Battery Contactor Diode?

"RST Engineering" wrote in message
...

"Morgans" wrote in message
news

"Scott" wrote in message
.. .

Nope, not really a concerning factor. Any of the "garden variety"
diodes in the 1N400X series should be fine. Just be sure to put the
cathode to the +12 or +24 volt side of the contactor coil and the anode
to the ground side (assuming a negative ground system).

Scott, at and above the 1N4004, the chip inside the case is physically
more
robust and will take a spike of current a little better than, say, a
1N4001.





I am not very good with the electron understanding, but I would be
grateful
to understand this whole discussion.

Is the cathode normally the downstream side of the diode? What does the
installation like this, do, to help with the current spike?


Jim, as you know, a diode conducts current in one direction and blocks it
from conducting in the other direction. A positive voltage on the anode
forward biases the diode and the voltage will appear as a positive voltage
(less some small conduction drop of a volt or so) at the cathode. A
negative voltage on the anode will reverse bias the diode and it will not
appear as a negative voltage at the cathode.

Likewise, a negative voltage at the cathode will appear as a negative
voltage at the anode.

When a coil has a collapsing field, it produces a spike of energy as the
field attempts to keep the current through the coil constant. THis large
negative spike can and will cause some of the aircraft electronics to fail
catastrophically. One way of making sure that negative spike doesn't kill
the avionics is to shunt it to ground through a forward biased diode.
That
way the most that can sneak through is a volt or so as opposed to several
hundred volt spikes without the diode. How do you forward bias a diode
with
a negative voltage? You connect the CATHODE to the "hot" +12 volt
terminal
of the coil and the ANODE to ground. THe negative spike is effectively
clipped at a volt or thereabouts.

Jim


I totally missread the initial question when this thread first started, and
somehow I suspect that I may not have been the only reader who did so.

The battery contactor, usually operated by the master switch, should
normally be a trivial case with regard to voltage spikes the circuit is
protected by the battery. It does no harm to protect the circuit and, if
you want to be really anally retentive, you could measure the current drawn
by the coil of the contactor and size the diode for a little more current
than that. Once the current is flowing in an inductor, such as a contactor
coil, it will try to continue flowing at the same rate when you first turn
it off--which causes a voltage spike several times the normal operating
voltage. The protection diode simply allows the current to flow around in a
circle--through the diode and around back through the coil for a few
milliseconds until the resistance of the wiring in the coil uses up the
energy. Thus, presuming that the diode is located at the coil, the voltage
spike
should be limited to the forward voltage drop of the diode--usually about
six tenths of a volt.

With regard to possible damage to avionics, the contactors which have their
coils powered from the master buss (which presumably is activated by the
battery contactor) are the greater issue and all should be protected by
diodes--although capacitors may also work. Technically this is airframe
wiring and therefore in the mechanic's job description--so that is who to
see in the case of a certified aircraft.

Otherwise, nearly every EAA chapter has members who are or were mechanics as
well as members who are or were avionics technicians--and a couple of cups
of coffee with the three of you together, and maybee a couple of additional
members, will quickly solve a lot of problems. Remember that is a lot of
the original and continuing reason for chapters!

BTW the 1N400X series of diodes are 1 amp, and I believe that the next
common size is/was around 3 amps.

Regards,
Peter

I could'na said it better myself.

Jim


wrote in message
ups.com...


Any sensitive
electronic device (like an expensive radio) without its own internal
protection goes poof.

Poof. THat's a technical term. Stick around long enough and you'll get it.



That magneto won't
work worth a hoot without the capacitor (sometimes called condenser).


Hoot's a technical term also. See answer above.

For the advanced student, do the calculation of the inductive reactance of
the magneto coil, the capacitive reactance of the magneto capacitor, the
frequency of the coil spark (RPM adjusted for seconds and the number of
cylinders) and see where the resonant frequency of that inductor and
capacitor are.

For the Master's thesis, factor in the resistance of the coil and see how
broad the Q of the resonant circuit is.

For the PhD dissertation, calculate the velocity of the flame front and
demonstrate the maximum cylinder pressure for two magnetos timed "x" degrees
apart.

Jim

Jim

Please do not listen to this person. (S)he understands nothing about
reverse emf, nor about reverse voltage spikes. The forward current drawn by
the coil has no bearing on the multiple hundreds of amps that a reverse
polarity diode needs to conduct to keep a large NEGATIVE voltage spike from
frapping (that's a technical term, you'll get used to it) your radios.

Jim


The battery contactor, usually operated by the master switch, should
normally be a trivial case with regard to voltage spikes the circuit is
protected by the battery. It does no harm to protect the circuit and, if
you want to be really anally retentive, you could measure the current
drawn
by the coil of the contactor and size the diode for a little more current
than that.

The reverse emf spike, just like any other emf, is measured in volts only.
The reverse current required to damage electronics is very small--that means
very very small. The coils of those little Ice Cube relays, with contact
ratings less than about 10 amps, usually have coil currents of less than
100ma; which is why 1N4004 diodes are so popular--and really overkill but
they weigh less than a gram. Larger relays, used to control more power, are
part of the mechanical airframe wiring and I continue to recommend that you
talk to one of the mechanics in your local chapter--one who also holds an
IA. There are more than enough small type certified aircraft in the rental
fleet to generate a highly reliable set of service bulletins with which a
mechanic with IA will be quite familiar. I also continue to recommend that
you include an electronics technician (preferably avionics) in the
conversation because all three on you, and any other builders who attend,
will come away with greater knowledge.

Disclaimer: Clearly, my technical knowledge is gravely wanting, as I have
no idea what "frapping" means.

Peter

"RST Engineering" wrote in message
...
Please do not listen to this person. (S)he understands nothing about
reverse emf, nor about reverse voltage spikes. The forward current drawn
by
the coil has no bearing on the multiple hundreds of amps that a reverse
polarity diode needs to conduct to keep a large NEGATIVE voltage spike
from
frapping (that's a technical term, you'll get used to it) your radios.

Jim


The battery contactor, usually operated by the master switch, should
normally be a trivial case with regard to voltage spikes the circuit is
protected by the battery. It does no harm to protect the circuit and,
if
you want to be really anally retentive, you could measure the current
drawn
by the coil of the contactor and size the diode for a little more
current
than that.

I couldn't agree more.

Jim Weir
Avionics Technician 1963 - 1967
Avionics Engineer 1967 - 20-
A&P, IA



"Peter Dohm" wrote in message
news

I continue to recommend that you
talk to one of the mechanics in your local chapter--one who also holds an
IA. There are more than enough small type certified aircraft in the
rental
fleet to generate a highly reliable set of service bulletins with which a
mechanic with IA will be quite familiar. I also continue to recommend
that
you include an electronics technician (preferably avionics) in the
conversation because all three on you, and any other builders who attend,
will come away with greater knowledge.

"Peter Dohm" wrote:

Disclaimer: Clearly, my technical knowledge is gravely wanting, as I have
no idea what "frapping" means.

Peter


Acronym....

**** FRAP Fluorescence Recovery After Photobleaching
**** FRAP Fire and Resource Assessment Program (California)
**** FRAP Facilitated Risk Analysis Process
**** FRAP Frame Relay Access Probe (Tylink)
**** FRAP Free Route Airspace Project (aeronautical)
*** FRAP Frenetic Random Activity Period (dog behavior
*** FRAP Fuel Rod Analysis Program
*** FRAP Fragmenting Payload
*** FRAP Flat Rate Access Promotion (Sprint)
*** FRAP Fleet Reliability Assessment Program
*** FRAP Facilitated Risk Analysis Procedure
* FRAP Frame Relay Access Point



American Heritage Dictionary -

Middle English frapen, to strike, from Old French fraper. 8o)


- Barnyard BOb -