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Carling switch resistance for micro currents?

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Aug 25, 2014 12:35 pm Post subject: Carling switch resistance for micro currents?

At 03:05 PM 8/25/2014, you wrote:


Hi Bob,



Last night I was metering a circuit (for total resistance) not including the

load at the end of the wire.  The circuit consists of about 10 feet of 18AWG

wire, an ATC fuse, fuse block and a carling SPST switch all obtained from

B&C, and two alligator test leads. I was surprised to find the meter (my

trusty Fluke) read several megohms when I closed the switch.  I was

expecting to see something less than 5 ohms (probably less than one).  It

would appear that there is quite a high resistance when only the very small

current of the metering circuit is passing through the switch.  Is this

normal?



Note:  The meter showed 0.2 ohms with the test leads shorted so the meter

seems to be functioning correctly.



    Okay, you've cited a number of devices in series

    all of which feature several 'metallic' joints

    normally expected to be a few milliohms each.



    Divide and conquer.



    Clip the ohmmeter to the fuseblock treaded

    power input post and then march downstream

    with the other lead looking for the first

    instance of "too much" resistance.



    Somewhere along the way you're going to find

    a bad crimp or perhaps a switch with contacts

    so 'self-worn' that they need to be cycled

    clean. Is the ship's battery available to this

    circuit?  Put a sacrificial 20A fuse in the

    fuseholder, dead short the far end of the

    wire to ground then 'rattle' the toggle until

    the fuse blows.  Pop a couple of fuses. Then

    put the proper fuse back in and march off smartly

    forward.



    I've used a similar technique to clean the shelf-

    fuzz off of several high dollar, mil-spec switches

    in biz-jets. In both cases, the switches had never

    been asked to SWITCH a significant load for years.

    Do a little arc-therapy on them and they good to

    go for another 5-10 years.

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Aug 26, 2014 8:11 am Post subject: Carling switch resistance for micro currents?

At 04:28 PM 8/25/2014, you wrote:

 Thanks for all the replies.  I have not been back to the hangar to do anymore since posting my original question but I will add a little more info here.

  

 This particular circuit has never operated any load yet.  It has been sitting for a couple years since originally crafted.   The plane is in a heated/insulated space.  Nothing else rusts or corrodes in this space.\



  EVERYTHING corrodes. For sure, the layer may be only molecules thick but its resistance can be significant and it rises with time.  

 The fuse is 5amp.  I believe I also clipped the meter's alligator directly to the screw terminal of the fuse block thereby eliminating the block and fuse but will retry tonight.

  

 I don't have a battery in the plane yet.  I use a 30 amp DC power supply that plugs into a piper style remote power port.   It puts out 14VDC and has run all my other loads just fine.  When the DC power supply is powered up, I get the full voltage at the load end of the subject wire which is what I expected to see.  Since there has never been any load driven by this circuit (no current flowing) there has never been and arky sparky happening at the switch.  So, is some arky sparky required to get the switch to indicate 0 ohms when in the on position?



  Yes . . . under certain conditions.   

 I wouldn't think so but I seem to recall something about these switches not being designed for extremely small currents.



  Few switches are. Such devices are gold plated and perhaps feature bifurcated contacts, etc. The catalogs that offer such switches will speak to these features and charge you more money for them. Run-of-the-mill power handling switches, even the $high$ devices can demonstrate corrosion induced open circuits when not 'exercised' for long periods of time. It's the nature of the beast.  

 What I do see is open circuit with the switch off,  dropping to several megohms when on.  I may try powering a medium load (5-10 amps) to see...

  

 A) If the load powers up as expected while metering current through the circuit.

 B) Check the switch to see if it generates any noticeable heat during this test

 C) Re measure the closed circuit resistance after the test to see if it now appears more normal.



  Forget the resistance measurements. Just load the far end of the wire with something and then run the switch through a few dozen cycles. The arcing at ANY current level is never zero. You could deliberately pop some 5A fuses . . . SOME arcing is expected to keep corrosion products cleared from the contacts of ALL power switches. "

 

  A busy switch is a happy switch."





    Bob . . .    [quote][b]

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fvalarm(at)rapidnet.net
Guest

Posted: Tue Aug 26, 2014 8:36 am Post subject: Carling switch resistance for micro currents?

OK.  Very good.  I will put a load on and out of  curiosity, check the total circuit resistance again after a few dozen  cycles.

  

 Bevan



   From:  owner-aeroelectric-list-server(at)matronics.com  [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert  L. Nuckolls, III

Sent: Tuesday, August 26, 2014 9:11 AM

To:  aeroelectric-list(at)matronics.com

Subject: RE: AeroElectric-List:  Carling switch resistance for micro currents?

 

At 04:28 PM 8/25/2014, you wrote:

Thanks for all the  replies.  I have not been back to the hangar to do anymore since posting my  original question but I will add a little more info here.

 

This  particular circuit has never operated any load yet.  It has been sitting  for a couple years since originally crafted.   The plane is in a  heated/insulated space.  Nothing else rusts or corrodes in this  space.\



    EVERYTHING corrodes. For sure, the layer may be only molecules    thick but its resistance can be significant and it rises    with time.  

The fuse is 5amp.  I believe I also  clipped the meter's alligator directly to the screw terminal of the fuse block  thereby eliminating the block and fuse but will retry tonight.

 

I  don't have a battery in the plane yet.  I use a 30 amp DC power supply that  plugs into a piper style remote power port.   It puts out 14VDC and  has run all my other loads just fine.  When the DC power supply is powered  up, I get the full voltage at the load end of the subject wire which is what I  expected to see.  Since there has never been any load driven by this  circuit (no current flowing) there has never been and arky sparky happening at  the switch.  So, is some arky sparky required to get the switch to indicate  0 ohms when in the on position?



    Yes . . . under certain conditions.  

I wouldn't  think so but I seem to recall something about these switches not being designed  for extremely small currents.



    Few switches are. Such devices are gold plated and    perhaps feature bifurcated contacts, etc. The catalogs    that offer such switches will speak to these features    and charge you more money for them. Run-of-the-mill    power handling switches, even the $high$ devices can    demonstrate corrosion induced open circuits when not    'exercised' for long periods of time. It's the nature    of the beast.  

What I do see is open circuit with the  switch off,  dropping to several megohms when on.  I may try powering  a medium load (5-10 amps) to see...

 

A) If the load powers up as  expected while metering current through the circuit.

B) Check the switch to  see if it generates any noticeable heat during this test

C) Re measure the  closed circuit resistance after the test to see if it now appears more  normal.



    Forget the resistance measurements. Just load the far    end of the wire with something and then run the switch    through a few dozen cycles. The arcing at ANY current    level is never zero. You could deliberately pop    some 5A fuses . . . SOME arcing is expected to keep    corrosion products cleared from the contacts of ALL    power switches. "



   A busy switch is a happy switch."



   

  Bob . . . [quote]



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[b]

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