isolation diode

[gcyeaw]

I was trying to remember how the big diode is connected for the generator battery charging/isolation. It has three terminals, but only two are used. Which one does the coach battery connect to? (the center or one of the outside terminals? I replaced mine with a Blue Sea unit, but this is for a friend with a similar setup.

Sounds like someone's using a transistor as a diode which is okay but you first need to determine if it's a pnp transistor or an npn transistor. The pinout will be pin_1: collector; pin_2: base; pin_3: emitter. The diode junction is emitter to base, but to get the bias right you need to know if it's pnp or npn. You should be able to use an ohm meter to tell. Connect the ohm meter to pin_2 and pin_3. If the reading is about 1K ohm you have the bias right. If the reading is inifinity, you have the bias reversed.

This webpage show's some common package pinouts:

http://www.expertcore.org/viewtopic.php?f=19&t=1713

EDIT: Scratch what I said above. From Gardner's earlier description I thought he was talking about a discreet device (diode, transistor, etc). From the picture in post #9, it's clear that he's talking about a battery isolator. Sorry for injecting any confusion.

[gcyeaw]

David,
I believe it is just a diode. Why the three teiminals I don't know, but on all the FC's only two terminals were ever used. The wiring diagram also shows only two. Unfortunately I don't have my diagram and am quoting someone elses observaton.

[michael3536]

Gardner is it a isolator like this one?

David I was not able to follow your explanation.

Originally Posted by gcyeaw

David,
I believe it is just a diode. Why the three teiminals I don't know, but on all the FC's only two terminals were ever used. The wiring diagram also shows only two. Unfortunately I don't have my diagram and am quoting someone elses observaton.

Hi Gardner,

Does the package look like any of those pictured in my previous post?

[gcyeaw]

Michael,
I believe it is, but your connections are different from the old FC's as far as I remember. Perhaps it really is a transistor and has been missused all these years??? I know on my old '78 it used only two connnections and did charge the gen battery. I just can't remember the connection markings or which was which.

Looking at the size of the wires on your unit, and the miniscule visible diode, I can't imagine what purpose it serves.

Ok, there is one possible purpose, to absorb a reverse voltage transient from a relay coil. When the power to a relay coil is removed, the magnetic field around the coil collapses and creates a reverse voltage. This can result in a very high voltage spike, but very low current. Sensative components can be damaged so a diode is used to short the reverse current.

[michael3536]

That diode was used as an exciter just to send 12v to the alternators so the would know to charge??. The center post was power in.

[Clark]

Gardner has been helping me work on April Smith's coach, and the isolation question was mine. I've tried to attach a photo showing the location. It's mounted in the top right section of the battery compartment of a 1984 FC35. My photo only shows one end of it. It does look like the one Michael posted above, but I do not have the diode. The cables were connected like this-

Left post - cable from the genny battery, at least I think so. It is marked white, which matches the schematics I have.
Center post - the cable that runs from the shunt, where the other side of the shunt connects to the alternator. (not sure that description makes any sense)
Right post – a cable with battery voltage all the time.

The original issue was with the coach running, I had no voltage at the + post of the alt. I moved the cable connected to the center post of the isolator over to the right post, where I have battery voltage. This corrected the no-charging issue, as I now have voltage (~14 volts) at the + post of the alt. while the motor is running. But, from looking at the wiring schematics I found on this site, I see the alternator connection should be on the center post, along with the cable connected to the coach batteries. I’m going to move the cables to the center post, unless someone tells me not to.

Someone seems to have connected the cables incorrectly, I think. But looking at Michael’s photo, I see cables on all three posts. Can anyone describe how the isolator is connected in your FC?

Thanks, Clark

[gcyeaw]

Clark,
I have been reading and looking into this a bit. This is what I believe is the situation:

The diode isolator unit in the bird has two diodes. The input voltage from the alternator goes to the center post. Assuming the alternator is 'self exciting' it needs battery voltage at this post either by connecting the coach battery here as well, or a diode like Michaels. The outside posts are for the two battery loads (coach and generator). The reason, I believe, that the coach battery and alternator are connected to the center post in older FC's is to eliminate the .7 volt loss across the diode when charging the coach batteries. The generator battery would be connected to either one of the other posts.

More modern diode isolaters use a Shotkey type diode, which has a significantly lower voltage drop. I am wondering if that is what Michael has, which would answer the question of how he is getting a full charge on his battery banks.

Michael,
Do you have separate house and start batteries?

[michael3536]

Clark good to hear from you..

I've included the link to a post I started a few weeks ago when I broke the exciter diode wire.

It is wired like you said. Alt. to shunt to middle pin, which charges both house and generator batteries. The exciter wire is necessary in my case for the alternators to charge.

How long hasn't the coach been charging? Did anyone notice that the Gauge was reading low? It didn't all of a sudden stop working, did it?

I have to spend some more time following wires for my schematic.
I have to still verify a couple of things on my own set up

I think I have two relays /battery combiners. One is controlled by the Aux Start switch. I still have to verify that the other side of the isolator is the Generator.



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