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EFIS brownout capacitor

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user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Thu Dec 01, 2011 12:44 am Post subject: EFIS brownout capacitor

Bob and all,

Is it feasible to provide brownout protection for an EFIS during engine cranking with a large electrolytic capacitor?  A capacitor would not have to be replaced periodically and it might not weigh as much as a backup battery.  Each brand of EFIS might have a different supply voltage requirement.  But for this discussion, let's assume that 9 volts are required to prevent the EFIS from rebooting.  How long is the battery voltage below 9 volts when starting the engine?  The starter will draw the most current  when it is first energized.  Once the starter is rotating, back EMF will reduce the current draw.  So for the first several milliseconds, the battery voltage will be at the lowest.  If a large capacitor could support the EFIS voltage for one second, would that be long enough?  If an EFIS draws 1 amp, what size capacitor would be required?  Some EFISs have dedicated terminals for a backup battery.  I assume that those EFISs have an internal diode to prevent discharge of the backup battery into the aircraft electrical system.  Otherwise an external diode is needed.

  Could a capacitor provide brownout protection?  Or would it have to be so large as to make it impractical?

Joe

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 01, 2011 6:59 am Post subject: EFIS brownout capacitor

At 02:44 AM 12/1/2011, you wrote:




Bob and all,

Is it feasible to provide brownout protection for an EFIS during 

engine cranking with a large electrolytic capacitor?  A capacitor 

would not have to be replaced periodically and it might not weigh as 

much as a backup battery.  Each brand of EFIS might have a different 

supply voltage requirement.  But for this discussion, let's assume 

that 9 volts are required to prevent the EFIS from rebooting.  How 

long is the battery voltage below 9 volts when starting the 

engine?  The starter will draw the most current  when it is first 

energized.  Once the starter is rotating, back EMF will reduce the 

current draw.  So for the first several milliseconds, the battery 

voltage will be at the lowest.  If a large capacitor could support 

the EFIS voltage for one second, would that be long enough?



      Probably. It might even be as short as 250 milliseconds. See:



http://www.aeroelectric.com/Pictures/Curves/99_Saturn_SL1_2.gif



http://www.aeroelectric.com/Pictures/Curves/99_Saturn_SL1.jpg



http://www.aeroelectric.com/Pictures/Curves/95_GMC_Safari_3.gif



http://www.aeroelectric.com/Pictures/Curves/95_GMC_Safari_1.gif



      The traces above were taken from the two vehicles

      we were driving at the time. The Saturn has a PM

      starter motor, the Safari has a wound field. The

      larger depression of battery voltage in the Saturn

      trace can be partially attributed to the the

      greater inrush current for the PM motor.



If an EFIS draws 1 amp, what size capacitor would be required?  Some 

EFISs have dedicated terminals for a backup battery.  I assume that 

those EFISs have an internal diode to prevent discharge of the backup 

battery into the aircraft electrical system.  Otherwise an external 

diode is needed.



Could a capacitor provide brownout protection?  Or would it have to 

be so large as to make it impractical?



    It's been done. Consider that the rate of change

    across a capacitor is defined by V/S = A/F



    1 Amp of charge (or discharge) on a 1 Farad

    capacitor produces a voltage change rate

    of 1 Volt per Second. Of course, few devices

    draw a constant current as the voltage varies

    so this relationship has a practical limitation

    that falls apart for more than short durations.

    Of course, that's what we're talking about.



    Of the top of our head, we could estimate that

    a capacitor charged to .7 volts below battery

    voltage (12.5 - .7) = 11.8 volts. The goal is

    to limit voltage drop to 2.8 volts over a .25

    second interval



    So, V/S = A/F



        (V/S)F = A



        F = AS/V = .25/2.8 = .09 Farads



    There's an RC vs. V calculator at



http://www.bowdenshobbycircuits.info/rc.htm



    which can be used to do the same math. In

    this case, plug in some essentially constant

    current combination, say 100 volts and 0.1K

    ohms (1A). Then plug in time of 250mS and

    voltage change of 2.8 volts. The tool calculates

    88029.7978 microFarads or 0.088F.



    Unfortunately, fat-caps don't come in the

    range of incremental sizes as their smaller

    cousins. Also, 16V ratings is in the realm

    of high voltage for a super-cap. Here's a

    device that would fit our hypothetical:



http://search.digikey.com/us/en/products/BZ12GA124ZAB/478-6281-ND/2506257



    It's a 120 milliFarad device described in this

    data sheet:



http://www.avx.com/docs/Catalogs/bestcap.pdf



    This exercise suggests to the practicality of using

    more 'active' forms of bus voltage regulation. If

    I were writing specs for a modern EFIS system

    where design goals called for improved immunity

    to starter brownout, I'd call for the switchmode

    power supply output to be stable down to 7 or 8

    volts. Of course, input currents to the supply

    would double during the brownout interval but

    it's a short duration.



    Bottom line is that its a lower cost of ownership

    to design brown out protection into a device than

    to paste it on outside.



    I proposed a scheme to a customer some years

    back where we explored the addition of a 3 volt,

    DC to DC converter to be switched in as a bus

    voltage booster during cranking times. The customer

    tried it in the lab with positive results . . .

    don't know if it ever went into production.



    A 3v, 1A supply is an ittty-bitty critter compared

    to a super-cap.

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Thu Dec 01, 2011 9:36 am Post subject: Re: EFIS brownout capacitor

Bob,

Thank you very much for taking the time to reply.

Digikey has some other large capacitors that are not as expensive:

$16  http://search.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=ECE-T1EP104FA

$12  http://search.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=ECE-T1CP104FA

One of these capacitors might be a viable alternative to replacing an expensive internal backup battery for someone who only needs brownout protection and not a longer backup time.

Joe

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Thu Dec 01, 2011 9:41 am Post subject: Re: EFIS brownout capacitor

When the master switch is turned on, would the inrush current to a 0.1F capacitor be a problem?

Thanks, Joe

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 01, 2011 12:08 pm Post subject: EFIS brownout capacitor

At 11:41 AM 12/1/2011, you wrote:

 	  Quote: 	
		  



When the master switch is turned on, would the inrush current to a 

0.1F capacitor be a problem?

Thanks, Joe


	




   Good question. If the cap is fed with say,

   4' of 22AWG (16 mOhm/ft) then there is

   potential for a 150+ Amp inrush. You might

   want to consider some ballasting resistance

   or perhaps an inrush limiter sized to your

   operating loads.

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 01, 2011 2:37 pm Post subject: EFIS brownout capacitor

At 01:35 PM 12/1/2011, you wrote:

 	  Quote: 	
		  

<nuckolls.bob(at)aeroelectric.com>


	


After some sifting through the gray matter archives, I retrieved

the core principals behind an active 'bus booster' that I

crafted some years back. Making this technology work for brown-out

protection would look something like this:



http://aeroelectric.com/Pictures/Schematics/Royer_Oscillator_Bus-Booster.pdf



I've used this style power oscillator in dozens of products.

It's designed around a saturable core transformer that operates

in the 1000-4000 Hz range. These are usually tape-wound toroidal

cores.



This particular implementation uses 'upside down PNP' power

transistors with grounded collectors. The assembly staff really

liked that . . . no insulating washers and grease!



This would be a DC-DC power supply that produces say 3.5 volts

for a 12 volt input. The instant the starter contactor is

energized, battery voltage applied to the protected device

is boosted to about 16.0 volts. When the starter inrush

hits, battery voltage drops to say 9 volts with a

proportional drop in boost level to about 2.4 volts

which is added to the 9 volts for a 'protected' output

of 11.4 or so.



When the starter is de-energized, the oscillator is

unpowered, boosting is replaced by normal bus power through

the upper, diode-isolated input. To handle a 1A load, output

power would be under 5 watts. This is a really small oscillator.

   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Fri Dec 02, 2011 6:57 am Post subject: Re: EFIS brownout capacitor

Bob,

Thanks for the Bus-Booster circuit.

I was thinking that an inductor in series with an EFIS would limit the inrush current to a large capacitor.  And when the main battery voltage dropped during cranking, that an inductor will try to maintain current flow to the EFIS while the capacitor tries to maintain the voltage.  I do not know what size inductor is required.  Inductors are heavy compared to capacitors.  It is starting to look like a small backup battery is the easiest solution to brownout protection.

Joe

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Dec 02, 2011 11:57 am Post subject: EFIS brownout capacitor

At 08:57 AM 12/2/2011, you wrote:

 	  Quote: 	
		  



Bob,

Thanks for the Bus-Booster circuit.

I was thinking that an inductor in series with an EFIS would limit 

the inrush current to a large capacitor.  And when the main battery 

voltage dropped during cranking, that an inductor will try to 

maintain current flow to the EFIS while the capacitor tries to 

maintain the voltage.  I do not know what size inductor is 

required.  Inductors are heavy compared to capacitors.  It is 

starting to look like a small backup battery is the easiest solution 

to brownout protection.


	


    An inductor would have that effect . . .

    but on a scale of weight and volume that

    would be exceedingly unattractive.



    Yes, batteries are effective energy

    storage devices when their weight and

    volume are compared with other technologies.



    The cost-of-ownership driver for standby batteries

    is maintenance. What would be really cool is to

    craft a cell technology that might offer a

    fraction of the energy storage capability

    for popular cells but high longevity numbers.

   Bob . . .

klehman(at)albedo.net
Guest

Posted: Fri Dec 02, 2011 12:23 pm Post subject: EFIS brownout capacitor

 	  Quote: 	
		   The cost-of-ownership driver for standby batteries

 is maintenance. What would be really cool is to

 craft a cell technology that might offer a

 fraction of the energy storage capability

 for popular cells but high longevity numbers.

 Bob . . .




	


pretty hard to beat a couple of D cell flashlight batteries (and a mini 

relay) for a couple of seconds of 1 amp 3 volt boost if that is still 

the situation being discussed.



One could even leave the batteries in the flashlight so that it can be 

used as a --- flashlight.



Ken

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Dec 03, 2011 5:15 am Post subject: EFIS brownout capacitor

pretty hard to beat a couple of D cell flashlight batteries (and a 

mini relay) for a couple of seconds of 1 amp 3 volt boost if that is 

still the situation being discussed.



One could even leave the batteries in the flashlight so that it can 

be used as a --- flashlight.



   Absolutely! I can recall the use of alkaline

   or ni-cad cells being suggested while mulling over

   design options. I think zero-maintenance circuitry

   consistently won out over the use of components

   with a shelf or service life.



   Of course, that was the T/C market which is driven

   by different design goals.

   Bob . . .

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