Help me understand VB921's

haenszel · Post by **haenszel** » Tue Jul 25, 2006 5:30 pm

Ok. I'm attempting to run a VW 20v Wasted spark COP setup. Each coil has 4 pins with the ignitors already built into each coil.

1) switched power to primary side of coil
2) ground for built in Power Output Stage
3) 5v DC from ECU to control POS switch
4) Secondary Coil Ground

It is my understanding, from conversations with others that pin 3 is grounded out thus triggering the coil to fire. The other 2 grounds are permanent. Now, the +5v DC does NOT actually have to be 5v. It can be 0v, and simply grounded out when it needs to be fired.

My questions is, I was told as per Steve@RSAutosport:
"The VB921 modules switch ground. So in a normal coil application you hook 12v to a coil and hook the negative side to the Megasquirt."

does this mean that if I hook up pin 3 to the VB module it will ground out, thus firing?

It does not send a +12v signal to the coil, correct?

Help me wrap my head around this, i've been at this all day.

Thanks.

mops · Post by **mops** » Tue Jul 25, 2006 5:54 pm

ok, ok....
so here is the deal....

If it was just a normal coil then it has permanent (+12) attached to the positive side. and to charge it you need to ground it, and once you disconnect the ground again it will spark

Thats easy.

however your coild have ignitors built in.
Now, a big assumption, I assume that your coild have 'dumb' ignition moodules, i.e. they do not control dwell on their own.
In which case NORMALLY ignitor module is just a big transistor, so to make it ground the coil you need to apply +ve (in some cases +12V, in some cases +5V, or pretty much any other voltage in between... but most likely +12V)

so basically the situation is the same as normal coil, except you instead grounding it, you provide +12V....

best would be take out coil and plug, set it up on the bench and then try it out yourself. Mind you coil charging time is about 3ms, so you dont really want to permanently connect that pin3, you just want to flick it together with ground, if that doesnt work flick it across +12 terminal...

mops · Post by **mops** » Tue Jul 25, 2006 5:56 pm

ah, ok... second part of the question....

VB921 grounds the coil, yes.

while you want to send +12

I had the same issue when i was installing my custom ignitor. What i did was built a simple transistor circuit, which inverts ground into +12V for the ignitor

haenszel · Post by **haenszel** » Tue Jul 25, 2006 6:03 pm

no no, i do NOT want to send a 12+.

the coil will NOT fire if pin 3 has voltage. It has to be grounded for 3ms(dwell).

My question is weather or not VB921 actually sends power through the wire connected do it, or if it is simply grounding out? -- from which i believe you answered, it simply grounds, correct?

mops · Post by **mops** » Tue Jul 25, 2006 6:45 pm

oh yeach. vb921 just grounds it, which is what u want...

haenszel · Post by **haenszel** » Tue Jul 25, 2006 6:55 pm

you're a savior, thanks