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Initial amp spike on PTT

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bbradburry(at)bellsouth.n
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Posted: Tue Dec 04, 2012 8:45 pm Post subject: Initial amp spike on PTT

My hangar mate and I checked the current in the wire pulled to ground by the

PTT and found an initial spike of almost 2 amps.  This seems excessive to

me. After the initial spike it dropped down to a low number that I don't

currently remember, but close to what I would think is acceptable in the

millamp range.



The radio is a Garmin GNS 430W.



Any comments on this?



Bill B

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Dec 04, 2012 10:00 pm Post subject: Initial amp spike on PTT

At 10:43 PM 12/4/2012, you wrote:

 	  Quote: 	
		  

<bbradburry(at)bellsouth.net>



My hangar mate and I checked the current in the wire pulled to ground by the

PTT and found an initial spike of almost 2 amps.  This seems excessive to

me. After the initial spike it dropped down to a low number that I don't

currently remember, but close to what I would think is acceptable in the

millamp range.


	


   What is the duration of the spike? The PTT

   line coming out of the radio may have some

   form of RFI ingress protection that includes

   capacitance to ground. Any capacitor rapidly

   discharged through the PTT circuit will produce

   some large 'inrush' currents but they're typically

   very short duration hence low energy.

   Bob . . .

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bbradburry(at)bellsouth.n
Guest

Posted: Wed Dec 05, 2012 4:51 am Post subject: Initial amp spike on PTT

Short duration, but long enough to read it on the VOM.  Maybe a second??



I have some trim indicators that are connected to this same ground (it is on

my stick) and they dim out when I hit the PTT.  I am going to separate the

PTT from them today.



Bill B



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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Dec 05, 2012 9:25 am Post subject: Initial amp spike on PTT

At 06:50 AM 12/5/2012, you wrote:

 	  Quote: 	
		  

<bbradburry(at)bellsouth.net>



Short duration, but long enough to read it on the VOM.  Maybe a second??


	


  Wow . . . I'm not sure we have enough data

  to craft good hypothesis.



 	  Quote: 	
		  I have some trim indicators that are connected to this same ground (it is on

my stick) and they dim out when I hit the PTT.  I am going to separate the

PTT from them today.


	


   Hmmmm . . . it would be interesting to

   dig into this further. I cannot imagine

   why the radio would impose such a stress

   on the PTT line . . . take that back . . .

   try this one for an "off the wall" notion.



   If the PTT current presented by the radio

   was limited to that which was necessary

   (perhaps a few mA) then PTT switches and

   relay contacts NOT characterized for very

   low currents tend to gain contact resistance

   due to corrosion. Switches designed to switch

   significant loads (1A or more) tend to get

   'cleaned' with the tiny arc that forms during

   each opening of the contacts. A 'power' rated

   switch can become flaky with age when loaded

   at very low values.



   Many upper-crust relays and switches come with

   gold plated contacts. As long as the contacts

   are never loaded beyond the "low level specs"

   the layer of gold preserves contact integrity.

   The same device can be used in 'power' control

   but the arcing will burn the gold away and

   the contact morphs into a 'power only' device.



   Some years ago, my employer's receiving inspection

   decided that it would be a good deal to 100%

   test all incoming, crystal can relays at their

   rated loads. Thousands of devices were put into

   a fixture and 'tested' at 2A.



   Problem is that many of these relays were destined

   for jobs as managers of low level signals. The gold

   plating was burned away during receiving inspection.

   The crippled relays began to fail in large numbers

   sometimes years after the airplane was delivered.



   It's just possible that some thoughtful designer

   at Garmin built a feature into the PTT line that

   reduces the need for PTT switches with gold

   contacts.



   Any chance you could get a 'scope trace of this

   current pulse?

   Bob . . .

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bbradburry(at)bellsouth.n
Guest

Posted: Wed Dec 05, 2012 3:54 pm Post subject: Initial amp spike on PTT

Bob,

No I have no access to a scope.  



As we were checking this problem with keying the mike out today, we found

that the RAC trim indicators were going to one end of the indicators and

dimming out when we keyed the mike.  We have checked out the power and

ground wires for these indicators and they seem ok.



I called the Ray Allen Co about this and he suggests that I sounds like an

RF problem, but we fabricated a coax that ran from the radio to the antenna

by going completely outside the plane to insure that we were not getting RF

interference and it did not change the indicators performance.



I keep thinking that someone previously on the list had a problem with the

indicators a year or so ago and possibly found a fix???



B2



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