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LED Light for SD-8 On Indication

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rossmickey(at)comcast.net
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Posted: Sun Jun 07, 2015 4:57 pm Post subject: LED Light for SD-8 On Indication

I wanted to install an LED "ON" indicator light for when I switch on the SD-8 alternate power supply.  I rigged an LED so it works with the 12 Volt system and tried it on the switch which goes to ground for ON.  It works fine when it is the only thing on the switch.  When I hook up the SD-8 relay to it ( as wired for the original Z-8 diagram with NO self excite) which is also a switch to ground for on, the LED lights up even in the off position.  Since the LED ground and SD-8 relay ground share the same terminal, my only explanation is that the LED is finding a ground somewhere along the SD-8 relay pathway.  I have stared at the drawing until my eyes are crossed and can't figure out what's happening.



Is my SD-8 wiring screwed up or is there actually a way for the LED to find a ground along the pathway?  If so, I guess I am SOL on hooking an on indicator light.



Ross

 [quote][b]

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user9253

Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan

Posted: Sun Jun 07, 2015 7:09 pm Post subject: Re: LED Light for SD-8 On Indication

Can you attach a drawing of your schematic showing how the LED is wired?  I can not visualize how the LED is connected.  LEDs require very little current to illuminate.  Sometimes a solid state switch (when off) will leak enough current to turn on an LED.  A schematic will be very helpful to others who are trying to answer your question.

Joe

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Joe Gores

nuckolls.bob(at)aeroelect
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Posted: Mon Jun 08, 2015 4:59 am Post subject: LED Light for SD-8 On Indication

At 07:54 PM 6/7/2015, you wrote:



 I wanted to install an LED "ON" indicator light for when I switch on the SD-8 alternate power supply.  I rigged an LED so it works with the 12 Volt system and tried it on the switch which goes to ground for ON.  It works fine when it is the only thing on the switch.  When I hook up the SD-8 relay to it ( as wired for the original Z-8 diagram with NO self excite) which is also a switch to ground for on, the LED lights up even in the off position.  Since the LED ground and SD-8 relay ground share the same terminal, my only explanation is that the LED is finding a ground somewhere along the SD-8 relay pathway.  I have stared at the drawing until my eyes are crossed and can't figure out what's happening.



    If I am interpreting your narrative correctly,

    then for some reason, there is a 'sneak path'

    to ground with the switch open. One fix for

    this phenomenon is to add a diode in the relay

    control lead that breaks sneak-path to ground

    looking at the relay.

  [img]cid:7.1.0.9.0.20150608075201.01f02418(at)aeroelectric.com.0[/img] 



   The 64 dollar question is where the sneak-path

   is coming from. As shown, I too am unable to

   identify the problem . . . UNLESS . . . your

   SD-8 is not tied to the always-hot side of

   the battery but instead tied to the switched

   side and is finding a sneak path through the

   system components on the main bus.



 

   Bob . . .

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rossmickey(at)comcast.net
Guest

Posted: Mon Jun 08, 2015 6:48 am Post subject: LED Light for SD-8 On Indication

I followed an article Jim Weir wrote back in 2000 for Kitplanes. 



http://www.rst-engr.com/kitplanes/KP0009A/KPtext.doc



http://www.rst-engr.com/kitplanes/KP0009A/KP0009A.htm



I know the switch is not leaking because the light works as intended when it

is hooked up to the switch by itself.  It is only when I connect the wire

that goes to the SD-8 relay that the light turns on apparently finding a

ground pathway through that system.



Ross



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