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  #11  
Old Wed 1st June 2016, 11:57 PM
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Lightbulb Sticky ...

Awesome write-up there Sam ...

If I ever consider doing this in the future, be sure to bookmark this for future reference ...

P.S. - Someone should make this a sticky as well ...

Ciao, Bantum ..
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  #12  
Old Mon 4th July 2016, 10:14 PM
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Explanation of the OE ignition coil and tach

After some experimenting with the EDIS coil pack and stock tach I found it is not possible to drive the stock MY tacho with the coil pack. At least not with diodes anyway, I'll explain below but beware! technical jargon ahead, you don't have to read it mainly to get it out of my own head hehe

I was almost right in my explanation of how the ignition coil works, and how it drives the tacho. The way the ignition coil actually works is that it is always "on", as in it always has 12v going to it and is always grounded *in between spark firing events* which builds up a magnetic field in the primary winding of the ignition coil. A spark event is caused by the points (or ignition module) breaking the circuit for the coil (in this case the coils connection to ground) which causes the built up magnetic field to collapse. As this magnetic field collapses an extremely high voltage is produced in the secondary winding of the coil which is what fires the spark plug(s) The same things happens in an A/C transformer which is how you can turn low voltage A/C into high voltage A/C or vice-versa. The ignition coil is basically a transformer, but using DC constantly switched on and off.

In addition to the secondary voltage, a spike of high voltage is also sent down the negative side of the PRIMARY windings of the coil. If you have any understanding of how inductors work then this will make sense. And this is where the tacho comes in; this spike of high voltage is sent down the negative side of the coil which the tach is connected to. The circuitry in the tach takes this spike of high voltage for each spark event and turns it into a needle movement with a simple circuit and IC chip inside the cluster. My theory is that as soon as you put a diode in this line to the tacho, that high voltage spike is wiped out by the diodes and the tach sees nothing. So when you connect the 2 separate negatives of the EDIS coil pack to the tach through some diodes, of course it won't work! And you can't just joint the 2 coil negatives together to the tach without diodes as that defeats the whole purpose of the 2 coils and might even damage the EDIS module.

So my plan is to build the circuit which I've added to the main posts. Using a gutted relay (using the relay coil only) and a transistor, hopefully this will simulate the original ignition coil (relay coil still makes a magnetic field, which will collapse when switched off and make the same high voltage spike) The signal to turn the transistor on and off will now have to come from the Megajolt ECU, not the coil pack.

Fingers crossed this will work, if it doesn't then I'm out of ideas on how to drive the stock tacho with EDIS
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  #13  
Old Tue 5th July 2016, 08:44 AM
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Why not use a simple op-amp in inverting mode from the tach output of the edis module and use it to turn the positive pulses into negative pulses?
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  #14  
Old Tue 5th July 2016, 06:54 PM
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Quote:
Originally Posted by Gannon View Post
Why not use a simple op-amp in inverting mode from the tach output of the edis module and use it to turn the positive pulses into negative pulses?
In theory you'd think that would work, but I'm willing to bet it wouldn't work with the stock tach because the tach needs that high voltage spike (maybe 20v or so in this case?) from the negative side of a coil as its magnetic field collapses. Also my previous attempts at making circuits with op-amps hasn't been very successful the transistor+relay coil idea maybe I can handle

Also, for anyone still reading; the above spiel about collapsing fields in electrical coils is exactly the reason why it is good to have a diode across the coil of any relay (some relays you can buy with this built in, or sometimes it is a resistor in cheaper versions) to protect any circuitry that might be downwind of a relay that could be damaged by that high voltage spike. Probably not so important in cars, very important in PCB's with relays.
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  #15  
Old Mon 13th March 2017, 11:08 AM
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I know this conversion is done and dusted now but I just thought I should update the stock tachometer situation in this thread. As I said in earlier posts, the stock tacho doesn't work on the EDIS signals - I didn't end up doing the hacked relay trick because I was convinced of a better option. I ended up pulling the instrument cluster apart, removing the stock tacho PCB and making my own from scratch using the 555 timer IC chip. The 555 tacho circuit is as old as the hills; tried, tested, simple and effective. I initially had some problems after I made the circuit with it not working, or the needle being very jumpy and vibrating. It took me a long time to work out why - all the circuits for this you'll find online are just what some electronics whiz has dreamed up and what he thinks should work, most of them haven't even been tested on the bench let alone on a car

Here is the circuit diagram I initially followed:



This circuit has a number of problems:

1. The input resistor (15R) is completely wrong, with 15 ohms here the 9v zener is dissipating something like 4-5 watts! With most zeners being 1-2w rated I went through a few zeners before I realized what was happening. A 5k resistor here is much better.

2. The timing resistor and capacitor on pins 6 and 7 are are bad choice (especially for this application) This choice of values gives an output pulse of .001 seconds duration. The end result of this is a shaky/vibrating needle since there's too much off time between the output pulses going to the meter. I worked out the maximum on time I could have for each pulse at 10,000rpm was something like .012 seconds, so I changed the resistor to 9.1k ohm and the capacitor to 10 microfarad to get .010 second pulses. This is when taking a signal from 1 coil only out of 2 so I'm getting half the pulses than if I was reading off both coils.



After this it worked much better. To calibrate the circuit on the bench, I found a 240v to 16v a/c transformer and connected one line of the 16vac to the input, the other line to ground of the circuit. This triggered my circuit at 60hz mains frequency, which on a regular 4cyl car would be 3000rpm. On mine because I'm reading off of one coil, I calibrated it to 1500rpm.

I borrowed the oscilloscope I use at work and it can save screen grabs to USB so:



The blue line is the 60hz mains sine wave (stepped down to 16v) you can see as soon as the sine wave goes negative, the tacho circuit triggers and makes a positive pulse at the output for the set duration. If you count the squares, you will see it is exactly .010 seconds The constant pulses going to the needle movement is smoothed out by the movement giving a stable reading at the needle. As the input frequency increases (higher revs) the output frequency will increase and the needle will move higher.

If you're interested, this is the signal coming from the ignition coil:



This was with the probe at 10x division so each square represents 10v. You can see the coil sits at 12v approx until the EDIS module disconnects it from ground for about 3 milliseconds. At this time, the coil fires from the secondary to the spark plug but also there is a huge 40v spike on the primary. This is the spike I was talking about it earlier posts, you get this when you turn off any inductive load (coils) e.g relays which is why they have supression diodes. The 5v zener, 1k resistor and .1uf capacitor in the tacho circuit prevents this spike from damaging the 555 IC.




Okay so the last little bit was probably not necessary to explain and maybe too technical but I thought I'd post the scope screen shots anyway, so you guys can see what is really going on when I'm talking about pulses and duration etc etc. Oscilloscopes are great fun
Attached Images
File Type: gif 555-tacho-fig1.gif (3.5 KB, 18 views)
File Type: gif 555-tacho-fig2.gif (12.9 KB, 18 views)
File Type: jpg Tach (1).jpg (50.6 KB, 18 views)
File Type: jpg Signal.jpg (43.8 KB, 18 views)
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Last edited by Silverbullet; Mon 13th March 2017 at 02:57 PM.
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  #16  
Old Mon 13th March 2017, 04:45 PM
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Nicely explained Sam.

When is the car going to be finished

TOONGA
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  #17  
Old Tue 14th March 2017, 08:28 PM
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That's crazy mate. Where did you learn all that stuff?

It was almost like learning a new language for me!

Top effort once again!

Cheers

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  #18  
Old Fri 17th March 2017, 06:28 PM
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Bennie - it's part of my job to understand this stuff at least to some degree Trust me it was like a new language to me too, took me ages to wrap my head around what was going on and this is a really basic circuit.

Jules, it will be finished when it's finished Hopefully in the next 6 months. There's alot of fiddly little things to do that take ages (like the tacho circuit ) I'm nearly ready to put the carpet in, just have to do 1 more front seat mount. Then it's all interior stuff...and exhaust...and front struts...well there's a bit more to do yet
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