[s-cars] PSO

[Paul Gailus]

----- Original Message -----
From: Aaron <aaront at cox-internet.com>
> The one thing I did not discover is what the hell is the purpose of
> these POS'S.
> Can anybody explain what they do?

Well, on the most basic level the PSO's act as amplifiers
to boost up the low level signals from the ECU in order
to drive the ignition coils. They are similar to the power
output devices on a audio amp, although in a different
wiring configuration.

Some more long-winded details if interested:

Before the ECU directs a cylinder to fire, it drives the
input to the PSO stage high so that the PSO output, which is
tied to one side of the coil primary, is driven low (to ground).
Since the other side of the coil primary is at battery voltage,
the current begins to ramp up, storing energy in the inductance
of the coil. To fire the cylinder, the ECU drives the input
to the PSO low, turning off the PSO stage and allowing the
stored coil energy to discharge through the coil secondary.
The collapse of the coil's magnetic field causes a "flyback"
voltage to ramp up on the coil secondary until it reaches the
breakdown voltage on the plug. Through transformer action,
the voltage at the coil primary (and PSO output) goes up
to several hundred volts by the time the plug fires. So the
PSO's are highly stressed devices, and the operating temps
under the hood don't help either.

The PSO output performs exactly the same function as
mechanical points did on the old traditional Kettering ignition
system. It's just that the spark timing and dwell are controlled
by the ECU. With the old mechanical points the dwell (the time
the coil primary is charging) was typically a fixed number of
crankshaft degrees. So at higher RPMs the actual time duration
of the coil charging was reduced, leading to a weaker spark.
With ECU control things are much more flexible.
"High energy" coils with low primary resistance can be used
without burning things up by appropriately controlling the
dwell time under various conditions.

Given the >10 year old vintage of our PSO design, I'm
sure the technology used is NPN type bipolar transistors,
possibly in a Darlington configuration (two transistors in
cascade).

Paul

----- Original Message -----
From: Aaron <aaront at cox-internet.com>
To: Audi - Scars <s-car-list at audifans.com>
Sent: Friday, September 06, 2002 4:34 PM
Subject: [s-cars] PSO


> I spent a few hours diagnosing a failed PSO (power stage output) devive.
> This list is superb.  Thanks to archive tips, I saved hrs and the cost
> of buying a PSO device by utilizing the extra unused channel.
> Many thanks to those who contributed this info to the list!
> The one thing I did not discover is what the hell is the purpose of
> these POS'S.
> Can anybody explain what they do?
> Thanks
> Aaron - sans Bently in Idaho.
>
>
> _______________________________________________
> S-CAR-List mailing list
> S-CAR-List at audifans.com
> http://www.audifans.com/mailman/listinfo/s-car-list