Driving fuel injection solenoid valves
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Driving fuel injection solenoid valves
Some one said recently that a diode across the coil of a fuel injector fuel
injector
solenoid could increases the time it takes to release.. How could this be?
Seems like
once the coil has a magnetic field built up from the last time it was fired, the
only way
to dissipate this field is to cause it to collapse. This is exactly what the
diode across
the coils does, helps the field to collapse and also prevents a large emf
buildup from
this field's collapse. The field must collapse before it build back up again to
fire the
solenoid again, so how can a diode cause a problem here? Maybe I am just
looking at the
wrong angle or something.. Please help in the understanding of this..
Leroy
Some one said recently that a diode across the coil of a fuel injector fuel
injector
solenoid could increases the time it takes to release.. How could this be?
Seems like
once the coil has a magnetic field built up from the last time it was fired, the
only way
to dissipate this field is to cause it to collapse. This is exactly what the
diode across
the coils does, helps the field to collapse and also prevents a large emf
buildup from
this field's collapse. The field must collapse before it build back up again to
fire the
solenoid again, so how can a diode cause a problem here? Maybe I am just
looking at the
wrong angle or something.. Please help in the understanding of this..
Leroy
Comments
writes:
> Driving fuel injection solenoid valves
>
>
> Some one said recently that a diode across the coil of a fuel injector fuel
> injector
> solenoid could increases the time it takes to release.. How could this be?
> Seems like
> once the coil has a magnetic field built up from the last time it was
> fired, the only way
> to dissipate this field is to cause it to collapse. This is exactly what
> the diode across
> the coils does, helps the field to collapse and also prevents a large emf
> buildup from
> this field's collapse. The field must collapse before it build back up
> again to fire the
> solenoid again, so how can a diode cause a problem here? Maybe I am just
> looking at the
> wrong angle or something.. Please help in the understanding of this..
>
> Leroy
>
I am the one who originally INCORRECTLY suggested the diode. Somebody else
CORRECTLY posted not to use the diode. In the automotive industry, I have
never seen a diode across the injector (so why did I suggest it??? probably
thinking of suppressing the spike, but did not take the time to think it
through)
I am not sure exactly why the diode across an injector will cause it to stay
open longer but .....I believe the reason the injector will stay open longer
is as follows:
If you use a "spike diode" when the injector (inductor) turns off, the large
voltage spike produced goes through the diode and back through the injector
until it dissiptes. I summize that the back emf routed through the spike
diode/injector is what keeps the injector open longer.
As far as collapsing the field, the diode does not have anything to do with
the collapsing action, The diode only "steers" or give the back emf spike a
place to go...that is back through the injector.
The back emf spike is a function of how QUICKLY you reduce the current in the
injector (inductor ...assuming all other factors equal) and that is
controlled by how quickly you can make the driving transistor turn off.
V inductor = inductance* delta amps/ delta time. Time is in the denominator,
therefore, the less time (quicker turn off of current) the bigger V inductor.
You double EE's out there agree?
[noparse][[/noparse]Non-text portions of this message have been removed]
fuel injector
> solenoid could increases the time it takes to release.. How could this
be? Seems like
> once the coil has a magnetic field built up from the last time it was
fired, the only way
> to dissipate this field is to cause it to collapse. This is exactly what
the diode across
> the coils does, helps the field to collapse and also prevents a large emf
buildup from
> this field's collapse. The field must collapse before it build back up
again to fire the
> solenoid again, so how can a diode cause a problem here? Maybe I am just
looking at the
> wrong angle or something.. Please help in the understanding of this..
*******************************************
As many of you know, the voltage/current in an inductor is given by:
V=L*(delta I)/(delta t), where I is current, L is inductance, and V is
inductor voltage. This holds whether current is increasing or decreasing.
When you turn off the inductor, the current will decay according to this
equation. You can rewrite the equation as
(delta I)/(delta t) =V/L.
Notice that (delta I)/(delta t) is the rate of current collapse. If you use
no snubber diode, voltage rises extremely high, (limited only by circuit
capacitance), and current drops rapidly. If you use a diode, inductor
voltage is the forward diode voltage, less than 1 volt, and the rate of
collapse is very slow. With most simple relay applications, the slow current
decay and slowed relay release doesn't matter.
When faster release is needed, you just allow more voltage to build across
the coil. One approach is to use a zener diode that allows voltage to rise
within safe limits for the transistor being protected. Another method is to
place a resistor in series with a snubber diode. As with the zener, pick the
resistor to keep the voltage in safe limits. The larger the resistor, the
higher the voltage will rise, and the faster the current will decay.
Hope this makes sense.
Ray McArthur
>Some one said recently that a diode across the coil of a fuel injector
>fuel injector
>solenoid could increases the time it takes to release.. How could this be?
Think of it as 2 separate things.
1) An inductor resists a change in current. If you suddenly reduce or
interrupt the current, the voltage rises in an attempt to keep the current
at the same level.
2) The magnetic field is proportional to the current flowing.
If you put a clamping diode across the coil, the voltage cannot rise. The
current continues to flow.
If you allow the voltage to rise, the current cannot continue to flow.
Put all the above together and you should see how clamping diodes increase
the release time.
Hope this helps!
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
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injection
valves. The math explanation really helped to make head or tails of the
concepts
involved.
regards,
Leroy