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Peak current handling of S604 gold "DB" style contacts

 
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nuckollsr(at)cox.net
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PostPosted: Wed Jan 25, 2006 5:22 am    Post subject: Peak current handling of S604 gold "DB" style contacts Reply with quote

At 05:43 PM 1/24/2006 -0800, you wrote:

Quote:

<jmfpublic(at)comcast.net>

Bob, or anyone,
I'm wiring my engine compartment to the instrument panel with a CPC2
series Amp connector to allow the panel to be removed from the plane and
assembled on the bench. One of the wire pairs from the engine
compartment is the 5 amp breaker switch to the field terminal on the BC20
alternator. I recall that Bob has suggested a mean current draw of 4
amps max for the gold plated 0.040 DB style pins and sockets, which have
the part number S604 on the B&C site. Could I use a pair of these to
carry the current through the field circuit breaker and switch? If the
overvoltage crowbar on the LR-3 regulator shorts the field, there will be
a momentary current draw of much larger magnitude. The data sheets for
this series of breakers at 5 amps suggests 50 milliohms internal
resistance, and if I use 12 feet of 20 gauge wire, this will add 120
milliohms to the circuit. I don't know the internal resistance of the
crowbar. If set to zero, this leaves 170 milliohms plus the contact !
resistance of two series pairs of the pin and socket. I have not
measured this resistance, but the Tyco (Amp) site suggests 10 millohm
maximum. After doing some reading, I find that the CPC series with "size
20" pins and sockets is rated at a single circuit of 8.34 amps continuous
with a 30 degree C rise in temperature. The breaker will open in 0.2 to
0.6 sec at 1000% load.

When the crowbar fires, the instantaneous current before heating is 16
volts/0.18 ohms, or 89 amps. The bimetal thermal breaker should raise its
electrical resistance much faster than the wiring because it is
enclosed. I'm not sure. The convenience of routing all wires through
this one or two CPC connector set is attractive, but I could run the wires
through a grommet in the panel, directly to the breaker. It is harder to
take apart this way, but at this point, I have convinced myself that the
direct route to the Fast-On tab on the breaker is the best way. Having
spent several hours researching this, I thought I'd share it with the list.

The two Tyco PDF documents of interest are: instruction sheet 408-7593 and
108-10024-3 on the web site.

The first thought I had was to wonder what considerations
drive the need to put a connector in this line (or any others
that will share the connector). The most common reasons are
a matter of convenience. An oft cited rationale is, "to be
able to unplug an engine harness as the engine is lifted out
of the airframe." Or, "I want to do all the wiring inside the
cockpit first and save all the wiring forward of the firewall
until later."

Consider that every connector adds three new joints to every
wire along with the (1) attendant concerns you've correctly identified
above, (2) increased parts count [reduced reliability], (3) increased
$time$ to procure, install and maintain.

We at RAC, (like the automotive industry) break major wire bundles down
for assembly through pressure bulkheads, at airframe/avionics interfaces,
etc, etc. These decisions are not made lightly. The #1 goal is to
NOT install any new hardware except where there is a return on investment
for costs of assembly.

A builder sent me this picture some years ago:

http://www.aeroelectric.com/Pictures/Wiring_Technique/term_strip_not.jpg

. . . very neatly installed, all wires labeled, high degree of
craftsmanship. When I asked him how much this installation saved him
in cost of ownership ($time$ to procure, install and maintain). He
didn't have a very convincing answer. Even when you might consider
a connector in an engine harness, keep in mind that the connector
has to live in worst environment that the airplane can throw at it.
The average engine has less than a dozen wires attached to it and
probably get lifted out of the airplane perhaps once every ten years.
I'll suggest that connector "convenience" is more likely to morph into
"maintenance nuisance"/

Another question that comes to mind is your choice of connectors is
this is going to mount on the firewall. Check the archives for threads
on firewall integrity and check out:

http://aeroelectric.com/articles/Firewall_Penetration/firewall.html

Finally, and specific to your question I'll suggest that the wire
between bus and regulator be as contiguous as practical and avoid
any unnecessary breaks in connectors. You're correct in your
deliberations that current flowing during the crowbar event is
on the order of 50-150 amps and while average current in this
line is 1 to 3 amps, the occasional but urgent requirement to
operate reliably as the design goals intended cannot be made
better by running the wire through any connectors whether rated
for this peak-event or not.

Finally, there ARE ways to practically parallel pins in small
connectors for improvements in current carrying capability. I've
done this in several projects, one of which has gone through
the full range of environmental and operational testing to qualify
for flight on this bird:

http://www.aeroelectric.com/Pictures/Misc/1st_Operational_GQM.jpg

To drive costs out of a throwaway power distribution assembly
pictured at:

http://www.aeroelectric.com/Pictures/Misc/power_dist.jpg

I used D-subs to carry as much as 20A continuous at 70C.
This was accomplished by first de-rating a pin to 3A and
then "ballasting" the paralleled pins with lengths of
wire in one or both sides of the connector junction as
shown in:

http://www.aeroelectric.com/Pictures/Schematics/D-Sub_Power_Dist_1.jpg

By adding approximately 16 milliohms of resistance in series
with each pin in the form of 22AWG wire, the pin-to-pin variability
of contact resistance is washed out in the totals and the
pin paralleled very nicely and survived elevated temperature
testing to qualify for the task.

In the case cited, I replaced a $10K+ relay based assembly in
a previous target design (used $super$ connectors) with this
all solid state version with 1/10th the weight volume and
cost.

Bob . . .


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