Theory of operation (was Diode in Lock Module)

["Walter" <Whalt@xxxx>]

I got curious and started looking at that schematic
http://www.dmcnews.com/Techsection/graphics/lockcirc.jpg
and my OEM door lock module. I've made the following observations:

1) There is an error in Dave Swingle's schematic. The brown/slate wire at
terminal #2 should be erased from the junction linking R1 and the emitter of
Q1 and instead be connected to the junction linking R2, D3 and RL1. This
would make it match the sister circuit below it.

2) The diode D4 is also toast on my module. It melted the insulation on the
violet wire and the leads are corroded on both ends.

3) The apparent function of D4 in this circuit is to drain the battery.
Well, this isn't far from the truth. I think the designer meant for it to
protect the transistors from surges. If you are going to try to improve
this module, it would be easier to just start from scratch with a new
design. But if you really want to fix it, I would recommend (and I'm
starting to get in a little over my head here) a diode that has a high
reverse impedance and can take a forward current of 1 amp or so. A ziener
diode would be better if the avalanche voltage is well enough over 12 volts
so that it doesn't conduct normal operating voltages. But I would choose
something really beefy. It is a trade off -- the beefier the diode, the
more it will drain the battery. A better choice may be to put a capacitor
or an MOV on there if they make one that operates at a low enough voltage to
do any good.

Here is one of my typical long winded explanations. This one is a 'walk
through' on how the module operates. If you read this, you have too much
time on your hands. If you understand any of it, you are a geek. If it is
useful to you then you probably could have figured it out on your own
anyway. But it never hurts to have a second opinion.

When the 'door unlock' switch is closed, electrons come from ground through
the switch and through the brown/pink wire into the module at terminal #3.
Current flow proceeds forward biased through diode D5. From here it goes
through the electromagnet of the relay RL11 energizing it. From the other
end of the relay coil, current proceeds forward biased through diode D12 and
to the emitter of the transistor and one side of resistor R11.

>From here two things happen:

First, current flows through the transistor to terminal #4 and through the
violet wire to the positive side of the battery completing the circuit. The
transistor acts like a latching switch permitting the relay to stay
energized even when the door unlock switch is opened. This is accomplished
by biasing the transistor through diode D13 and resistor R12. Here it is
evident that the function of diodes D5 and D13 placed back-to-back are to
allow the transistor to be switched on without the door unlock switch
interfering with the transistor's bias.

The second thing that happens is that current begins flowing from ground
though capacitor C11 and resistor R11 and on through the transistor along
with the current that came through the relay coil. R11 and C11 make up what
is called an R/C timing circuit. This is a cheap simple way of making an
electronic timer. The purpose of resistor R11 is to limit the amount of
current flowing through capacitor C11 so that it charges slowly. How long
it takes capacitor C11 to charge is how long the timer lasts. Once this R/C
circuit has charged to a certain point, it upsets the bias on transistor Q11
causing it to switch off. The end purpose is that the amount of time that
this circuit is active determines the duration of the pulse sent to the lock
solenoids. The reason for diode D12 is to stop the R/C circuit from
charging through resistor R12 and to keep the battery from draining while
the circuit is inactive.

The purpose of diode D11 is to stop surges generated by the lock solenoids
from spiking through transistor Q11 and damaging it. When the solenoids are
through moving and the current applied to them is shut off, the magnetic
field set up around the solenoids' coils collapses. As it collapses, it
induces a current in the coils making them appear as very high voltage
sources in a polarity opposite to that which they originally had. Even
though they seem isolated from the transistor when the relay contacts
release, they can still zap the transistor as the contacts are letting go.
The easiest way to deal with this is to let a diode shunt it. The same
technique is also used on A/C compressor clutches.

This is my OPINION on how the 'unlock' side of the door lock module operates
based on Dave Swingle's diagram. Remember, this my opinion and I have been
wrong before. The lock side works the same way, but if you are looking at
Dave Swingle's diagram, the error on the 'lock' part may confuse you.

Walt Tampa, FL