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teblejw
Joined: 19 Nov 2013
Posts: 14
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 Posted: Tue Apr 08, 2014 7:32 pm Post subject: When are diodes needed? |
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In adapting Z-07 and/or Z-19, I'm confused about the use and purpose of diodes. As I understand them, diodes allow current to flow in one direction, with a small voltage drop, but not in the opposite direction, except for a small leakage. The voltage drop times the forward current yields a power loss that results in heat, which may need to be dissipated to keep the heat from damaging the diode or other nearby equipment. The power lost may also be an issue in a battery-only situation after loss of an alternator, for example. O.K. so far?
Pairs of diodes are used in Z-19 for the two battery feeds for the main fuel pump, for example. When both feeds are on, this would keep a "good" battery from feeding a "bad" one. Is that the primary purpose? In Z-07, only one feed to the bus with the fuel pump has a diode because judicious switching can isolate either battery. Correct? Is there something more that I'm missing? For example, if I'm not concerned about one battery "feeding" another, do I still need diodes to "smooth" the feed to a bus from two different batteries? Thanks.
Tom
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user9253
Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan
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| Posted: Wed Apr 09, 2014 6:54 am Post subject: Re: When are diodes needed? |
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It is unlikely that a battery will be bad enough that the other "good" battery would feed it. A more important reason for isolating the two fuel pump circuits in Z-19 is to prevent alternator charging current from back-feeding through small wires and fuses intended to carry only the fuel pump load.
There is more than one version of Z-07. Looking at revision 5, the diode in the "Engine B" wire prevents engine starting current and main bus load current from flowing through the "Engine B" wire from the Aux battery.
I do not know what you mean by diodes "smoothing" the feed.
Joe
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teblejw
Joined: 19 Nov 2013
Posts: 14
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| Posted: Wed Apr 09, 2014 9:06 am Post subject: When are diodes needed? |
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Thanks Joe,
I now understand the potential problem with feeding through small wires and fuses. Embarrassed that I missed it. "Smoothing" was not really what I should have said. I have been told that, when two supplies through diodes are connected to a component, the diodes ensure that the one with the higher voltage supplies the current. As the voltage in that one drops, the transition to the other is seamless. True? Without the diodes, I have two batteries in parallel, which yields an average voltage, assuming similar internal resistances. Not clear to me the advantage of one over the other.
Thanks again for your help.
Tom
[quote]
From: user9253 <fransew(at)gmail.com>
To: aeroelectric-list(at)matronics.com
Sent: Wednesday, April 9, 2014 8:54 AM
Subject: Re: When are diodes needed?
--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>
It is unlikely that a battery will be bad enough that the other "good" battery would feed it. A more important reason for isolating the two fuel pump circuits in Z-19 is to prevent alternator charging current from back-feeding through small wires and fuses intended to carry only the fuel pump load.
There is more than one version of Z-07. Looking at revision 5, the diode in the "Engine B" wire prevents engine starting current and main bus load current from flowing through the "Engine B" wire from the Aux battery.
I do not know what you mean by diodes "smoothing" the feed.
Joe
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Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=421822#421822
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user9253
Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan
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| Posted: Wed Apr 09, 2014 10:18 am Post subject: Re: When are diodes needed? |
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It is true that a load will draw current from whichever source has the higher voltage.
Joe
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jluckey(at)pacbell.net
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| Posted: Wed Apr 09, 2014 12:17 pm Post subject: When are diodes needed? |
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Joe,
Not sure how to answer without a little more context. Can you give us a sketch of the circuit in question?
In the mean time, maybe this will help:
Crude diode symbol: ->|
Current will flow thru the diode (left to right) in the following circuit:
5 volts 4 volts
---------- ->| ------------
Current will NOT flow backward (right to left) thru the diode in the following circuit:
4 volts 5 volts
---------- ->| ------------
The diode acts as a "check-valve" and prevents current from flowing backwards
Hope this helps
-Jeff
From: user9253 <fransew(at)gmail.com>
To: aeroelectric-list(at)matronics.com
Sent: Wednesday, April 9, 2014 11:18 AM
Subject: AeroElectric-List: Re: When are diodes needed?
--> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>
It is true that a load will draw current from whichever source has the higher voltage.
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=421830#421========================http://www.matronics.com/N= --> [quote][b]
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Bob McC
Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON
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| Posted: Wed Apr 09, 2014 12:38 pm Post subject: When are diodes needed? |
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Yes.
[b]All else being equal if you connect both a 12V and a 6V source in parallel to a single load without any other intervening components then the 12V source will supply the load and the 6V source will sit idly by doing nothing.
Bob McC[/b]
> --> http://www.matronics.com/Navigator?AeroElectric-List
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[quote][b]
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Bob McC
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bob.verwey(at)gmail.com
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| Posted: Wed Apr 09, 2014 11:56 pm Post subject: When are diodes needed? |
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Is there a practical application for this?
On 09/04/2014, Bob McCallum <robert.mccallum2(at)sympatico.ca> wrote:
| Quote: | Yes.
All else being equal if you connect both a 12V and a 6V source in parallel
to a single load without any other intervening components then the 12V
source will supply the load and the 6V source will sit idly by doing
nothing.
Bob McC
> Subject: Re: When are diodes needed?
> From: fransew(at)gmail.com
> Date: Wed, 9 Apr 2014 11:18:58 -0700
> To: aeroelectric-list(at)matronics.com
>
>
>
> It is true that a load will draw current from whichever source has the
> higher voltage.
> Joe
>
> --------
> Joe Gores
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=421830#421830
>
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Best...
Bob Verwey
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wschertz(at)comcast.net
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| Posted: Thu Apr 10, 2014 4:13 am Post subject: When are diodes needed? |
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I believe that the 12V source will try to charge the 6V source, overcharging the 6V battery if a diode is not present.
From: Bob McCallum (robert.mccallum2(at)sympatico.ca)
Sent: Wednesday, April 09, 2014 3:37 PM
To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)
Subject: RE: Re: When are diodes needed?
Yes.
[b]All else being equal if you connect both a 12V and a 6V source in parallel to a single load without any other intervening components then the 12V source will supply the load and the 6V source will sit idly by doing nothing.
Bob McC[/b]
> Subject: Re: When are diodes needed?
[quote]
href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List
href="http://forums.matronics.com">http://forums.matronics.com
href="http://www.matronics.com/contribution">http://www.matronics.com/c
[b]
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Bob McC
Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON
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| Posted: Thu Apr 10, 2014 5:06 am Post subject: When are diodes needed? |
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Yes absolutely ! (assuming the sources are batteries. If they are diode isolated power supplies (alternators) the original statement stands)
I wasn't suggesting that this was a practical thing to do. Mearly that the higher voltage source will supply the load. Normally we would be talking paralleled batteries or a battery and an alternator. In either case the higher voltage source will supply the load. All else being equal, the lower voltage source will be idle until the system voltage sags to equal the lower. (even this is an oversimplification, but hopefully gets the idea across. The higher of two paralleled differing voltages will be supplying the loads, including attempting to raise the lower voltage source. That's the basis of how a charging system works)
Bob McC
From: wschertz(at)comcast.net
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Re: When are diodes needed?
Date: Thu, 10 Apr 2014 07:12:02 -0500
I believe that the 12V source will try to charge the 6V source, overcharging the 6V battery if a diode is not present.
From: Bob McCallum (robert.mccallum2(at)sympatico.ca)
Sent: Wednesday, April 09, 2014 3:37 PM
To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)
Subject: RE: Re: When are diodes needed?
Yes.
[b]All else being equal if you connect both a 12V and a 6V source in parallel to a single load without any other intervening components then the 12V source will supply the load and the 6V source will sit idly by doing nothing.
Bob McC[/b]
> Subject: Re: When are diodes needed?
[quote]
href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List
href="http://forums.matronics.com">http://forums.matronics.com
href="http://www.matronics.com/contribution">http://www.matronics.com/c
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http://forums.matronics.com
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Bob McC
Falco #908
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