Today's Message Index:
----------------------
1. 11:22 AM - Re: Re: OVM-14 MkIII (Jeff Luckey)
2. 12:50 PM - Re: OVM14 MkIII, rev P1 (Eric Page)
3. 01:40 PM - Potential for Overvoltage Issue (M Wilson)
Message 1
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| Subject: | Re: OVM-14 MkIII |
Crayons?=C2- I like crayons.=C2- I do some of my best work with crayon
s ;)... and snacks?=C2- Who doesn't like snacks?
-Jeff
On Thursday, September 21, 2023 at 07:29:44 PM PDT, Neal George <neal.g
eorge@gmail.com> wrote:
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Just send me back to my corner and crayon snacks =F0=9F=98=8A.
=C2-
I looked at the proposed circuit, but didn=99t study it, and in not s
tudying, didn=99t grok it =93 I can look at the simplest of sch
ematics for an eternity and still not get it.=C2- Not pairing the words w
ith the proposed schematic, I imagined one light, multiple functions (I see
m to remember a device (LR3x, maybe?) =C2-that could set a lamp ON for ov
er-voltage, blinking for low voltage).
=C2-
Mea Culpa didn=99t mean to roll a grenade.
=C2-
Neal
=C2-
At 11:38 AM 9/21/2023, you wrote:
Neal - yes, latching is essential and my sketch assumes that it is possible
.=C2- (and I believe that Bob confirmed that)
=C2- I'm lost . . . with the crowbar OV management
=C2- system, an OV event triggers a low resistance
=C2- load downstream of the alternator field breaker
=C2- forcing it to open thus depriving the alternator
=C2- of field excitation.
=C2- This even is inherently latching . . . the breaker
=C2- is not going to close itself. A light wired across
=C2- the open breaker would illuminate until the
=C2- breaker is re-closed.
=C2- I'm not seeing value in adding circuitry to
=C2- sense/latch based on the ov event.
=C2- Bob . . .
=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2
-=C2-=C2-=C2-=C2-=C2- ////
=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2
-=C2-=C2-=C2-=C2- (o o)
=C2-=C2- ===========o00o=(_)=o00o====
=====
=C2-=C2- < Go ahead, make my day . . .=C2-=C2- >
=C2-=C2- < show me where I'm wrong.=C2-=C2-=C2-=C2-=C2- >
=C2-=C2- ====================
============
=C2-
=C2-=C2- In the interest of creative evolution
=C2-=C2- of the-best-we-know-how-to-do based
=C2-=C2- on physics and good practice.
Message 2
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| 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 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
5.9k 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.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=511474#511474
Message 3
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| Subject: | Potential for Overvoltage Issue |
I've been luring on this list for years and this is my first post.I've rec
ently completed my plane (Tailwind) and have if flying.I used the previous
version of the OVM crowbar circuit and I have modified an automotive altern
ator to use an external Ford-type voltage regulator.All is working well.=C2
- But, in going through a FMEA in my mind I have identified a potential i
ssue:=C2- I modified the alternator using the method in Kitplanes, Novemb
er 2019.=C2-In this method one of the brush terminals is isolated with in
sulating washers and an insulating sleeve.=C2- In my case=C2-I have gro
unded the other brush to case ground.=C2- The issue is that if ever the i
nsulating washer or sleeve were breached the B--lead would directly feed th
e field through the brush and an overvoltage would result that the crowbar
could not shut down. Turning off the master switch would disconnect the bat
tery but the run away alternator would still be feeding the bus.I post this
in case anyone else has this setup.=C2-=C2-
Solutions are:1. Add a Alternator disconnect contactor connected to the cro
wbar (in which case, why use an external regulator?)2. Externally ground th
e B-lead side brush with a fusible link and feed the field current from the
VR into the other (currently grounded) brush.
Any other thoughts?
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