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dhoadley

Diode?

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dhoadley

I'm chasing a (lack of) spark problem on my 4108 and I had a small wire going under the flywheel that had a fuse connector that was junk. In replacing that I found a brown diode and, without thinking about it, took that out when replacing the fuse holder. In a different topic (http://www.simpletractors.com/club2/topic.asp?TOPIC_ID=137265) Tarheel mentioned a diode in line. I'm thinking that was maybe more important than I previously realized. Now I can't find it anywhere in my shop. It was brown with stripes. Do I really need it and if so, what is its value if I have to replace it. A friend of mine has bins and bins full of them. Thanx, Dave

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HubbardRA

You probably have an alternator mounted under the flywheel. The diode only lets current go in one direction, thus changing AC current to DC current before it goes to the voltage regulator. Just a guess, since I am not familiar with the charging system on your tractor.

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donmoore1904

Dave - if there was a diode, it is "important" and something will not function right without it. If it is the charge circuit as Rod suggests it may be, it could be a fairly high (for diodes) current - I saw someone here mention recently that a 10hp or 16hp tractor may have an 18A rating on the charging system. Was this brown part attached to anything? A diode passing 10A or more of current may require a heat sink to dissipate energy. Just so you know, diodes may be rated for 100 milliamps, 1A, 3A, 10A etc. The voltage rating will probably not be a factor once you find one with a sufficient current capability.

Once you find the right diode, the line on it (cathode) goes towards the battery - the other end (anode) towards the source of current (alternator).

See where some two cylinder motors use diodes to isolate the coils in the kill switch circuit, which would be another reason to find them under the flywheel.

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dhoadley

Donmoore1904- the diode was at one end of the fuse holder, there was no heat sink. I've got a bag full of old diodes from a guitar amp repair. I'll ID them and throw an appropriate one in line. Thanx guys, Dave

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steve-wis

On my B206E, the diode was missing, which allowed the current to flow from the battery back to the alternator and to ground, thus draining the battery overnight. It also burned out the alternator, I believe, so I had to replace the alternator, the rectifier, and the wire with the diode. I was fortunate to find them all on ebay as they were nla for my tractor.

Steve

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GaryinTexas
quote:Originally posted by dhoadley

I'm chasing a (lack of) spark problem on my 4108 and I had a small wire going under the flywheel that had a fuse connector that was junk. In replacing that I found a brown diode and, without thinking about it, took that out when replacing the fuse holder. In a different topic (http://www.simpletractors.com/club2/topic.asp?TOPIC_ID=137265) Tarheel mentioned a diode in line. I'm thinking that was maybe more important than I previously realized. Now I can't find it anywhere in my shop. It was brown with stripesid="red">. Do I really need it and if so, what is its value if I have to replace it. A friend of mine has bins and bins full of them. Thanx, Dave


id="quote">
id="quote">The discription you give sounds more like a resister than a diode.

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donmoore1904
quote:Originally posted by GaryinTexas

The description you give sounds more like a resister than a diode.


id="quote">
id="quote">You have a point - this is true from the description "brown with stripes". Most older resistors were Buster Brown with stripes - more specifically colored stripes. The color code was black, brown, red, orange, yellow, green, blue, violet, gray, white (0-9). Silver and gold for less than 1 ohm. I can't remember ever seeing a brown diode - many are black, a high current one may have a silver metal case. In the old days (40+ years ago), I remember zener diodes (not what would be used here) were in copper colored metal cases. A diode would have only one stripe (usually white or silver), which is the cathode, and represents the side of the diode with the vertical line in BLT's diagram (on the right). Don't know why there would be a resistor in this area, but it is possible.

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dhoadley

So, does this wire just feed juice back to the battery to charge it? If so, can I just try to get it running with just the inline fuse? I'm pretty good with mechanical stuff, but fairly inept with electrical problems. I appreciate all the help. I'll be getting to this project this weekend, it's supposed to get into the low 50's.^ Thanx, Dave

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BLT

What you have is a 28V 6A + or - AC circuit running to the diode. The diode blocks the (-) side and lets (+) plus side pass and charge the battery. Half the voltage and amperage is let thru, 12V 3A. When the engine is shut off the diode blocks the (+) side from feeding back. When the diode fails, I believe you lose your charging capability and let the battery drain back.

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powerking_one

David,

There are typically 3 types of failure modes of a silicon diode in these 1/2 wave charging circuits:

1. It becomes totally shorted; resulting in FULL +12v DC from the battery accross the alternator/stator windings and usually burning up these windings, boiling the battery when running and deep discharging/possibly ruining the battery left in this state.

2. The diode goes open circuit and results in zero charging current; no damage to the battery or stator windings.

3. The diode PIV(peak inverse voltage) rating of it is breaking down resulting low to no charging of the battery and passing some A.C. current to the battery; though this is not a common scenario. Once a diode starts breaking down like this, it will usually either go full shorted or open in short order.

Hope this helps,

Tom (PK)

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donmoore1904
quote:Originally posted by BLT

What you have is a 28V 6A + or - AC circuit running to the diode. The diode blocks the (-) side and lets (+) plus side pass and charge the battery. Half the voltage and amperage is let thru, 12V 3A. When the engine is shut off the diode blocks the (+) side from feeding back. When the diode fails, I believe you lose your charging capability and let the battery drain back.


id="quote">
id="quote">I am not sure the amperage would halve - I believe it would be 14V and 6A on each half cycle. The PIV of the rectifier shouldn't be a factor here, as my guess is most high current diodes will be rated for a PIV of at least 50V. I think I would go for a 6A diode.

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donmoore1904
quote:Originally posted by dhoadley

The "Shack"'s biggest was 3A 50V, so I went with that. gotta start with something. Dave


id="quote">
id="quote">That should get you started. You may want to try the following, if you have an ohm meter, before you install it. A diode conducts in one direction only, electrical potential-wise. You should get low resistance in one polarity, and higher in the other (switch the red and black leads). If you can verify this now, you can check later to see if it behaves the same way in the event something changes. Diodes can short (low resistance regardless of polarity) or go open (high resistance) when they fail.Remember it goes in one way only - anode (no line) towards charging source (alternator, coil), cathode (line) towards load (battery). This assumes you actually removed a diode - I didn't go back to see if you have verified this. Good luck!Good luck!

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dhoadley

It's missing, so there's no way of knowing. Soldered it in yesterday (after checking an earlier post on direction) and left the battery hooked up. If it holds a charge, then I'll move on for now. No spark, so on to the points...;)

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BLT

You should find a solid black wire by the throttle control and another stranded wire going to ignition on the same terminal. Disconnect stranded and or solid wire and then check for spark. If you get it a spark, work backwards to the switch for the problem.

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donmoore1904
quote:Originally posted by dhoadley

It's missing, so there's no way of knowing. {snip}


id="quote">
id="quote">What's I meant was if you independently verified somehow this is actually a wire in the charging circuit, where a diode should be. Like someone who knows your model. Sounds like you decided this is what it must have been.Good luck tracking down the spark.

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