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Voltage drop puzzle

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andrewbutler(at)ireland.c
Guest

Posted: Tue Aug 11, 2009 4:22 am Post subject: Voltage drop puzzle

Hello All,



I have more or less finished my wiring. I have impelemented Z13/8 on my project. I have a switch to engage the relay that isolates the feed from the SD-8 during normal operations. I have installed a bulb in this circuit to tell me that the relay is engaged.



While I have 12V plus on my main bus, I am getting a much lower voltage (the order of 4 volts) through the bulb (I was alerted to this by the bulb being very dim). Why is this? Sure enough I should have a drop through 1N5400 diode that is in the circuit, but surely not that much? Does the relay itself contribute to the drop aswell? Attached is the diagram with a circle around the bulb in question. The circuit is installed as shown.



Is this even a problem? The relay engages to the NO position as required when I flip the switch (number 12) and stays there.



Best regards,



Andrew Butler,

RV7 EI-EEO

Firewall Forward

Galway Ireland.

Voltage_Drop_Query.pdf

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mprather(at)spro.net
Guest

Posted: Tue Aug 11, 2009 6:14 am Post subject: Voltage drop puzzle

 	  Quote: 	
		   Hello All,



 I have more or less finished my wiring. I have impelemented Z13/8 on my

 project. I have a switch to engage the relay that isolates the feed from

 the SD-8 during normal operations. I have installed a bulb in this circuit

 to tell me that the relay is engaged.



 While I have 12V plus on my main bus, I am getting a much lower voltage

 (the order of 4 volts) through the bulb (I was alerted to this by the bulb

 being very dim). Why is this? Sure enough I should have a drop through

 1N5400 diode that is in the circuit, but surely not that much? Does the

 relay itself contribute to the drop aswell? Attached is the diagram with a

 circle around the bulb in question. The circuit is installed as shown.



 Is this even a problem? The relay engages to the NO position as required

 when I flip the switch (number 12) and stays there.



 Best regards,



 Andrew Butler,

 RV7 EI-EEO

 Firewall Forward

 Galway Ireland.


	




As drawn, you have the bulb in series with the coil - a voltage divider. 

This probably isn't a very good thing as the series resistance of the bulb

is reducing the current through the coil which will reduce the turn-on

margin on the relay.  If the bus voltage drops, you might not be able to

get the relay to close.  Once closed, it will probably stay there until

the controlling switch is opened - which is fine.



It seems like it's not a good idea to insert anything in the circuit to

the coil.  If you wanted to monitor the position of the switch you could

select a double pole (DPST) switch to control the relay and wire the lamp

through the extra pole.

Matt-

andrewbutler(at)ireland.c
Guest

Posted: Tue Aug 11, 2009 6:21 am Post subject: Voltage drop puzzle

Having just stared at it a while, I realise that I need to take the unreliable bulb out of the circuit. Any suggestions on how I could wire one in to tell me whether or not the circuit is live?



Andrew.



---

jay(at)horriblehyde.com
Guest

Posted: Tue Aug 11, 2009 6:46 am Post subject: Voltage drop puzzle

Hi there,



I think that the voltage drop through the relay is the problem.  Maybe

change the aux power switch to a 2 pole one (a 2-3) and then wire the second

pole with a 'clean' 12V from the 'upstream' side of the relay - say from the

same terminal as where the yellow wire of the OV crowbar goes to; then

through the switch and via the light to ground.



Jay

--

andrewbutler(at)ireland.c
Guest

Posted: Tue Aug 11, 2009 6:47 am Post subject: Voltage drop puzzle

Thanks Matt. My own bulb went off in my head while thinking on it a bit more and figured just that about the DPST switch.



Andrew.



---

flyboybob1(at)gmail.com
Guest

Posted: Tue Aug 11, 2009 6:48 am Post subject: Voltage drop puzzle

Matt wrote: <<  If you wanted to monitor the position of the switch you

could

select a double pole (DPST) switch to control the relay and wire the lamp

through the extra pole. >>



It would be a weak design goal to know the position of the switch.  A better

goal would be to know the position of the relay.  A still better goal would

be to know the function the relay controled was working.  Lectric Bob has

presented an inexpensive and elegant solution to the best design goal here.

In a recent thread with the subject: Pitot heaters . . . warn or not, that

IS the question, bob posted his solution on the web at:



http://www.aeroelectric.com/articles/Failure_Detection_and_Annunciation.pdf



Hope this helps.



Regards,

Bob Lee

N52BL  KR2

Suwanee, GA  USA

92% done only 67% to go!

andrewbutler(at)ireland.c
Guest

Posted: Tue Aug 11, 2009 7:49 am Post subject: Voltage drop puzzle

Thanks Jay,



What about if using the 2-3 switch I connected the light to the live terminal on the capacitor? That would allow me to test the relay in pre-flight by engaging the switch (and checking for illumination) while also allowing me to monitor that I have flipped the switch during flight........



Andrew.

---

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Aug 11, 2009 7:49 am Post subject: Voltage drop puzzle

At 07:16 AM 8/11/2009, you wrote:

 	  Quote: 	
		  Hello All,



I have more or less finished my wiring. I have impelemented Z13/8 on 

my project. I have a switch to engage the relay that isolates the 

feed from the SD-8 during normal operations. I have installed a bulb 

in this circuit to tell me that the relay is engaged.



While I have 12V plus on my main bus, I am getting a much lower 

voltage (the order of 4 volts) through the bulb (I was alerted to 

this by the bulb being very dim). Why is this? Sure enough I should 

have a drop through 1N5400 diode that is in the circuit, but surely 

not that much? Does the relay itself contribute to the drop aswell? 

Attached is the diagram with a circle around the bulb in question. 

The circuit is installed as shown.



Is this even a problem? The relay engages to the NO position as 

required when I flip the switch (number 12) and stays there.


	




   I'm surprised that the relay still pulls in but

   that's a function of bulb size. As others have noted

   you have the bulb wired incorrectly for the purpose

   of showing that the relay is "closed". Wired as shown,

   it only says that the relay coil has some current flowing

   through it . . . but does not speak to actual connection

   (or functionality) of the alternator.



   Further, for you lamp to function as a  "relay energized"

   notification, the bulb needs to be in PARALLEL with the

   relay coil, not series.



   May I suggest that your active notification of low voltage

   is a much better way to report condition of the WHOLE alternator

   system? The fact that you KNOW the relay is energized

   is not very informative. There are lots of things that

   could cause alternator output to be inadequate or

   completely missing. The fact that bus voltage is too

   low to avoid discharging the battery (13.0) is the

   DEFINITIVE indicator of alternator performance.



        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------

jay(at)horriblehyde.com
Guest

Posted: Tue Aug 11, 2009 8:49 am Post subject: Voltage drop puzzle

Hey there Andrew,



That would work fine- remember though that all you really are indicating is

that the pole on the switch is working    Hopefully the other pole is

working as well...



The LV monitoring that Bob describes is also a good idea- normally this is

part of the Z13/8 circuit and perhaps you plan to include it in any case.

On the Sling I used the MGL EFIS to monitor the voltage; you can set an

alarm that can also give you a digital output to switch something.  If you

are using an EFIS perhaps it has similar features?  If not I would use the

LV warning cct and light.



Another idea is to reverse the wiring for the S704-1 relay and use the

normally closed contact to illuminate a light when the relay is de-energised

(see the attached diagram); to my mind this also tells you that the dynamo

is producing a voltage when it's not in use.



Jay



--

Visio-Z-13_8_Dynamo_alternative_cct.pdf

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user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Thu Aug 13, 2009 5:52 am Post subject: Re: Voltage drop puzzle

What is the purpose of the diode connected to the over voltage relay coil?  It appears that the intent is to raise the trip voltage by 0.7 volts, for instance from 16 to 16.7 volts.  If the purpose of the diode is for arc suppression, then the diode should be connected in parallel with the coil, not in series.

Also, the 1A fuse inside of the red circle will blow before the 2A circuit breaker will trip.  The Fuse should be removed from the circuit.

After shutdown, leaving the Aux Power switch on will drain the battery.

Joe

_________________
Joe Gores

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