Measuring dwell / charge current

hobieboy · Post by **hobieboy** » Fri Aug 18, 2006 5:35 am

The megamanual suggests to place a 0.1ohm between the emitter & ground of the VB921 to measure the current ramp. In my case, it would be MUCH easier if I can put that resister between MS output and collector of the VB921.

I don't think it will work but can anyone confirm please?

thanks.

Philip Lochner · Post by **Philip Lochner** » Fri Aug 18, 2006 7:09 am

hobieboy wrote:The megamanual suggests to place a 0.1ohm between the emitter & ground of the VB921 to measure the current ramp. In my case, it would be MUCH easier if I can put that resister between MS output and collector of the VB921.

I don't think it will work but can anyone confirm please?

thanks.

This resistance will of course add to that of the coil but it should have minimal effect on the operation of the MS and the coil. Be VERY careful about connecting up the scope IF you place this res between output and collector. The earths of the scope channels are connected so you could end up shorting something and I suspect you will battle getting the scope triggered. It would therefore be better to put it between emitter and ground such that your scope earth will then also be connected to ground.

Why would you want to do this anyway? You can accurately calculate the coil primary current with the following formula:
i = V/R(1-e^(-R/L*t))
V= volt, R = coil resistance, t = time, L is inductance of coil in Henry. I have measured 3 coils, and they all have inductance of between 3.9 and 4.8mH.

hobieboy · Post by **hobieboy** » Fri Aug 18, 2006 7:24 am

I have measured the resistance & inductance already so theoretically I don't have to do this but thought I should verify in real time that the rest of the circuit is functioning properly?

Maybe all I need to do is to make sure the VB921 is not hot - but then I also have a not-so-accurage temp sensor to warn me.

jsmcortina · Post by **jsmcortina** » Fri Aug 18, 2006 7:53 am

Philip Lochner wrote:Why would you want to do this anyway? You can accurately calculate the coil primary current with the following formula:
i = V/R(1-e^(-R/L*t))

Due to the voltage correct and any discrepancy between the commanded dwell and actual dwell (something of an issue with MS1/Extra at higher rpms and wasted spark) doing the empirical test is valid.

The V3 board already includes the sense resistor to measure the coil current.

James

ptegler · Post by **ptegler** » Fri Aug 18, 2006 8:12 am

...but overall coil current is NOT the objective to
measure here... it's the rise time of the coil current so
the proper dwell can be set.

The .1 ohm resistor is simply to give you something to
have a voltage drop across so you can measure the signal.
Probing the drain of the of the VB will give you a
differentail back to ground to see the signal. BUT you'll
be looking at an inverted spike rather than a 'rise' in
the scope trace.
do-able... bot not neccessarily as easy to read.

ptegler

On Fri, 18 Aug 2006 08:09:15 -0700
"Philip Lochner" <philip@gpltel.com> wrote:

hobieboy wrote:
The megamanual suggests to place a 0.1ohm between the
emitter & ground of the VB921 to measure the current
ramp. In my case, it would be MUCH easier if I can put
that resister between MS output and collector of the
VB921.

I don't think it will work but can anyone confirm
please?

thanks.
This resistance will of course add to that of the coil
but it should have minimal effect on the operation of the
MS and the coil. Be VERY careful about connecting up the
scope IF you place this res between output and collector.
The earths of the scope channels are connected so you
could end up shorting something and I suspect you will
battle getting the scope triggered. It would therefore
be better to put it between emitter and ground such that
your scope earth will then also be connected to ground.

Why would you want to do this anyway? You can
accurately calculate the coil primary current with the
following formula:
i = V/R(1-e^(-R/L*t))
V= volt, R = coil resistance, t = time, L is inductance
of coil in Henry. I have measured 3 coils, and they all
have inductance of between 3.9 and 4.8mH.

------------------------
Kind regards
Philip
Land Rover Discovery MS-2/V3/2.36/LC-1/4.6 V8i, VR(6
BTDC), TP100, std coil - EDIS install in progress.
Jaguar XJS V.12 5.3, MS-2/Dual WBO/MSD6A/ignition +
fuelling

Paul Tegler
ptegler@cablespeed.com
http://www.teglerizer.com

Posted by email.

hobieboy · Post by **hobieboy** » Fri Aug 18, 2006 9:33 am

Paul - understood... its the rise/fall time that I was hoping to scope. Since I'll be doing 1 coil at the time, rising/falling should be the same wrt looking at time.

James, I'm running 6 off-board VB921 so the V3 board provision cannot be used unfortunately

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