 |
Matronics Email Lists
Web Forum Interface to the Matronics Email Lists
|
| View previous topic :: View next topic |
| Author |
Message |
nuckolls.bob(at)aeroelect
Guest
|
 Posted: Mon Feb 14, 2011 5:32 pm Post subject: Permanent Magnet Alternators - Over Voltage Protection |
 |
|
At 04:49 PM 2/14/2011, you wrote:
| Quote: |
I live in the UK where the Rotax 912 with its 18 amp Permanent
Magnet Alternator (PMA) is ubiquitous.
Many folks are now fitting very expensive avionics in their Rotax
engined aircraft and are keen to guard against over voltage (I know
how rare that is with PMA regulators - but it does occasionally happen).
|
Recommend you use the parts supplied in the BC505-1 kit
and wire per
http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z16M.pdf
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
 |
markcs
Joined: 14 Feb 2011
Posts: 3
Location: England
|
| Posted: Tue Feb 15, 2011 2:15 pm Post subject: Re: Permanent Magnet Alternators - Over Voltage Protection |
|
|
Bob,
Many thanks for your reply. Your time is always much appreciated.
Regards
Mark
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
|
nuckolls.bob(at)aeroelect
Guest
|
| Posted: Sun Feb 20, 2011 9:19 am Post subject: Permanent Magnet Alternators - Over Voltage Protection |
|
|
At 10:20 AM 2/20/2011, you wrote:
Yes, the two-transistor emulation of the MBS4991 was
crafted after the former part went obsolete.
| Quote: | | Question: Can I substitute a zener (8.7 or 9 volt) diode for the MBS4991? |
No. If you study both the specs for the 4991 and noodle out the
behavior of the PNP/NPN transistor pair, you'll deduce that
the device is both voltage sensing AND avalanche-trigger
that dumps the energy stored on the time delay capacitor
into the gate of the crowbar SCR.
| Quote: | | I did find a replacement for the MBS4991 at Mouser: the NTE6403 although it is relatively expensive at $8. I am not sure how to connect it because your schematic only shows 2 of the 3 leads connected. |
If it's a bilateral device, it doesn't matter which
two outside leads are connected where. The center
lead (gate) is cut off.
There is a new device (at least new to me!) offered
by Digikey. See:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=835-1126-1-ND
It's typically an 8v device so you might have to diddle
the voltage divider values a tad to get a trip point
well inside the pot travel. The temperature drift
numbers look good.
As you can see, the 2-transistor+zener
circuit is a synthesis of 1/2 of the bilateral
trigger diode. All the parts are readily available
and the circuit as shown has been constructed by many
OBAM aircraft builders.
Here's a handy test fixture for setting your finished
OVM.
http://tinyurl.com/4k455p5
The relay+lamp/resistor should draw about 1A when energized.
Set the bench supply for 16.3 volts. Adjust the
trigger calibration pot on the OV module so that
the relay "clicks" about once per second.
Bob . . . [quote][b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
|
nuckolls.bob(at)aeroelect
Guest
|
| Posted: Sun Feb 20, 2011 9:19 am Post subject: Permanent Magnet Alternators - Over Voltage Protection |
|
|
At 10:54 AM 2/20/2011, you wrote:
| Quote: |
> who could confirm whether or not they agree that the 505 seems
suitable as a generic PMA OV device
|
correct
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
|
nuckolls.bob(at)aeroelect
Guest
|
| Posted: Sun Feb 20, 2011 10:18 pm Post subject: Permanent Magnet Alternators - Over Voltage Protection |
|
|
At 07:31 PM 2/20/2011, you wrote:
| Quote: |
I read about the Silicon Bilateral Switch on the internet
today. Evidently it is not necessary to connect the gate because
the SBS will start conducting when the voltage reaches a certain
value without the gate connected.
I am still wondering if a zener diode could be used instead of
the SBS. If not, it would be interesting to know what the
disadvantages of the zener diode are, or if it would even work at all.
|
It MIGHT work sort of if you used a sensitive gate
SCR but that offers a potential for other problems
like nuisance tripping due to noises on the bus.
The non-sensitive crowbar SCR needs a swift kick in
the gate to insure a good trigger at all temperatures.
The simple zener has a soft conduction 'knee' in it's
V/I curve so is a poor voltage reference at the
threshold of conduction. Further, the current that
can be delivered to the gate of the SCR due to
zener conduction is limited by the source impedance
of the calibration voltage divider.
The diac has a more predictable trigger voltage
which produces an avalanche conduction mode that
dumps the time delay capacitor into the gate
of the SCR. That pulse is hundreds of mA to
over 1A. This same characteristic is also desired
for the triggering of triacs in dimmers . . . the
#1 commercial use for diacs of any voltage.
See:
http://aeroelectric.com/Pictures/Products/9003_B&C/OVM_14_Prototype.jpg
This was the first crowbar OVM we offered many moons
ago. This configuration was modified early in
the production experience to add the 10 ohm gate
to cathode resistor to negate nuisance tripping.
The diac in this version visible on the left was
a really cool part. An SD10 as I recall with very
good tolerances on a 10v trip calibration. This part
went obsolete and we went to the MBS4991. That part
went obsolete and I synthesized the diac functionality
with the pnp/npn pair. At this time, I think B&C has
both the diac and transistor versions in production
on the fleet of products. They made a life-time buy
on the MBX4991. I bought a couple hundred too so that
I can support my own production though it's probable
market life. It is similar to this configuration
http://aeroelectric.com/Pictures/Products/9003_B&C/OVM-14_A.jpg
http://aeroelectric.com/Pictures/Products/9003_B&C/OVM-14_B.jpg
except that we now offer a 15 turn calibration potentiometer
that can be accessed in the field.
This is why the DIY project features the transistor
pair. But it appears that the new kid on the block
from Digikey would allow one to fabricate the diac version.
Your choice.
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
|
user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
|
| Posted: Mon Feb 21, 2011 11:36 am Post subject: Re: Permanent Magnet Alternators - Over Voltage Protection |
|
|
Bob,
I checked out that bilateral switch from Digikey. Unfortunately it is not stocked and minimum order is 2500 pieces. I did find some other interesting devices that could be used in an O.V. protection circuit.
http://www.fairchildsemi.com/ds/LM/LM431B.pdf used in this circuit:
http://en.wikipedia.org/wiki/File:Crowbar_Circuit.jpg
And here is a 16.1 volt zener diode that might work in your circuit without voltage dividing resisters:
http://www.mouser.com/Search/ProductDetail.aspx?R=NZX16B,133virtualkey66800000virtualkey771-NZX16B,133
It might be fun playing around with some of these components. On the other hand, it is hard to beat the proven circuit that you have provided.
Thanks for your advice.
Joe
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
_________________
Joe Gores |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
