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markcs
Joined: 14 Feb 2011
Posts: 3
Location: England
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 Posted: Mon Feb 14, 2011 2:49 pm Post subject: Permanent Magnet Alternators - Over Voltage Protection |
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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).
The only device that I have come across to meet this need is the excellent B&C 505-1 (I have B&C alternators on my RV - outstanding pieces of kit). I spoke to TJ at B&C who could not have been more helpful but did explain that the 505 had not been tested an any other PMA other than their own alternators and so he could not formally recommend it for the Rotax PMA or any other PMA for that matter - a position which I understand.
As I understand it the 505 OV module controls the relay which takes the output from the PMA regulator. If an OV event occurs the OV module releases the main relay contacts and so cuts off the supply voltage to the main bus. If my understanding is not wildly out it would appear that the 505 could be used successfully on almost any PMA (Relay current limitations not withstanding) including the Rotax.
I would be grateful for any experiences of members who have used the 505 on PMA's such as the Rotax etc, or any another comments about the 505 generic suitability, and who could confirm whether or not they agree that the 505 seems suitable as a generic PMA OV device.
Many thanks
Mark
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Sun Feb 20, 2011 7:20 am Post subject: Re: Permanent Magnet Alternators - Over Voltage Protection |
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Bob,
I intend to build an over-voltage circuit for my Rotax powered RV-12. I found a couple of designs on your website:
http://www.aeroelectric.com/DIY/DIY_Crowbar_OVP_F.pdf
and
http://www.aeroelectric.com/Catalog/AEC/9003/CbOV-14_Installation_A.pdf
I like the latter circuit due to its fewer parts count. Unfortunately, the MBS4991 Silicon Bilateral Switch is no longer available.
Question: Can I substitute a zener (8.7 or 9 volt) diode for the MBS4991?
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.
Question: Do the Gate and the A1 terminal of the bilateral switch connect to the pot?
Thanks for your help.
Joe
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Joe Gores |
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Sun Feb 20, 2011 7:54 am Post subject: Re: Permanent Magnet Alternators - Over Voltage Protection |
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| Quote: | | who could confirm whether or not they agree that the 505 seems suitable as a generic PMA OV device |
Mark,
I agree with your thoughts on this. But I have no experience with the B&C 505-1 to back up my opinion. You could build your own over-voltage protection using one of Bob's schematics mentioned in my post above.
Bob sells over-voltage protection for $30 on his website:
http://www.aeroelectric.com/Catalog/AECcatalog.html
If you go to there and scroll down and click on CBOV-14, the link takes you here: http://www.aeroelectric.com/Catalog/AEC/9003/CbOV-14_Installation_A.pdf
I believe the aircraft schematic at the bottom of the PDF will work for a permanent magnet alternator. I am sure that someone will correct me if I am wrong.
Joe
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Joe Gores |
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Sun Feb 20, 2011 4:31 pm Post subject: Re: Permanent Magnet Alternators - Over Voltage Protection |
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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.
Thanks for any advice.
Joe
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pmather
Joined: 17 Sep 2009
Posts: 33
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| Posted: Mon Feb 21, 2011 12:44 pm Post subject: Permanent Magnet Alternators - Over Voltage Protection |
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I've built my own overvoltage crowbar with the MC3423 which is a dedicated
chip for the purpose - very easy to use and cheap enough. See
http://tailwindbuild.blogspot.com/2009/11/15th-november-2009-overvoltage-cro
wbar.html for details
Best regards
Peter
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