Every wonder how an MSD works?

[rick-l]

I was curious how they put out multiple sparks with only the points as an input. I got the schematic from http://www.zublin.com/msd/msd6a_02.pdf and did a spice simulation.
I uploaded the LTSpice files here http://tech.groups.yahoo.com/group/LTspice/files/%20Temp/ (MSD_Timing.asc). You have to be a member of Yahoo LTspice users group to download it however.

This is the clever feature (Timing One Shot). This one-shot outputs a pulse (labeled tach in the SPICE traces) whose pulse width is a function of input frequency or rather input pulse period. This triggers a free running multi-vibrator that is active while this pulse is high. This puts out pulses that triggers the SCR at about a 1.1 kHz rate (labeled trig_r in the SPICE traces)

Timing One Shot


When the one shot is triggered the timing capacitor C7 discharges through R17 until Q6 turns back on and puts it in the stable state. When it is in this state the timing capacitor C7 charges through R16 until the next points opening triggers it (labeled mag_in) repeating the cycle. The longer it stays in the stable state till it is retriggered the bigger the charge on the timing cap and the longer the triggered pulse will be.

The voltage on the capacitor is shown as V(N013, one_shot_tim) N013 is the colector of Q4.

IDLE 800 RPM


3000 RPM


7000 RPM


I have heard these units are less than completely reliable. What gives out?

[neilca]

The only time I have killed an MSD is by welding on the chassis without disconnecting the unit. Other than that I say they are bulletproof. I am currently running a 6T box that is about 15 years old.

[RWebb]

Rick - thanks much for your information.

I was never very conversant with this stuff, and it was decades ago... can you refresh my memory a bit?

Is it astable? bistable?

and....

the input pulse flips the multivibrator, which consists of Q? and Q?2, to initiate the output pulse train. The end of the pulses occurs because _____?

And what do all the other active devices do? Q7 and D3 so if you can make a m/v out of 2 transistors, the other 5 or so must be amps? signal conditioning? are the diodes for protection or what?

my bet is that few people remember much about analog circuits anymore...

[rick-l]

Here are some definitions from Wikipedia.

• astable, in which the circuit is not stable in either state—it continuously oscillates from one state to the other.

• monostable, in which one of the states is stable, but the other is not—the circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. A common application is in eliminating switch bounce.

• bistable, in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. Such a circuit is important as the fundamental building block of a register or memory device. This circuit is also known as a flip-flop.

This is the circuit that generates the multiple spark pulses. It is very similar to the one shot that generates the TACH signal.



When TACH goes high it turns on Q9 which turns off Q11 (base current shunted through the cap). This turns on Q10 (Q11 off - base current through R29 and R28) and holds Q10 on until C10 charges to Vbe of Q11. The charge current for C10 being provided by the biasing of Q12. With Q10 on current flows through the base of the PNP Q7 and R25 which turns Q7 on which then turns on Q8 and holds the trigger (base to Q9) low.

When Q11 turns back on (end of pulse) Q7 remains on due to the base current supplied through R25 and C10 until the voltage on C10 gets close to 5 volts (Vbe of Q7). When C10 is charged and Q7 shuts off this in turn shuts off Q8.

With Q8 off if TACH is still high Q9 turns on which retriggers the one shot and the cycle repeats. If not it sits and waits for the next TACH signal.

[rick-l]

Has anyone taken these things apart? Can you access the components inside?

I was going to buy one to play around with.

[RWebb]

Yes. Drill out the rivits etc. and you can see this:
http://forums.pelicanparts.com/porsche-911-technical-forum/450862-msd-ignition-unit-internals.html#post4412371

I'll pull it apart more when I have enuff free time.


I am still not clear on what type of multi-vibrator it is. I know the def'ns but am not conversant enuff to figure it out fromt he circuit topology or description you posted...