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OVM14 MkIII, rev P1
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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Sep 20, 2023 6:46 pm Post subject: OVM14 MkIII, rev P1

Here 'tis.



 You guys with sharp pencils and greased calculators

 are encouraged to flush out the 'bugs'.



 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

9003-620-1P1,_OVM-14_Mk_III,_SCHEMATIC.pdf

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Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Fri Sep 22, 2023 11:50 am Post subject: Re: OVM14 MkIII, rev P1

A few things that I noticed...



1. Absolute maximum cathode-to-anode voltage on the LM431 is 37V (see TI and onsemi datasheets).  A significant OV event could exceed that limit until the crowbar brings bus voltage under control.  This might not be an issue with a lead-acid battery, but the OV behavior of the BMS in lithium batteries is unknown.  If a lithium BMS disconnects pronto, bus voltage could rise farther and more quickly than with lead-acid on the job.



Q: Should there be a 33V Zener in parallel to protect the LM431?



-----



2. The recommended maximum voltage on LM293 inputs is Vcc minus 2V (see table 6.2 in the TI datasheet).  In this case Vcc is 8V, so the max input voltage is 6V.

- The 7V reference exceeds 6V on the inverting inputs of both comparators.

- Any bus voltage >13.64V will exceed 6V on the non-inverting input of U16A.

- Timing capacitor C11 on the non-inverting input of U16B will charge to 8V.



Q: Should the comparator reference be adjusted downward (perhaps delete R4, adjust R3/R5, and use the LM431's 2.5V reference for the comparators)?

Q: Should the non-inverting inputs be protected with 5.1V Zeners?



-----



3. In recent discussion of the OVM task you mentioned your intent to make the trip delay 500mS +/- 50mS.  This schematic provides ~200mS.



Q: Have you reconsidered the 500mS trip delay?



-----



4. Schematic Note 1 contemplates a crowbar current of 100-200A DC for 25mS.  The BT139-600E SCR datasheet shows a peak non-repetitive on-state current of 155A, but that specification assumes an initial junction temperature of 25degC (unlikely under an engine cowling), a pulse duration â‰¤20mS and a full-sine-wave AC pulse, not DC.



Q: Can the BT139-600E handle this crowbar task?  If not, could you swap U16B's inputs and interpose a P-FET to trigger a beefier SCR?



-----



Could you describe how to calculate the effect of closing the PTT switch (connecting 5.9kÎ© from 8V to the junction of R8 and R7)?  I'm at a loss as to how that works; it might as well be witchcraft!  Thanks.

user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Sat Sep 23, 2023 2:33 pm Post subject: Re: OVM14 MkIII, rev P1

I think that most people would expect the crowbar overvoltage device to trip

the breaker when the push-to-test button is pressed, regardless of whether 

the alternator is running or not, especially during an annual condition inspection

by a mechanic or new owner.

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Sep 23, 2023 3:49 pm Post subject: OVM14 MkIII, rev P1

At 05:33 PM 9/23/2023, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>



 I think that most people would expect the crowbar overvoltage device to trip

 the breaker when the push-to-test button is pressed, regardless of whether 

 the alternator is running or not, especially during an annual condition inspection

 by a mechanic or new owner. 	


    The product would be supplied with instructions

    calling out a procedure for validating

    continued airworthiness. Leave the button

    off if you will. Do not label it anything.



    At a propitious time verification procedures

    say, join the two wires together. Start

    the engine leaving alternator OFF. Field

    breaker should remain closed. Turn alternator

    ON. Breaker should open. Shut down engine.

    Disconnect the two wires and secure for

    service.



    The procedure not only verifies gross

    functionality but makes a good ball-

    park check of calibration. A push

    button precludes inadvertently leaving

    them together . . . but you'd know

    about it early on when the alternator

    fails to stay on line . . . so it's

    pretty much fail-passive either

    way.



 

 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Sun Sep 24, 2023 6:37 pm Post subject: Re: OVM14 MkIII, rev P1

If the overvoltage module ground wire is connected to ground using a screw in an easily accessible location, 

then it would be fairly easy to test the OVM using 3 "D" cell batteries in series per this thread:

http://forums.matronics.com/viewtopic.php?p=480365

The advantage of this method is that every component of the OVM is tested including all resistors and the LM431.

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Sep 25, 2023 9:37 am Post subject: OVM14 MkIII, rev P1

At 09:37 PM 9/24/2023, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>



 If the overvoltage module ground wire is connected to ground using a screw in an easily accessible location, 

 then it would be fairly easy to test the OVM using 3 "D" cell batteries in series per this thread:

  http://forums.matronics.com/viewtopic.php?p=480365

 The advantage of this method is that every component of the OVM is tested including all resistors and the LM431. 	


    You are correct . . . I published a similar technique

    in the CBOVM DIY project



  http://aeroelectric.com/AEC_Legacy_Products/9003/DIY_Crowbar_OVP_F.pdf 



    The proposed 'jiggering' of the ov trip setpoint

    resistors also confirms full functionality of all

    components along with a rough but reassuring

    validation of calibration.



    Normal ops centers the design point trip value

    of, in this case, 16.0 Volts. Adding the little

    tug on this setting via the testing circuit

    centers the trip at what is an accepted

    value for low voltage warning . . . 13.5

    Volts.



    This is a value at which the battery neither

    takes on more energy (charges) nor does it

    significantly deliver energy (discharge).



    Test voltages above and below this value

    are readily provided by temporary operation

    of ship's power with alternator ON (14.2

    or higher) and alternator OFF (12.8 or lower).



    The marketing goal for the test feature is

    to encourage owner/operators to periodically

    verify functionality of the ov management

    system. We KNOW how easy it is to allow some

    critical feature of our machines to languish

    especially when 'things have been running

    along nicely for a long time'. 



    Batteries are a good example. Timing belts

    on engines are another. Here on the List we

    once studied a constellation of what I

    called 'dark-n-stormy-night' stories published

    in aviation rags. In EVERY story related

    by someone who survived a bad time in the

    cockpit, the after-action follow up never

    spoke of poor preventative maintenance,

    lack of planning or pilot ignorance as to

    the condition of the equipment he/she was

    flying.



    Of course, those who did not survive similar

    events were unable to favor us with an

    "I learned about flying from that" story.



    The crowbar ov module was produced for decades

    by both AEC and B&C. AEC warranty on products

    is LIFETIME. 



  https://matronics.com/aeroelectric/Catalog/warranty.html



    Over the lifetime of CB OVM module production

    I've had two returns. Both modules were toast.

    Seems they were mis-installed with the wrong

    upstream circuit protection. In neither case

    did the customer cite damage to the rest of

    the airplane. Both modules were replace along

    with my thanks for the field-service feedback.



    OV events are rare but not zero. The probability

    of any particular OV module failing to rise

    to duty is also very small but (it's

    a manufactured device) NOT zero. It seems

    a good idea to offer some simple, minimal

    effort feature that tends to drive that risk

    toward zero.



 

 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Mon Sep 25, 2023 11:29 am Post subject: Re: OVM14 MkIII, rev P1

There is more than one way to test the OVM.  Most people would prefer a simple push button test.

A perfectionist might want to go to the trouble of testing the complete circuit with actual voltage that is too high.

Some people want an indicator light to tell if the OVM tripped the alternator field breaker, thus saving troubleshooting time.

Maybe there is way to accomplish that using solid state components instead of a latching relay.

_________________
Joe Gores

mike_tailwind

Joined: 06 Feb 2017
Posts: 12

Posted: Mon Sep 25, 2023 12:31 pm Post subject: OVM14 MkIII, rev P1

If you really want an OV indicator it is easy to do.  I think this was shown already.  Just connect an LED fed by a separate source of 12 volts and a 270 ohm 1/2W resistor to the RED20 terminal of the OVM.  When the SCR trips and the breaker opens the LED will light and it's current will keep the SCR tripped - latching.  The data sheet for the BT139-600E specifies a 45mA hold current, so the LED and resistor should be sized to supply that much current.  But, the typical hold current is 4mA, so you can probably get by with a larger resistor and smaller LED.

This LED:  Dialight 5596101027F  can handle 50mA.  I would probably start with a 470 ohm resistor instead of 270 ohm.

                  

                                                                    On Monday, September 25, 2023 at 02:38:51 PM CDT, user9253 <fransew(at)gmail.com> wrote:                 

                 



                 



                 --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>



There is more than one way to test the OVM.  Most people would prefer a simple push button test.



A perfectionist might want to go to the trouble of testing the complete circuit with actual voltage that is too high.



Some people want an indicator light to tell if the OVM tripped the alternator field breaker, thus saving troubleshooting time.



Maybe there is way to accomplish that using solid state components instead of a latching relay.



--------



Joe Gores



Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=511490#511490







=================



http://www.matronic==================



http://forums.matronics.com



https://matronics.com/contributio======================

jluckey(at)pacbell.net
Guest

Posted: Mon Sep 25, 2023 12:31 pm Post subject: OVM14 MkIII, rev P1

Joe,

You wrote:



"Maybe there is way to accomplish that using solid state components instead of a latching relay."





There definitely is, however the latching relay has a few advantages over logic circuit latch: 

1. it's cheap (~$2)



2. simple circuitry



3. relays are bullet proof (very tolerant of "noisy" environments)





-Jeff



                  

                                                                    On Monday, September 25, 2023 at 12:35:22 PM PDT, user9253 <fransew(at)gmail.com> wrote:                 

                 



                 



                 --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>



There is more than one way to test the OVM.  Most people would prefer a simple push button test.



A perfectionist might want to go to the trouble of testing the complete circuit with actual voltage that is too high.



Some people want an indicator light to tell if the OVM tripped the alternator field breaker, thus saving troubleshooting time.



Maybe there is way to accomplish that using solid state components instead of a latching relay.



--------



Joe Gores



Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=511490#511490







http://www.matronics.com/Navigator?AeroElectric-L;          - MATRONICS WEB FORUMS -



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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Sep 25, 2023 12:41 pm Post subject: OVM14 MkIII, rev P1

At 02:29 PM 9/25/2023, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>



 There is more than one way to test the OVM.  Most people would prefer a simple push button test. 	


    The current configuration COULD be

    implemented with a panel push button.

    However, without specific instructions

    on usage, what's likelihood of it being

    'over used'?  I am aware of no ov management

    system on TC or EX aircraft with any

    sort of a in-situ test feature.



    So installing a button labeled press-to-test

    will be deduced by 99.9% of pilots as

    something you can punch any time as

    often as you like. That's what the avionics

    and instrumentation tests do.



  	  Quote: 	
		  A perfectionist might want to go to the trouble of testing the complete

 circuit with actual voltage that is too high. 	


    Perhaps an individual who doesn't know

    how the thing works in the first place?



  	  Quote: 	
		  Some people want an indicator light to tell if the OVM tripped the

 alternator field breaker, thus saving troubleshooting time. 	


    Really? What's the hurry? If the breaker

    is out, something is broke. A simple

    process of manipulation + observation

    will deduce root cause. If the breaker is

    out and the device checked good in

    recent weeks, you're 90+ percent sure

    it's an ov event.



    Like any in-flight breaker operation,

    you can generally reset one time. Watch

    bus voltage. If it's an ov condition,

    there will be a slight delay in the breaker

    re-opening and a corresponding bump in

    bus voltage exceeding 15v. If it's a dead

    short, the breaker trips immediately with

    no 'bump'. If it's an intermittent or

    soft overload, the breaker may stay in

    and bus voltage comes rises from battery to

    normal alternator voltage and no 'bump'.



    In the later case, you've got a bigger

    can of worms. In the first two cases,

    choice of root causes is pretty limited

    but the process takes the same amount

    of time irrespective of which gremlin

    is being outted.



 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Mon Sep 25, 2023 2:23 pm Post subject: Re: OVM14 MkIII, rev P1

 	  nuckolls.bob(at)aeroelect wrote: 	
		  The current configuration COULD be implemented with a panel push button. 	


I imagined the PTT mounted on short wires, tied into the wiring harness for the OVM and inaccessible to the pilot in flight.  During oil changes or condition inspections -- when the cowling is already off -- close the PTT, conduct the test, then open the PTT.  No risk of over-use and if properly documented in the Operating Handbook, future owners will know what to do with it too.

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Mon Sep 25, 2023 3:02 pm Post subject: Re: OVM14 MkIII, rev P1

 	  Joe Gores wrote: 	
		  There is more than one way to test the OVM.  Most people would prefer a simple push button test.

A perfectionist might want to go to the trouble of testing the complete circuit with actual voltage that is too high. 	


That's what's so clever about Bob's PTT: it relies on the complete circuit to accomplish the test by changing the sensed voltage slightly when the button is pushed.  It tests everything, but there's no need to disconnect the module or apply an actual overvoltage to the bus.  As he mentioned elsewhere, it targets a point just below normal bus voltage and trips when the charging system is turned on, which roughly verifies calibration as well as operation.



 	  Quote: 	
		  Some people want an indicator light to tell if the OVM tripped the alternator field breaker, thus saving troubleshooting time.

Maybe there is way to accomplish that using solid state components instead of a latching relay. 	


Just for the sake of conversation, and leaving aside the argument about whether or not it's a good idea, yes, there is: in addition to triggering the SCR, the delay timing comparator could also activate other output circuitry.  No additional logic or relays required.



-----



Regarding the last couple of sentences in my comments on Bob's schematic...



 	  Eric Page wrote: 	
		  Could you describe how to calculate the effect of closing the PTT switch (connecting 5.9kÎ© from 8V to the junction of R8 and R7)? I'm at a loss as to how that works; it might as well be witchcraft! 	


...I found a nice set of tutorials...



https://www.allaboutcircuits.com/textbook/direct-current/chpt-10/what-is-network-analysis/



...that explain several ways to calculate the values in his PTT circuit.  It turns out not to be witchcraft after all!



-Eric

user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Tue Sep 26, 2023 6:56 am Post subject: Re: OVM14 MkIII, rev P1

At first, Mike_Tailwind's comment, "the LED will light and it's current will 

keep the SCR tripped - latching" seemed like a good idea.  But what happens 

when the pilot resets the circuit breaker?  Won't it just trip again since the 

SCR is still conducting?

_________________
Joe Gores

mike_tailwind

Joined: 06 Feb 2017
Posts: 12

Posted: Tue Sep 26, 2023 9:32 am Post subject: OVM14 MkIII, rev P1

Yep, you would have to cycle master power first.

                  

                                                                    On Tuesday, September 26, 2023 at 10:09:42 AM CDT, user9253 <fransew(at)gmail.com> wrote:                 

                 



                 



                 --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>



At first, Mike_Tailwind's comment, "the LED will light and it's current will 



keep the SCR tripped - latching" seemed like a good idea.  But what happens 



when the pilot resets the circuit breaker?  Won't it just trip again since the 



SCR is still conducting?



--------



Joe Gores



Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=511509#511509



http://www.matronics.com/Navigator?AeroEle            - MATRONICS WEB FORUMS -



/wiki.matronics.com" rel="nofollow" target="_blank">http://wiki.matroni=======================

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user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Tue Sep 26, 2023 1:40 pm Post subject: Re: OVM14 MkIII, rev P1

How about using a second SCR for an over-voltage tripped light?  Maybe 

something like the attached circuit.  Keep in mind that I am not an electronic 

engineer.  I don't even play one of TV.

OV indicator light__.png

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_________________
Joe Gores

Last edited by user9253 on Tue Sep 26, 2023 5:31 pm; edited 1 time in total

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Tue Sep 26, 2023 3:48 pm Post subject: Re: OVM14 MkIII, rev P1

 	  user9253 wrote: 	
		  How about using a second SCR for an over-voltage tripped light?  Maybe something like the attached circuit. 	


See edits...

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user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Tue Sep 26, 2023 4:00 pm Post subject: Re: OVM14 MkIII, rev P1

I thought about that at first Eric, but it only takes 50 microamps to turn on the BT169D SCR.  

I was concerned that much current would come via R12.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Tue Sep 26, 2023 5:37 pm Post subject: Re: OVM14 MkIII, rev P1

I fixed the LED polarity.  I kept the schematic simple by not drawing the test 

circuit, but I think it is a good idea and should be included.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1910
Location: Riley TWP Michigan

Posted: Wed Sep 27, 2023 1:51 pm Post subject: Re: OVM14 MkIII, rev P1

Eric, how about this circuit for an over-voltage indicator light?

I added a Schottky diode to prevent R12 current from turning on the BT169D SCR.

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Joe Gores

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Wed Sep 27, 2023 5:13 pm Post subject: Re: OVM14 MkIII, rev P1

The output of the LM293 comparator is a bit strange.  It can "sink" current (draw current out of the circuit, to ground), but it can't "source" any current (supply current to the circuit).  It's a transistor that can either be on ("low," pulling anything connected to the output down to ground), or off ("high," doing nothing).  When the output is low (again, transistor on), the current that flows through resistor R12 is bled off to ground and can't do anything in the circuit.  When the output is high (transistor off), R12 can "pull up" the output node to the supply voltage and the current from R12 turns on the crowbar SCR.



The diode that you added will block the comparator's output transistor from being able to bleed R12 current out of the circuit, so the crowbar SCR will be permanently on.  Likewise, it will prevent any current from R12 from reaching the SCR in the LED circuit, so it will never turn on.  In this application, you can think of the diode as an electrical check valve; its symbol tells you which way current can flow (with the arrow).



To do what you want, the diode should be removed and both SCRs' gates should be connected to the node that's common to the comparator's output and the bottom of R12.  When the output goes high, current from R12 will turn on both SCRs, crowbarring the field breaker and turning on the LED.



Each SCR's gate will only absorb as much current as it needs to turn on.  Bob specified a 750-ohm resistor because the crowbar SCR is a high power device and it wants a stout gate current to trigger.  The SCR you selected for the LED requires so little current that it will trigger just fine from the same pull-up resistor as the bigger one.



-Eric

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