I have not even opened my kit yet (it came today) but I would like to get started on the coil driver circuit design so I can order parts if I need to.

I am going to be building a replacement for the CDI module on my old motor cycle (parts no longer available), and I am wondering if 3 parallel mosfets from the kit should be able to safely drive a 12v ignition coil, and what sort of circuit protection would be best... I have spares of everything, but I don't need coil feedback killing stuff.

Should be some interesting programming once I get the circuitry figured out.

Hi huzbum,

Welcome to the forums!

We used a inductive spark generator. I also no longer have the details of that project, but there was some discussion here. I doubt that three of the 2N7000's in parallel would have enough current-handling capacity to produce a good spark.

CDI = Capacitor Discharge Ignition -- is that correct for your motorcycle? Are you trying to fix the CDI system, or to replace it with an inductive-type spark generator? It sounds like the latter, but I just wanted to check.

Mike

I am intending to replace the CDI (Capacitor Discharge Ignition) system with a programmable micro-controller. I will use input from the pulse coil and alternator to estimate engine speed and position then dynamically determine spark timing in software.

The coil driver circuit needs to supply 12 volts to the ignition coil (from the motor cycle battery) until the calculated time of spark, in which it needs to brake the circuit quickly and precisely while isolating the sensitive micro-controller from any high voltage feedback coming out of the coil.

the discussion you referenced has a link to this The first schematic looks like about what I am thinking, but with a MOV in place of RC1 and maybe a diode or two between the transistor and the coil.

I will be adding more features later, but for now this is the important one. I'll probably get to it this weekend, maybe radio shack will have what I need and I won't need to wait.

I picked up a pair of 2n3055 transistors at radio shack. These babies are pretty heavy duty. Didn't have any zener diodes or MOV's, so I got a couple rectifier diodes rated for 400v. I figure one of those reverse polarity across the coil should quench most feedback.

Hi huzbum,

A reverse polarity diode across the coil may be good for protecting the rest of your circuit, but it might severly degrade your ability to generate a spark. You need a rapid change in primary coil current in order to generate a large voltage in the secondary winding of the transformer, and there's going to be an inductive part of that in the primary. If you "quench" the feedback, you're also allowing the current to decay slowly (related to the L/R time constant of the primary winding), instead of forcing the current to shut off quickly (but, as you point out, in a way that can be ugly / noisy / spiky). Does that make sense? Let us know how your experimentation proceeds!

Mike

I guess I don't follow...

My understanding is that if you stop the current going into the coil (done by the transistor switching) then the magnetic field collapses inducing a voltage in the secondary, but continues through the core and induces a reverse polarity "ringing" back into the primary. My thinking is I would just short out any reverse charge with a diode across the primary.

I don't see how this would prevent a build up of the magnetic field or prevent it's collapsing. what am I missing?

Yes, the clamping diode does short out the reverse charge, This is primarily to protect the rest of the circuitry connected to the same system.

The 'ring back', or inductive kick is useful in an ignition circuit. Ignition systems have circuitry to deal with the kick back so it causes no harm yet allows the coil to do its job effectively. If you clamp the primary with a diode, it will also affect the output of the ignition coil.

okay, so I changed my plan a bit.

I studied some CDI schematics and decided I needed to hop the voltage up with a transformer and discharge it through a capacitor like CDI does.
Here's my basic schematic:
There will be resistors in there (of course), but that's just my general concept. Please tell me if I have something terribly wrong. I can probably figure it out, but suggestions of resistance values and locations are certainly welcome too.

The software will send opposite pulses to the two 2N7000 mosfets which will drive the two 2n3055 transistors, each powering opposing coils in the transformer to produce an alternating 250-300v current in the secondary coil to charge the capacitor.

Check this link for a pretty decent schematic for a CDI ignition setup.

This one uses a clamping diode but it's because this is a true CDI system. The capacitor on the upper right in the circuit is the work horse.

CDI circuit

So after looking at several CDI schematics I think I have figured out how it works. I was under the impression that the capacitor discharged it's entire charge through the coil and timing is determined by when power is cut to the capacitor. This is not the case.

Correct me if I'm wrong, but my new assumption is that the capacitor holds a charge (and builds a magnetic field) in the coil until the SCR (which I did not recognize until I looked up it's schematic) is triggered shorting the capacitor to ground discharging the capacitor.

I also confirmed that my bike has a high voltage output from the alternator for the CDI system, so I eliminated the inverter and transformer. It's a much simpler circuit. If you refresh you will see the updated schematic above.

Am I correct in assuming that the SCR will protect the trigger circuit? Would it be safe to switch the SCR with a 2n7000 mosfet? If not, what sort of additional protection would you suggest?

Take a closer look at the polarities in the link I posted earlier.

The capacitor (C3) charges through D1 and D4 (and a little through the coil primary).

When the SCR fires, it discharges the capacitor across the primary side of the coil. It's a higher voltage than the normal 12V in a car, (around 800V to 1000V) so the capacitor value is relatively small. This allows for a rapid charge and discharge rate, useful in the ignition circuits. D4 doubles as a clamping diode and prevents reverse polarity spikes from getting back into the system but allows just enough to unlatch the SCR, making it ready for the next cycle.

The 2N7000 should be able to handle the trigger for the SCR. Just stick a resistor in line with it just in case the kickback has enough power to fry it. Maybe something like a 3.3K or something in that area.

The 470 uF cap might be a bit big for a fast rate but will have quite a punch.

I was looking at that schematic, and it looked to me like no significant amount of power would ever go through the coil, but I think I've finally got it figured out. Thanks for keeping with me, It's been a long time since I've worked with schematics and semiconductors.

C3 charges positive on the D1 side and negative on the D4 side, correct?
When the SCR trips, it shorts the positive side of the capacitor to the ground, making the ground positive (relative to the primary inductor), making D4 block flow, leaving the primary inductor the only path to the negative side of the capacitor.

I think I was hung up on the ground being the positive side of the coil.

I updated my schematic to reflect the changes.

LoL would you believe that Radio Shack doesn't have any SCRs and didn't even know what I was talking about? What would be a good place to find a 400V+ rated SCR? Or do I have to make one from 2 transistors? The stuff in my bike is all rated 400V.

One more thing you need to be aware of.

If you trigger the SCR in this configuration, it will also short out the power source.

R1 and D2 are the isolation form the power source at the ignition breaker. Don't let the line through the exciter coil confuse you, it is just an electromagnet across the same feed to ground. the 22K 2 watt resistor limits the charge current and also prevents the SCR from shunting the power source.

I looked up the wiring diagram for the bike, and you're right, one side of the exciter coil is hooked to ground. That being the case, I suppose I could get away with one diode instead of a rectifier and just use half of the wave to charge the capacitor... it looks like that's what most of them do anyways. Would it be necessary to use a diode and resistor to "burn off" the unused half of the wave?

An 'SCR' (Silicon Controlled Rectifier') is also known as a 'thyristor'. That should help you find what you're looking for :P. They are very useful.

BM

could just get the repair mainual for any automobile and take a look at the schematics section