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messydeer
Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA
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 Posted: Wed Jul 29, 2009 4:41 pm Post subject: Z-20: Fuse Links, Master Switch, Alt OV Disc. and OVM |
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Hello 
First, thanks to Bob his dedication to helping build dependable electronic systems for homebuilts. I am building a Sonex powered by a Jabiru 3300. I've read through the book and have some questions below. Unless noted otherwise, I am referring directly to your Z-20 diagram.
18awg and 16awg Fuse Link to Starter Contactor
On Z-20, connecting to the starter contactor is a 14awg wire from the Alt OV disconnect relay that has an 18awg fuse link inline with it. Although it meets the criterion of being 2 steps smaller than the 14awg, it also say to contact Bob about making them any bigger than with a 22awg line (protececting 18awg). Same issue occurs above this in the diagram with a 16awg fuse link called out to protect the 12awg master power line.
Are these fuse links of a different type? On the diagram, it says to refer to Note 4, which doesn't shed any light on this issue.
DC Master Power Switch (S1)
"Up" (keyway) position: As I understand it, the 2-10 switch is a 3-position dpdt switch. The top position (oriented according to the diagram) would connect terminal 2 to 1, connecting the main power distribution bus to the battery via a starter contactor terminal. At the same time, terminal 5 would connect to 4, where it connects both to the crowbar OVM and the alternator OV disconnect relay, and if then eventually, if all is well in the world, to the PM alternator.
This would be the 'normal' position for regular flying, right?
"Center" position: Toggling the switch to the center position would still connect terminal 2 to 1 (I had originally thought it made it open), and terminal 5 goes to 6, disconnecting the OVM and cutting power to the alt OV disconnect relay. The alt OV disconnect magnetic field collapses, causing the NO switch to move to the NC position.
Moving the toggle to the center position manually takes the alternator offline.
"Down" (opposite) position: Moving this switch to the down (opposite) position disconnects terminal 2, cutting off the connection to the battery. And terminal 5 is the same as the 'center' position, disconnecting the alternator.
ALT 5A Fuse: Current will flow through the 22awg wire between terminals 2 and 5 only when the switch is in the "Up" (keyway) position, i.e. both the alternator and battery are online. The fuse will blow when more than 5A passes through it, caused by an overvoltage condition. But if there is an OV condition, wouldn't the crowbar OVM take the alternator offline? Is this just redundancy, or am I missing something? Maybe so when the OV condition returns to normal it will keep the alternator permanently offline, instead of going off and on and off, etc, caused by the crowbar OVM cycling off and on?
Crowbar OVM
In Z-20, the crowbar OVM is shown sharing terminal 4 of the master power switch with the line to the alternator. For me, the crowbar OVM is a black box. All I need to understand is what the 'net' function of it is. When it senses a voltage above a certain value, does it allow a current path to be completed through it to the panel ground? If so, then there would be no current going to the alt OV disconnect relay, which would then take the alternator offline. Is that how it works?
Alternator OV Disconnect Relay
Am I right in assuming that in the normal engine operation with the power master switch in the 'up' position, the power is applied through the switch to the alternator OV disconnect relay, whose magnetic field then pulls down the alt OV switch from the N.C. position to the N.O. position? That had me confused at first, cuz I figured 'normal' operation is when both the alternator and battery are connected. I think I can interpret 'Normally Open, Normally Closed' as being the 'unenergized' switch position, right?
Dynamo, aka PM Alternator
Z-20 shows just two wire coming off the alternator. One connects to the voltage regulator at terminal 3, the other at terminal 1. There is also a tach signal wire spliced into the terminal 1 wire. So both of these wires are carrying current generated by the alternator to the voltage regulator? If so, why have two leads instead of one, and shouldn't there be a ground someplace?
Capacitor
Is the capicitor after the regulator for filtering out noise to the radio? I know of someone who wired according to Z-20, but left out the capacitor. I'm not sure why.
Cheers,
Dan
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messydeer
Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA
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| Posted: Fri Jul 31, 2009 9:09 am Post subject: Re: Z-20: Fuse Links, Master Switch, Alt OV Disc. and OVM |
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| Quote: | 18awg and 16awg Fuse Link to Starter Contactor
On Z-20, connecting to the starter contactor is a 14awg wire from the Alt OV disconnect relay that has an 18awg fuse link inline with it. Although it meets the criterion of being 2 steps smaller than the 14awg, it also say to contact Bob about making them any bigger than with a 22awg line (protececting 18awg). Same issue occurs above this in the diagram with a 16awg fuse link called out to protect the 12awg master power line.
Are these fuse links of a different type? On the diagram, it says to refer to Note 4, which doesn't shed any light on this issue. |
Also, B & C sells FLK-2 Fusible Link Kit, 20-16 AWG, but in their instructions, they likewise say use this method for 20 and 18 awg, no larger?
http://www.bandc.biz/pdfs/Fuselink.pdf
Another Way to Skin a Cat
Instead of using a fusible link, it looks like an inline fuse with a fuse holder could be used: http://forums.matronics.com/viewtopic.php?t=34954 In this post, Bob says to use MAXI fuses instead of ATC fuses to protect the alternator. I'm guessing this was either because ATC didn't offer them in a large enough size, or because the MAXI line was more dependable for this application.
The ATC spec page http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ATC_Specs.pdf shows a 25A fuse (the size Bob said would work for a 20A alternator, I believe) and a holder that would work up to 24A continuous.
The MAXI spec page http://www.aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/MAXI_Specs.pdf shows only 20A and 30A without a 25A fuse.
So how should I protect my 20A Jabiru PM alternator? Fusible link, 25A ATC fuse and holder, or 30A MAXI fuse and holder?
| Quote: | | DC Master Power Switch (S1) |
Bob answered my questions about this in 2007: http://forums.matronics.com/viewtopic.php?t=21238
Thanks,
Dan
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