"bias voltage" question...

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Tegger - 05 Nov 2007 23:09 GMT

I tried posting this in partial reply to a post by Comboverfish in the
thread "What does an igniter on an '89 Toyota do?", but it appears to have
got missed. I therefore ask again. (Has to do with Comboverfish's mention
of how "bias voltage" is part of how the ECM monitors igniter output.)
...

I'm not quite getting the "bias voltage" modification thing.

As I understand it from Googling, the ECM would put, for instance, +5VDC
on the monitor line (which implies some kind of permanent ground in the
igniter). The igniter would change that to -5VDC on a firing impulse.

Why wouldn't there simply be NO voltage on the monitor line until the
igniter put voltage of some type there? What is the advantage of
modifying a pre-existing voltage instead of just creating voltage where
there was none before?

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Tegger

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. - 06 Nov 2007 00:47 GMT

> I tried posting this in partial reply to a post by Comboverfish in the
> thread "What does an igniter on an '89 Toyota do?", but it appears to have
[quoted text clipped - 12 lines]
> modifying a pre-existing voltage instead of just creating voltage where
> there was none before?

http://en.wikipedia.org/wiki/Bias_voltage

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Tegger - 07 Nov 2007 11:45 GMT

>> I tried posting this in partial reply to a post by Comboverfish in
>> the thread "What does an igniter on an '89 Toyota do?", but it
[quoted text clipped - 17 lines]
>
> http://en.wikipedia.org/wiki/Bias_voltage

I saw that before, thanks. I just wasn't sure if it was applicable to the
automotive igniter in question.

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Tegger

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Comboverfish - 06 Nov 2007 04:31 GMT

> I tried posting this in partial reply to a post by Comboverfish in the
> thread "What does an igniter on an '89 Toyota do?", but it appears to have
[quoted text clipped - 3 lines]
>
> I'm not quite getting the "bias voltage" modification thing.

I'm not formally trained in electronic theory, so my use of the
terminology and my understanding have been moulded by the pitfalls of
self education. Bia voltage may not be the exact proper term in this
specific case, but I don't know what would be. A street term I hear
but don't tend to use is pulldown voltage.

> As I understand it from Googling, the ECM would put, for instance, +5VDC
> on the monitor line (which implies some kind of permanent ground in the
> igniter). The igniter would change that to -5VDC on a firing impulse.

There's no significant negative "DC" voltage in reference to chassis
ground in a DC sensor input circuit. When a field collapses, for
example when a coil full of energy is released, the voltage swings
above and below ground until it comes to rest, all in the course of a
few milliseconds. This shouldn't be confused with the subject of bias
(or whatever it's called) voltage.

On an ECM input monitor, the voltage on the sensor line is fixed by
the ECM, typically at 5, 8, or 12 volts (many times 12 volt circuits
in this case aren't really regulated, but rather simply B+ in series
with some amount of resistance). This voltage is generated and
controlled in such a way as to be capable of being grounded (or
partially grounded) without drawing much current through the circuit.
The ECM either looks for a single threshold, i.e. anything < 0.5 V =
"1" while anything > 0.5 V = "0", or a finite number of steps, i.e. a
voltage range that charts the position or temperature of a sensor.
Luckily, 10 Megohm input DVOMs are common and cheaply had. With such
a tool, you can parallel into a circuit and see what the ECM sees
without modifying the signal. It doesn't take much voltage potential
to screw up this type of circuit, but other than bad wiring or outside
influences, it is a hearty design used sucessfully in auto electronics
along with pretty much anything else in the DC electronics world.

> Why wouldn't there simply be NO voltage on the monitor line until the
> igniter put voltage of some type there? What is the advantage of
> modifying a pre-existing voltage instead of just creating voltage where
> there was none before?

I wish I knew. Refer once again to my self education AKA just enough
to get by :-)

Toyota MDT in MO

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Tegger - 07 Nov 2007 11:45 GMT

> On an ECM input monitor, the voltage on the sensor line is fixed by
> the ECM, typically at 5, 8, or 12 volts (many times 12 volt circuits
[quoted text clipped - 11 lines]
> influences, it is a hearty design used sucessfully in auto electronics
> along with pretty much anything else in the DC electronics world.

This sort of thing is a bit hard for my non-theoretical mind to grasp. I
think I'm going to have to get some practical data and hook up my DVOM to
my igniter output line and see what kind of readings I get.

>> Why wouldn't there simply be NO voltage on the monitor line until the
>> igniter put voltage of some type there? What is the advantage of
[quoted text clipped - 3 lines]
> I wish I knew. Refer once again to my self education AKA just enough
> to get by :-)

I read "."'s suggested Wiki page. Looks like even more study is needed
before I understand the practical aspects of this subject.

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