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

On Sun, 03 Sep 2006 13:40:12 GMT, "Skrud"
wrote:

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


Why wouldn't you use a capacitor?

Joaquin

"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

Skrud wrote:
What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?
'Depends. If the circuit would have adverse effect from a delay in
dropping out, it would be better to use a zener diode or transient
suppressor with appropriate voltage and current ratings. A diode across
a coil causes the current to flow longer than it would in an open
circuit, and so delays the drop-out of a relay. That may not be desired
where a motor is controlled by the relay, which would then run longer
and, perhaps, over-shoot a limit switch.

Skrud wrote:
What is the correct type of diode (p/n) to put across a garden-variety
battery contactor to prevent the inductive spike?
It's also a good idea to have the diode at the load, rather than at the
switch. This keeps the diode current circulating only at the load, and
not through the wiring, where it can induce transients in nearby
wiring. This, especially, if the wiring is not twisted pair, but
instead uses the airframe as one of the conductors. With this, there is
only one wire carrying the current, so it acts as an antenna, radiating
into nearby wiring both electromagnetically and electrostatically.
Twisted pair wiring minimizes these effects, since the magnetic fields
from the opposite-direction currents cancels.

Joaquin Murrieta wrote:
On Sun, 03 Sep 2006 13:40:12 GMT, "Skrud"
wrote:

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


Why wouldn't you use a capacitor?

Joaquin

A capacitor has limited capacity, and as Jim Weir pointed out,
we're talking about a lot of current here, even though it's a brief
pulse. The diode shorts the current during the inductive spike
generated when the master switch ( or starter switch) is opened,
keeping that spike from entering the rest of the aircraft's electical
system. A capacitor would store some of it, but there'd be enough left
over to cause trouble. A capacitor thus charged also keeps the
contactor closed for some time after shutoff as it discharges through
the coil.
I have measured a 600 volt spike across the master contactor
coil. Gives a person a really good jolt if he's touching the terminals
at the time. Makes one respectful of the system, and makes him more
aware of the need to shut off avionics before shutting off the master.
We can't really rely on that diode always being in good shape.
A Zener diode won't work here any better than a switching diode.
We need a diode that allows forward current flow for a very short
period, while preventing reverse flow during normal operation. A Zener
is designed to break down at a specific reverse voltage, and the only
reverse voltage this application offers is the usual 14 volts or so
during aircraft operation.

Dan

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