out the MOSFET. would this have the same symptoms? i.e if it not working would it still conduct. all this pnp and npn realy confuses me with my schoolboy electronics....

I will fit the transistor this week end and let you know the results.

regards

Andy

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Well its not Q1, i replaced it today and the symptoms are the same...

I carried out some tests on the trigger and i get 12volts when the revlimiter is NOT active, this drops to .6volts when the limiter is active. i assume this is correct which shows that the MJLT is performing correctly....

I can only think that UN1 is defective, (not looking forward to trying to replace that) I will try to get one localy an have a go..

I rechecked the ground and it is ok using the -ve off the MLJT socket as it goes into the board. Just wondering if it suffered static damage when out of the car for so long, does this sound reasonable?

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When activating / deactivating the rev limiter, what voltages are you seeing into the gate of the mosfet? It's that lone trace that goes to the mosfet device, viewable from the top. test from the vantage point of Q1.

It's not inconceivable that it was damaged when it was outside of the unit- but it's hard to tell after the fact. PM me if you have problems finding that part.

Regards,


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