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1. 01:49 PM - Re: Next generation Crowbar OVM (Finn Lassen)
2. 05:16 PM - Re: Re: Next generation Crowbar OVM (Robert L. Nuckolls, III)
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| Subject: | Re: Next generation Crowbar OVM |
On 8/14/2023 8:28 PM, Robert L. Nuckolls, III wrote:
>> Looks promising. Do you have a schematic with part numbers? I'm not
>> familiar with the zener diode symbol with a third pin (input) which
>> appear to stabilize the 10V reference.
>
> That's an LM431 'adjustable zener'. . . a
> big player in many of my contemporary projects.
>
> I'm reluctant to publish values and part numbers
> until the concept is, at least, bench tested
> for conformance to design goals.
>
> The motivation for publishing the 'cartoon' was
> to plant seeds of an alternative idea based on
> a couple decades of experience, allegiance to
> the legacy design rules and just perhaps, a
> bit of irritation at decisions driven more
> by economics and bureaucratic intransigence
> than a quest for the best we know how to
> do.
>
> The two ic's are lm311 style comparators.
> You can probably dope out a workable
> constellation of values based on the
> stated design goals.
>
> While the FET may indeed be suited to the
> 'crowbaring' of a circuit breaker, how
> do we insure that it maintains a saturated,
> on condition from the time it switches ON
> until the breaker opens?
>
> The gate voltage cannot fall below the minimum
> value for necessary conduction while the
> breaker is deciding to toss in the towel . . .
> probably 15 to 50 milliseconds.
>
> Thid critical performance question demands
> more prayer over the drawing board.
>
> Thank you so much for your interest . . .
>
> Bob . . .
>
> .
>
Thanks Bob, that's a handy device if one doesn't happen to have a zener
on hand with desired voltage, even if it doee require two resistors.
I'd forgotten that LM311's can have open collector outputs (when tying
output transistor emitter to ground). Still have several in my old parts
bins but more than four decades since last used one.
I'm thinking the SCR is still the better choice for a crowbar circuit.
But if we're changing the OVP circuit to a relay disconnect between
alternator and battery/bus a FET or other device that can drive the
relay may be OK. Assuming the OVP monitors alternator voltage and not
bus voltage.
Finn
Message 2
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| Subject: | Re: Next generation Crowbar OVM |
>So, if I understand this correctly, and read between the lines a
>little bit, the ~3mS delay in your legacy OVP circuit...
>
>http://www.aeroelectric.com/DIY/DIY_Crowbar_OVP_F.pdf
Good eye my friend! That's a typo. Just to be sure, I ran the
traps on circuit values and came up with approx 60mS to trip
for a more realistic rate of rise for a faulted alternator
driving a flightworthy battery. Still pretty fast having
sorta caved to legacy certified designs. Gonna quit that
and go for any event exceeding 16.0+/-0.5Volts AND 500+/-0.50Seconds; well
inside design goals cited by DO160/MIL-Std-704.
I'll go fix that posting . . . or maybe just take
it down. The very first CBOVM-14s I delivered
exploited a relatively precise 10.V trigger
diode that were 'calibrated' as-assembled.
First the diode went obsolete and now attractive
SCRs are more difficult to find. Time for the
BROAD BRUSH upgrade.
>...is shorter than ideal, and the relevant standards allow a much
>longer response time. If we adopt a longer delay, we get a system
>that's significantly more tolerant of bus transients but still
>protects our avionics when things truly go sideways. Does that sound right?
>
>
> > While the FET may indeed be suited to the 'crowbarring' of a
> circuit breaker, how do we insure that it maintains a saturated, on
> condition from the time it switches ON until the breaker opens?
> >
> > The gate voltage cannot fall below the minimum value for
> necessary conduction while the breaker is deciding to toss in the
> towel... probably 15 to 50 milliseconds.
>
>Yeah, good point. When the FET turns on, the supply is pulled HARD
>toward 0V, which kills the FET's gate drive.
Also good eye. Vcc for the electronics will need to be
diode isolated from the effects of the crowbar pull down.
The capacitor across Vcc will be large enough to hold
the crowbar action ACTIVE long enough to trip the breaker.
I'll have to check again but I think the circuit demands on
Vcc will be on the order of 1mA. Keeping Vcc at or above
5v for 200mS or so shouldn't be hard.
>Perhaps the following might work (see "Crowbar FET.pdf" below):
>Isolate the pullup resistor from the rest of the 10V circuit using a
>diode with low reverse leakage (i.e. BAS33: 1nA@15V), use a modestly
>low value pullup resistor (1k?) and a logic-level FET so the minimum
>Rds-on occurs at a low drive voltage.
>
>Something like the IRLZ44 (TO-220 through-hole) or BUK962 (TO-263-3
>surface mount) might work. Both are limited to a Vgs of +/-10V, but
>that's protected by the forward voltage of the isolating diode. I
>couldn't find any high current, logic level FETs with higher Vgs.
>
>FET datasheets:
>IRLZ44: https://www.vishay.com/docs/91328/irlz44.pdf
>BUK962: https://assets.nexperia.com/documents/data-sheet/BUK962R5-60E.pdf
>
>
> > You can probably dope out a workable constellation of values
> based on the stated design goals.
>
>Indeed, I took a stab at it with an eye toward using it as a relay
>driver for a PM stator (see "2-Comp OVP Relay Driver.pdf" below).
>- Used an LM393 (vs the LM311, the LM393 has two comparators in one
>package and it's cheaper).
>- P-FET on the output to drive relay coil(s).
>- N-FET to latch timer comparator's reference input to ground.
>- Separate reference divider for timer comparator so latching FET
>can't pull 431's REF pin to ground.
>- 7.5V Zener on both comparators to protect non-inverting inputs
>from exceeding Vsupply minus 2V (LM393's input voltage range).
>- N-FET to ensure timing capacitor is fully discharged during reset
>so comparator can't restart in latched state.
>- Timing: 10V charging 10uF through 10k should take ~69mS to reach
>the 5V reference.
>What do you think? Is a 69mS delay OK, or would you aim for
>something shorter or longer?
I'd go for the whole 500 MSec enchilada.
Yeah, need to resolve the issues with trip
dynamics to make sure the FET stays on
for at least 200mS to open the breaker.
I've toyed with the notion of a 3-wire
configuration where Vcc is derived from
say the battery bus (A draw of about 1mA
. . . insignificant) leaving only the
crowbar breaker on the main bus.
At the moment, I'm rather enamored of the
NEC NP110N03PUG FET. This little guy will
grunt over 200A at low temperatures with
a Vgs of only 5V.
Let's continue to continue . . .
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
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