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Diode between buss

cranland

I'm New Here
Please bear with me, did not build my plane and not an electrical genius. My EMS is reading my battery voltage at 12.5, however bat the battery and main buss it's 13.5. chased it down to a 1 volt drop when it goes through a square diode between the main and essential buss? There are 2 wires from the main buss to 2 prongs on this diode and a single wire going to the essential buss leaving 1 prong empty, does this sound right?
 
Yes, that sounds right. There are 4 diodes inside of the square case. The assembly is called a BRIDGE DIODE. The way that it is wired in your plane uses 2 of the 4 diodes. A 1 volt drop is normal.
 
The one volt drop does not hurt anything, except a little wasted energy that heats the diode. The avionics will run just fine on 13 volts instead of 14. Turning on the E-Bus switch (if there is one) will bypass the diode.
 
Why would a diode cause a voltage drop? Thought resistors did that? And I set my low voltage alarm at 12.3v and it comes on when using a lot of the electronics. And in actuality it's 13.3v. Is it necessary to use that bridge diode?
 
Why would a diode cause a voltage drop? Thought resistors did that? And I set my low voltage alarm at 12.3v and it comes on when using a lot of the electronics. And in actuality it's 13.3v. Is it necessary to use that bridge diode?

The function of the diode is to allow the alternator to charge the aux battery, but to block reverse flow from the aux battery to the main buss. If you need aux power presumably the main battery or buss has a short, and you do not want your aux battery shorted too!
Diodes are basically the junction of two dissimilar materials. At the junction there exists a potential 'hill' due to atomic forces, typically about 0.6 volts for doped silicon diodes. You get no forward current unless you have that 0.6 volts or more. In your 'bridge' set up you have two diodes in series, so you loose 1.2 volts. There are better ways to isolate your system so you only loose 0.6 volts. There are also 'Schottkey' diodes with a forward drop of only 0.3 volts or so, that are popular for this useage.
 
There are also 'Schottkey' diodes with a forward drop of only 0.3 volts or so, that are popular for this useage.

This is what I used to connect the main and aux buss in my plane.

Schottkey diodes used to be rather expensive, but they seem to have dropped in price well. Be cautious with their current rating.......

I am using dual alternators and batteries, so the placement of the diode in my plane is not designed to deal with the aux battery charging, but it is to allow either battery to feed power to the aux buss--------my EFIS and radios are directly off the aux buss, all other electron eaters are on the main buss. You could also call my aux buss the essential buss.

It has functioned well, and I would do it again if building another plane with similar electrical needs.
 
Why would a diode cause a voltage drop? Thought resistors did that?
Anytime current flows through a conductor, there is a voltage drop. That is why the wire size must be increased when current must travel a long distance. The longer the distance and the smaller the wire, the greater the voltage drop.
What kind of alternator and voltage regulator do you have? Ideal voltage output at cruise should be closer to 14.2, not 13.3.
This SCHOTTKY DIODE will drop 0.625 volts when conducting 20 amps according to the DATA SHEET graph. Power diodes should be mounted to aluminum heat-sink with heat conductive paste or film. Heat is the enemy of electronics.
 
It's hard to take EE101 via the forum...suggest you get Bob Nickoll's book and do some reading...he does a great job of explaining basic electronics used in aircraft without getting too deep in theory and math.

Also Bob came up with the essential bus scheme used by many RVs and explains the rational for the design and several others in the book.

Order the book, "The AeroElectric Connection" from B and C Specialities.
 
The function of the diode is to allow the alternator to charge the aux battery, but to block reverse flow from the aux battery to the main buss. If you need aux power presumably the main battery or buss has a short, and you do not want your aux battery shorted too!
Diodes are basically the junction of two dissimilar materials. At the junction there exists a potential 'hill' due to atomic forces, typically about 0.6 volts for doped silicon diodes. You get no forward current unless you have that 0.6 volts or more. In your 'bridge' set up you have two diodes in series, so you loose 1.2 volts. There are better ways to isolate your system so you only loose 0.6 volts. There are also 'Schottkey' diodes with a forward drop of only 0.3 volts or so, that are popular for this useage.

My older AeroElectric Z-11 schematic shows a bridge diode, but it does not have two diodes in series. It only uses one of the four diodes with one input connection to one of the AC INPUT pins and the other connection to the + OUTPUT.

The voltage drop is that of a single diode.

However,

Checking Bob N's latest diagram Z-11 (revision N) he now calls out a Schottky diode as mentioned above.

The part number mentioned is AEC9001-1 - an Aerolectric kit number -

http://www.aeroelectric.com/Catalog/AEC/9001/IM9001-700A.pdf
 
Rv8 diode setup

https://www.facebook.com/profile.php?id=100000093897350&fref=photo
10xg2tt.jpg
 
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Cranland's diodes could be wired in series, but it is much more likely that they are connected in parallel. If the two input wires are connected to the terminals marked "AC", then the diodes are connected in parallel.
 
You can buy the Schottky Diodes + at Mouser Electronics

Mouser Electronics has the Schottky diodes (VT6045CBP-M3/4W) for $3.11
A heat sink (567-421-K) for $15.00
Thermal Mounting Kit (4880MG) for $2.39
You will need some heat sink compound, I bought it at Radio shack for few bucks but I think Fry's has it.
 
I put an "Endurance" buss in my RV-10 like Bob Nuckolls shows. It used a diode purchased fron him. I put my voltage sensing wire on the endurance buss. As a result my normal battery voltage reading was about 12.9 V with the motor running due to the voltage drop of the diode.

This bugged me because I was expecting to see the alternator voltage and did not want to get use to the "abnormal" voltage as "normal ". Putting the sensor on the normal buss would mean no voltage reading if I was on the endurance buss.

EDITED

The solution is very simple, I just removed the diode assembly and replaced the SPST "off-on" switch with a SPDT "on-on" breaker switch. Down position feeds from the normal buss, up feeds from the battery via a fuse at the battery to protect the wire. Now my voltage reading is the true voltage available, minus some tiny drop thru the switch all the time.
 
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I put an "Endurance" buss in my RV-10 like Bob Nuckolls shows. It used a diode purchased fron him. I put my voltage sensing wire on the endurance buss. As a result my normal battery voltage reading was about 12.9 V with the motor running due to the voltage drop of the diode.

This bugged me because I was expecting to see the alternator voltage and did not want to get use to the "abnormal" voltage as "normal ". Putting the sensor on the normal buss would mean no voltage reading if I was on the endurance buss.

The solution is very simple, I just replaced the diode assembly with a SPDT "on-on" breaker switch. Not my voltage reading is the true voltage available, minus some tiny drop thru the switch.


** With the Skyview you can easily define the Endurance Bus voltage color alerts to be 0.8 volts less than the standard and create a custom label on the display "EN BUS". The colors are more useful than specific numbers when in use.

** A second switch is not needed if you are replacing the diode. Just use a two pole switch for the Endurance Bus feed and when the switch is ON, use the second pole to disconnect the "diode circuit". In normal use this second pole will replace the diode. This is even easier than adding a second switch.
 
VT6045CBP-M3/4W

I looked up the specs on the device ( VT6045CBP-M3/4W) and it said 30 amps max current each diode to equal 60 amps, not each diode tied together to then equal 60 amps. I'm not sure I would ever use this device type (TO220) for this application at 60 amps. If 60 amps were applied and one diode drew more than 30 amps the circuit might fail and then clean out the other diode.

Maybe this data sheet shows this application somewhere but I can't find it.

John
 
I looked up the specs on the device ( VT6045CBP-M3/4W) and it said 30 amps max current each diode to equal 60 amps, not each diode tied together to then equal 60 amps. I'm not sure I would ever use this device type (TO220) for this application at 60 amps. If 60 amps were applied and one diode drew more than 30 amps the circuit might fail and then clean out the other diode.

Maybe this data sheet shows this application somewhere but I can't find it.

John

You do bring up a good point, but I think in this case looking at the data sheet that Mouser provides the voltage drop increases as current increases so the load will balance between the 2 diodes. I also agree 60 amps does seem like a lot of current going through such a small device. I'm using this with both diodes tied together for my AV buss with a max current of about 20 amps the 2 diodes simply provide redundancy.
 
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** With the Skyview you can easily define the Endurance Bus voltage color alerts to be 0.8 volts less than the standard and create a custom label on the display "EN BUS". The colors are more useful than specific numbers when in use.

** A second switch is not needed if you are replacing the diode. Just use a two pole switch for the Endurance Bus feed and when the switch is ON, use the second pole to disconnect the "diode circuit". In normal use this second pole will replace the diode. This is even easier than adding a second switch.

Boy, that was too early. What I wanted to say was that I deleted the diode and put in a switch. Down the endurance buss is fed from the normal buss thru a 30 amp fuse, up it is fed directly from the battery which has a 40 amp fuse at the battery to protect the wire.

I just want to know the voltage available to the aircraft. More than 13.5 and the alternator is on line and working, much less and it is time to check the field breaker/switch and then possibly shed load and look for an airport.
 
Yes, that sounds right. There are 4 diodes inside of the square case. The assembly is called a BRIDGE DIODE. The way that it is wired in your plane uses 2 of the 4 diodes. A 1 volt drop is normal.

Newbie here. I've been trying for a few days now to understand the difference between a rheostat and a bridge rectifier but to no avail. I've tried to read about it on Wikipedia and even this article with a video about bridge rectifiers https://www.derf.com/how-a-bridge-rectifier-works-step-by-step-tutorial/ but they don't really explain the difference in layman's terms.. Too many long words :) Could somebody here please explain the difference like I'm 5? Any and all help will be much appreciated. Thanks in advance
 
A rheostat and a bridge diode are two completely different components not
related at all, except for being electrical components. A rheostat is a variable
resistor used to restrict current flow. A bridge diode only conducts electricity in
one direction and blocks current in the opposite direction. A rheostat has been
used to control the brightness of cockpit lights.
A water analogy: a rheostat is comparable to a water valve. A diode is
comparable to a check valve.
 
Maybe you meant to write rectifier, not rheostat. A rectifier and diode
are the same thing. A bridge diode contains 4 diodes inside of one case.
 
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