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Car clock in an aircraft

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trigo(at)mail.telepac.pt
Guest

Posted: Tue Apr 10, 2018 1:49 pm Post subject: Car clock in an aircraft

To the electrowizzards in this list.

(This is not a rocket science aircraft subject, but I need your help on this)



I bought one of those cheap (chinese) digital car clocks, with temperature indication, to install in the back of my RV-10, as a courtesy to the rear passengers. (please avoid any comments on this decision... ).

As one could expect, the miserable instructions leaflet which came with the clock, doesn’t have any information about the electric circuit, so I’m in the guess field here.



This critter has a 2-wire cable to be connected to 12V/24V, and also has 2 of those coin type batteries, so I suppose that the batteries are a backup to the ship’s power.

But I’m not sure...



This leaves me with a doubt on where to connect the + wire of the power cable.

Should I connect it to a circuit which is only powered when the aircraft Master switch is on, to avoid this critter to deplete the aircraft battery?

Or can I connect it to the always hot bus, hoping that when the Master is Off, the clock’s coin batteries will be powering the clock and only a very tiny current will be draining from the always hot bus, not depleting the aircraft’s battery?



Even knowing that without information on the clocks’ electric circuit it is difficult to know the answer, comments and suggestions are welcome



Thanks

Carlos

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BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Tue Apr 10, 2018 2:38 pm Post subject: Car clock in an aircraft

Carlos:

You can do either.

I would put it on a SWITCHED (On/Off) buss.

The 'watch battery' is as you guess for a 'keep alive' circuit for the clock.

The easy way of proving this would be to remove the watch battery while the unit is plugged into the planes power.  THEN!  Shut OFF the plane's power.  Since the keep alive battery is no longer in circuit the clock should die and loose time (keep the plane's power off for 5 minutes just for S&G's.  The clock should NOT hold memory and should either die or loose time...  The 5 minutes.

Why would I use a switched buss?  Because if you do not fly for a REAL long time the clock and what ever else you have connected directly to the battery would drain the battery.  Slowly for sure but, drain none the less.

I just had a fellow tell me his 18 Ah battery was dying every two weeks.

I checked the current draw on the battery with EVERYTHING Off.

There was a 45 ma draw.

When you work out the Ah and the hours in 2 weeks ..  The battery would be dead!

Problem identified...

Problem solved...  The plane had a CD player with a keep alive circuit.  

For what?  The CLOCK and the play list memory.

Pulled the CD player and the problem went away!

The CD player was illegal for a certified plane anyway!



Barry





On Tue, Apr 10, 2018 at 5:40 PM, Carlos Trigo <trigo(at)mail.telepac.pt (trigo(at)mail.telepac.pt)> wrote:

 	  Quote: 	
		  --> AeroElectric-List message posted by: Carlos Trigo <trigo(at)mail.telepac.pt (trigo(at)mail.telepac.pt)>

 

 To the electrowizzards in this list.

 (This is not a rocket science aircraft subject, but I need your help on this)

 

 I bought one of those cheap (chinese) digital car clocks, with temperature indication, to install in the back of my RV-10, as a courtesy to the rear passengers. (please avoid any comments on this decision... ).

 As one could expect, the miserable instructions leaflet which came with the clock, doesn’t have any information about the electric circuit, so I’m in the guess field here.

 

 This critter has a 2-wire cable to be connected to 12V/24V, and also has 2 of those coin type batteries, so I suppose that the batteries are a backup to the ship’s power.

 But I’m not sure...

 

 This leaves me with a doubt on where to connect the + wire of the power cable.

 Should I connect it to a circuit which is only powered when the aircraft Master switch is on, to avoid this critter to deplete the aircraft battery?

 Or can I connect it to the always hot bus, hoping that when the Master is Off, the clock’s coin batteries will be powering the clock and only a very tiny current will be draining from the always hot bus, not depleting the aircraft’s battery?

 

 Even knowing that without information on the clocks’ electric circuit it is difficult to know the answer, comments and suggestions are welcome

 

 Thanks

 Carlos

 

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user9253

Joined: 28 Mar 2008
Posts: 1926
Location: Riley TWP Michigan

Posted: Tue Apr 10, 2018 2:44 pm Post subject: Re: Car clock in an aircraft

Set your multi-meter to read milliamps and connect it in series with a 12 volt power wire.  Then measure the clock's current draw.  Once armed with the numbers, a decision can be made.  It is puzzling why a digital clock would even have a power wire unless it is for a back light.  Most digital clocks are powered by internal batteries.

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_________________
Joe Gores

JOHN TIPTON

Joined: 17 Sep 2006
Posts: 239
Location: Torquay - England

Posted: Tue Apr 10, 2018 11:31 pm Post subject: Car clock in an aircraft

Hi Carlos



Do you have an 'off aircraft' battery/power sauce to run a test on this clock, as for aircraft power, any low current item (instrument lights) could share its power supply and fuse/CB I would imagine



Regards



John



Sent from my iPad



     ----x--O--x----



 	  Quote: 	
		   On 10 Apr 2018, at 10:40 pm, Carlos Trigo <trigo(at)mail.telepac.pt> wrote:

 

 

 

 To the electrowizzards in this list.

 (This is not a rocket science aircraft subject, but I need your help on this)

 

 I bought one of those cheap (chinese) digital car clocks, with temperature indication, to install in the back of my RV-10, as a courtesy to the rear passengers. (please avoid any comments on this decision... ).

 As one could expect, the miserable instructions leaflet which came with the clock, doesn’t have any information about the electric circuit, so I’m in the guess field here.

 

 This critter has a 2-wire cable to be connected to 12V/24V, and also has 2 of those coin type batteries, so I suppose that the batteries are a backup to the ship’s power.

 But I’m not sure...

 

 This leaves me with a doubt on where to connect the + wire of the power cable.

 Should I connect it to a circuit which is only powered when the aircraft Master switch is on, to avoid this critter to deplete the aircraft battery?

 Or can I connect it to the always hot bus, hoping that when the Master is Off, the clock’s coin batteries will be powering the clock and only a very tiny current will be draining from the always hot bus, not depleting the aircraft’s battery?

 

 Even knowing that without information on the clocks’ electric circuit it is difficult to know the answer, comments and suggestions are welcome

 

 Thanks

 Carlos

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jim(at)PoogieBearRanch.co
Guest

Posted: Wed Apr 11, 2018 6:27 am Post subject: Car clock in an aircraft

Hm.  If it works without being wired in, why not just leave it powered

by the button cell batteries?  Seems like a "non-essential" item to me,

and I would hate for there to be ANY possibility it might drain the

battery over time.  



Jim Parker

------

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Apr 11, 2018 11:04 am Post subject: Car clock in an aircraft

At 09:26 AM 4/11/2018, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: <jim(at)poogiebearranch.com>



 Hm.  If it works without being wired in, why not just leave it powered

 by the button cell batteries?  Seems like a "non-essential" item to me,

 and I would hate for there to be ANY possibility it might drain the

 battery over time.  	


   If any accessory powered internally by a 'button cell'

   is fitted with an external power connection, I would

   wonder if external power isn't just for lighting.

   Button cell capacities are very low . . . digital

   devices that use them will have current drains 

   measured in microamps.  If the external power does

   indeed supplement the internal battery, then drain

   on that lead will be insignificant with respect to

   energy stored in the ship's battery. Similarly,

   electric clocks in car's used to be powered directly

   from the battery with no significant impact on

   the system's availability.



   If the external lead is to power panel illumination,

   then consumption will be much higher . . . but

   probably independent of electronics. Hence needs

   voltage only while the airplane is powered up.



 

 

   Bob . . .

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Eric Page

Joined: 15 Feb 2017
Posts: 247

Posted: Wed Apr 11, 2018 12:13 pm Post subject: Re: Car clock in an aircraft

Typically, Real Time Clock ICs will have two power input pins, a primary and a backup.  For example, see the MCP7940N datasheet...



http://ww1.microchip.com/downloads/en/DeviceDoc/20005010F.pdf



...where you'll see that Pin 4 is for normal operating supply and Pin 3 is for battery backup.  Generally speaking, the battery is there only to keep timekeeping underway if primary power is lost.  It's not uncommon for all other functions of the clock -- including time display -- to cease functioning when using battery backup.



So, chances are good that if you power your automotive clock from a switched bus, the coin cell will keep the clock accurate when the bus is turned off.  You'll know it's time to replace it when the clock doesn't keep time properly between flights.



Eric

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Mauledriver(at)nc.rr.com
Guest

Posted: Fri Apr 13, 2018 5:21 am Post subject: Car clock in an aircraft

Agreed.  And if it's needed, don't put it on an always-on bus.  I had 

this situation with an earlier GRT unit that had an internal clock that 

required an always-on connection.  Actually I had 3 units... the 

combined drain slowly but surely killed my AGM battery.  They don't like 

that kind of treatment (there's slightly more to the story).



But I would say "don't connect anything to always-on unless critical... 

then question why that product is in  your plane".



On 4/11/2018 10:26 AM, jim(at)poogiebearranch.com wrote:

[quote] 



 Hm.  If it works without being wired in, why not just leave it powered

 by the button cell batteries?  Seems like a "non-essential" item to me,

 and I would hate for there to be ANY possibility it might drain the

 battery over time.



 Jim Parker

 ------

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