Why High & Low Stator Windings on Mercury Ignitions - Related to Bias Circuit? How?

[VivaLaMigra]

Why High & Low Stator Windings on Mercury Ignitions - Related to Bias Circuit? How?

OK, first off, why does the Mercury stator have high and low AC outputs for the CDI pack, when OMC can get along just fine with one output? Ultimately the capacitor has to be charged from a single source, right? So, why two outputs, and why two voltages? I read on another thread that the bias circuit somehow controls timing and compensates for the trigger voltage increasing at progressively higher RPM which would result in advancing the timing since the 0.6V threshold for the switching transistor would be occur earlier. OK, why doesn't THAT happen on "Johnrudes?" And, on an L6 or V6, you have two packs linked together via the bias terminal. On a 3-cylinder, one TRIGGER wire is connected to the bias terminal. Is there a bias terminal at all on a 4-cylinder pack?

I'm just a bit in the dark here about this whole 'bias' deal and especially about the two stator voltages which is a feature of the 3, 4, and 6 cylinder motors. Anyone out there who can put this in terms that a relatively intelligent person, without an EE degree, can grasp? Thanks.

 

[GA_Boater]

Welcome aboard.

What problem are you working on? Most of the answers to your questions are in the minds of the designers.

 

[Scott Danforth]

two different companies, two different ignition design philosophies. most of the engineers that originally designed either have long since left the companies or have passed.

 

[Faztbullet]

On Mercury the stator has low speed charge coils and high speed charge coils, mounted on the stator under the flywheel. Each of these supply voltage to the switchbox, red, red/white and blue, blue/white wires. The High speed will vary depending on rpm from a low of 20V at cranking to as high as 300V at 4,000+rpm. Usually around 180-200V. The low speed will have an output of approx. 200V through it's rpm range.
OMC does not use multiple, output charge coils in most of their models.
This voltage from the charge coil is sent to a capacitor in the switchbox, where it is stored(DC VOLTAGE).There is another set of coils, the trigger coils also under the flywheel. As these coils discharge voltage they send it to a SCR in the switchbox which in turn acts to discharge the voltage stored in the capacitor.This voltage is in the 5 to 10v range.The voltage is then sent to the individual coils. This voltage is around 180-200V. In the coils it is stepped up to 20,000 to 40,000V.

A little light on the trigger/bias issue. An SCR fires when the gate reaches +0.6v with respect to the cathode. The cathode is at ground potential when the circuit is inactive (through the coil primary). However, the trigger signal is not referenced to ground. The trigger is cross-connected between switchboxes and is referenced to the white/black connection. The bias voltage on the white/black is negative with respect to ground. The trigger voltage has to overcome the negative bias on the white/black plus go 0.6v positive before the SCR fires. The more negative the bias, the longer it takes the trigger voltage to reach the 0.6v threshold and fire the SCR. So the bias circuit doesn?t really vary the amplitude of the trigger ? it varies the trigger reference.

The trigger amplitude is between 5 and 10 volts peak. It rises to 30 to 35 volts at 5000+ rpm. Likewise, the bias will be in the ?30 to ?35 volt range at the same rpm. The bias circuit is designed to keep the point at which the SCR gate voltage reaches +0.6v constant over the full rpm range. The trigger voltage increases with rpm and would advance the timing more and more if it weren?t for the bias voltage increasing (in the negative direction) at the same time.

The various advance/retard/idle modules monkey around with the bias voltage under certain conditions. For example, the typical high-speed spark advance module cuts the bias voltage in half (and thus advances the timing) above a certain rpm.

 

[gm280]

Faztbullet, that is a pretty good write up. And since I know all about electronics, it was interesting as well. Thanks! It does seem a convoluted method to accomplish such things, but whatever it took at that time I guess.

 

[VivaLaMigra]

Faztbullet: I got all THAT off another tread. Makes sense, as far as it goes. I get the part about the bias circuit compensating for the increased triggger voltage. Odd that OMC doesn't have that prioblem, eh? Wouldn't their trigger also increase in voltage with RPM?

OK, I still don't understand why TWO charge coils and different voltages. Are these voltages combined before going to the capacitor? Your post didn't answer that. You also didn't explain why the two packs on an L6 or V6 have to be bridged at the bias circuit. Wouldn't each one respond to that timing compensation individually? And what about a 3-cylinder? How does the bias work on those when it's connected to a trigger wire?

 

[VivaLaMigra]

Faztbullet: at least now I know why they call it a 'bias' circuit and what it basically does. Still sort of a mystery as to why a bad bias circuit in ONE pack will kill the spark on the OTHER pack, but I'll accept that It Is What It Is.

 

[Faztbullet]

Are these voltages combined before going to the capacitor? Your post didn't answer that.

No these go thru a internal rectifier inside switchbox and converted to DC voltage to capacitor......

You also didn't explain why the two packs on an L6 or V6 have to be bridged at the bias circuit. Wouldn't each one respond to that timing compensation individually?

The bia wire completes the current path and connects the pack bia's rails together and ensure both advance at the same time. As soon as I find my Amtech school book will post schematic...

 

[VivaLaMigra]

Faztbullet: Yes, I know a capacitor stores a charge using a DC voltage. So, you're saying the two AC voltages are summed going into that rectifier? I know that with increased RPM comes a higher total energy demand; the capacitor has a shorter and shorter time to get charged so it would need a higher voltage to saturate it.