Battery Contactor Diode?

What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?

"Skrud" wrote in message
.. .
What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Something like a 1N4004 (1A, 400V) should do just fine.

Rob

"Rob Turk" wrote in message
...
"Skrud" wrote in message
.. .
What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Something like a 1N4004 (1A, 400V) should do just fine.

Yep that what's I thought.
But doesn't it depend on the size of garden?
KA

"karel" wrote in message
...

"Rob Turk" wrote in message
...
"Skrud" wrote in message
.. .
What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Something like a 1N4004 (1A, 400V) should do just fine.

Yep that what's I thought.
But doesn't it depend on the size of garden?

Not really. The size of the chip above the 1N4003 is fairly hefty and
constant as opposed to the size down around the 1N4001. You aren't worried
about carrying reverse currents of an amp or so; what you are concerned with
is single-pulse forward currents of hundreds of amps for a microsecond. The
1N4004 ought to do you just fine until you get to the current level for a
contactor for a railroad diesel engine {;-)


Jim

"RST Engineering" wrote in message
...

"karel" wrote in message
...

"Rob Turk" wrote in message
...
"Skrud" wrote in message
.. .
What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Something like a 1N4004 (1A, 400V) should do just fine.

Yep that what's I thought.
But doesn't it depend on the size of garden?

Not really. The size of the chip above the 1N4003 is fairly hefty and
constant as opposed to the size down around the 1N4001.

Never knew this - thank you!

You aren't worried about carrying reverse currents of an amp or so; what
you are concerned with is single-pulse forward currents of hundreds of
amps for a microsecond. The 1N4004 ought to do you just fine until you
get to the current level for a contactor for a railroad diesel engine {;-)

Yep indeed!
KA

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


karel wrote:
"Rob Turk" wrote in message
...

"Skrud" wrote in message
...

What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?


Something like a 1N4004 (1A, 400V) should do just fine.


Yep that what's I thought.
But doesn't it depend on the size of garden?
KA

"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).

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 in NC

"Morgans" wrote

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?

Oh, I also want to add, that if Jim Weir responds, will someone reply to the
post with his entire post quoted?

For some reason, my ISP seems to have "blacklisted" his ISP, so the only
way I see his posts is in a response.

Also, to say I don't understand electrons, is a huge understatement! g
--
Jim in NC

"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

Hi Jim,
Schematically, the cathode is the bar and the anode is the arrow part.

With the diode connected with its cathode to the positive voltage and
its anode connected to ground, it is reverse biased and looks like an
open circuit during normal operation. When the contactor relay (master
solenoid) is turned off, the collapsing magnetic field will generate a
field of opposite polarity (the voltage spike we are talking about) such
that the side of the coil that is connected to the +12V bus will now be
at a large NEGATIVE voltage, putting the cathode of the diode more
negative than its anode and it will now conduct. The voltage across a
conducting diode is about 0.7 Volts, so the spike will be limited to
about a -0.7V along the +12V bus in the aircraft. Without the diode,
hundreds of negative volts could be placed on the bus (for a short
period of time, in the milliseconds), but solid state devices don't get
along well with these high voltages, ESPECIALLY since it is opposite
polarity of what they are designed to operate at.

Scott


Morgans wrote:

"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).


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?