Cooked circuit for the -ve coil signal to drive Tach :(

Tman · Post by **Tman** » Mon Apr 07, 2008 4:21 am

Ok so biult the curcuit,

,about a week back and it all looked good and worked well. Was made as the diagram with the 2x 1n4004 and the 18V zener.
car is still being biult but was doing a bit of tuneing the other day and left the ignition on for a couple of mins with out the engine running and next thing i know all the magic smoke that they put in the componints that makes them work was excaping....
So my question,

Why would this have happened?
How can I stop it fro happening?
Can I make it with 3A Diodes with the hope they can handle a bit more?

Thanks

Timon

MartinM · Post by **MartinM** » Mon Apr 07, 2008 9:13 am

Which components let out their smoke - all of them? Or just the Zener and one of the 1N4004? And any idea if the tacho is still working?

Then maybe some failure mode can be hypothesised - but more info is needed to start with...

Tman · Post by **Tman** » Mon Apr 07, 2008 9:58 am

Ok both the 1n4004 diodes have bruned them selves into non-existance. The zener was starting to get warm but by look of things id say it was just due to the heat transfer.
I had mounted it all on to a little pcb board and srinked wraped it all.
The diodes are totaly crispy and literly coming to bits

At the time I wasnt keen to put the ignition back on and have since removed the little circuit board from the car.

Thanks

david jenkins · Post by **david jenkins** » Mon Apr 07, 2008 10:14 am

The shrink-wrap might have 'helped' by not allowing the diodes to keep cool.

MartinM · Post by **MartinM** » Mon Apr 07, 2008 12:23 pm

The only way to get IN4004's to release their smoke is to have their 'pointy-ends' at, or via a pretty low resistance to, earth potential, whilst their blunt-ends are at more than about 0.8v

This is never the case in normal operation (ignition on, for however long, engine running or not) since the only possible connection to earth is via the tacho and that should have a high resistance to earth (if its the type that reads the flyback voltage)

Even if you replaced the tacho with a short circuit to earth the Zener will stop any current flow through the IN4004's unless your battery voltage is over 16-19 volts approx

...so it sounds like you had a short circuit somewhere

As far as I know there is nothing wrong with the circuit and it has been used many times - the only mod I've seen (which has been the difference between a working and non-working tacho) is to pull-down the junction of the blunt-end of the Zener and the tacho with a 1K resistor, so that the tacho sees a +ve voltage and real earth, rather than a +ve voltage and open circuit

Tman · Post by **Tman** » Mon Apr 07, 2008 9:37 pm

Ok thanks for that. I dont quite under stand your last statement with the 1K resistor to pull-down the junction of the blunt-end of the Zener and the tacho, but the tacho was working fine so i dont see the need to go there just yet.
Ill check for a short. The ciruit and the wires to the two -ve on the coils should be fine as it was all shrink wraped nice and tidy. It is conseavible that the original wire to the single wire could have shorted, but I was pretty sure this was not just sitting against the body of the car or anything. If it was then am i to understand that the zener should have protected against that anyway ? If it was would the extended time of the coils being charged and the short to earth on the other side have loaded up the circuit leading to heat and the melt down? I fine it strainge that it was the two diodes and not the zener that fryed???
*EDIT* Oh and it would be ok to make it with the higher rated 3Amp diodes instead of the 1Amp ??
Thanks