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3 phase Rectifrier Diode Testing?

S

SED

Just got it firing!
Joined
Jan 8, 2010
Messages
17
Hello Everyone,

I possibly posted my first post (about LMIII carb jetting) in the wrong spot. Got good help (thanks) and am back for more.

The LMIII has a weird charging problem where the Veglia voltmeter shows a discharge (11V) after having the headlight on for 10 minutes or so. The voltmeter jumps back up to 14v if I turn off the headlight. A double check with a digital voltmeter after a ride shows that the battery is being charged, but at a low voltage (about 13v) and over 100 miles of riding the battery will drop from 13v to 12.9v.

So I've checked the rotor, brush and stator resistances and cleaned the slip rings. I've checked and cleaned the connections at the back of the ignition switch and they now look good - as do the connections on the back of the voltmeter.

The rectifier suffered slight corrosion due to battery vapors so that is where I am now and I have a few of dumb questions about it.

Question one: do I have to remove (or somehow isolate) the individual diodes to test them? With the diodes in place, all 6 of the main diodes show about 550 ohms one way and infinite the other using the multimeter diode test. The other smaller diodes that are reachable give similar readings. Not only that, the pattern of continuity seems correct in that the continuity is in in one direction from the positive side of the rectifier and in the opposite direction on the negative side. Does this positively tell me the diodes and rectifier are good?

Question two: these results are totally different than those shown in Guzziology. Guzziology shows 20 or 60 ohm resistance not 550. Also Guzziology shows continuity where I have none and vice verse. Does anyone have experience with the diode table in Guzziology that can validate it. The table has at least one error. (i.e., should I trust my multimeter or Guzziology?)

Final question: Where does the ground wire attach? The rectifier is supposed to be grounded on the negative side with a ground wire. The mounting points appear to be insulated where the mounting bolts go through (though there is some continuity on the negative side). Is this a competent ground or should a ground wire be bolted directly to the negative side of the rectifier.

Thank you for your help!

Shawn
 
550 ohms is high for the forward resistance of any diode, but as they all read the same, it may be an anomaly in the measurement method rather than a fault.
A good earth to the rectifier is essential, make sure you have a good one from the rectifier body back to the battery negative ideally. Just make sure you connect to a negative part of the rectifier, as some parts may have different polarity.
 
Thank you Brian. 550 ohms does seem high, but the reading is consistent with a good diodes as far as the multimeter manual is concerned. If I only had a good diode board to compare!

In the brighter light of day I can see that what I thought were brown phenolic insulators on the negative side are in fact oxidized steel spacers. I will make sure that ground is secure.
 
The reading of any diode is very much subject to the type of meter you are using. If you have an older analogue meter, you will get one reading. If a digital one, you can use resistance and get one reading, or you can use a diode checker (if your meter has one) which should give the best reading. The forward resistance of any diode is subject to the voltage applied (ie the battery in the meter) and the type of diode it is.

Some diodes exhibit highish forward resistance and others seem to be lower. This also depends on whether the diode is made from Silicon or Germanium. This performs a static test only. When the diode is doing its thing, the results will again be variable. The best way to test a diode is to power up a lamp (say 12V type) in series with the diode and measure the voltage across the diode. A good/healthy Silicon diode will show around 0.4 to 0.6 volts across it while it is conducting. A Germanium (which should be fairly rare these days) will have a voltage drop around the 0.2 volts across it.

Hope this helps and hasen't confused you too much...
 
Oz1200Guzzi said:
The reading of any diode is very much subject to the type of meter you are using.

Some diodes exhibit highish forward resistance and others seem to be lower. This also depends on whether the diode is made from Silicon or Germanium. This performs a static test only. When the diode is doing its thing, the results will again be variable. The best way to test a diode is to power up a lamp (say 12V type) in series with the diode and measure the voltage across the diode. A good/healthy Silicon diode will show around 0.4 to 0.6 volts across it while it is conducting. A Germanium (which should be fairly rare these days) will have a voltage drop around the 0.2 volts across it.

Hope this helps and hasen't confused you too much...
Click to expand...

Actually this is quite helpful. It is a digital meter with a diode tester and it is the diode test function that would give 550 Ohms one way and none the other. The rectifier seemed to be ok with this test so I cleaned all the connections - especially grounds and reinstalled it. I'm waiting for a carb part and some dry weather and will try it out. If I had known I could hook up a lamp in series across the diode to test the voltage drop I would have done it.

Does the light only light one way across the diode?

Also, do diodes ever work cold and fail as they warm?

Thanks.
 
I would first look for a short in the headlight circuit prior to suspecting the rectifier.
 
motoguzznix said:
I would first look for a short in the headlight circuit prior to suspecting the rectifier.
Click to expand...

The headlight circuit has had relays installed and the wire from the battery has an inline fuse. I would think the fuse would blow if the circuit were shorted.
 
Just a little technical info here regarding diode testing with digital meters. The "diode check" function on all of the digital meters I have seen actually reads the forward voltage drop, not the resistance. It sounds like your meter might be reading in millivolts, 550 mV = 0.55 volt which is about right for a silicon diode. A Schottky diode will be around 0.2 to 0.4 volt. If you want to test an LED, it will probably give you about 1.5 V and you might even see a tiny bit of light right where the two leads meet inside the plastic. The polarity of a diode is usually indicated by a mark on one end of the package. The mark is the cathode, meaning if you hook the meter red lead to the un-marked end and the meter black lead to the marked end, you should see about half a volt. Switch the leads around the other way and the meter should indicate open or over range. Another thing to be aware of, an analog meter will usually use reverse voltage when reading ohms so the red & black leads might seem to work backward compared to a digital meter, but you should still be able to notice a change in the reading when switching polarity.

The marked end on all the diodes in your rectifier should be pointing towards the battery positive connection and away from the battery negative (or ground) connection.

Happy rectify-ing.

Howard
 
The voltage at each of the three legs at the diodes should be going between 0 and 13 (or so) volts with the bike running and the headlight on. (I suppose actually about -0.7 to +14 or so) If you take a DC reading, you should see around 6.5 volts with the bike running ( with the huge ripply content, it will depend on the meter ).
I would measure from ground to each of the three legs of the diodes. All three should be about the same. If you have a really odd one, you may have a bad diode.
Also, what is the voltage at the brushes at idle and the headlight on?
 
Wayne Orwig said:
john zibell said:
Wayne,

I know transistors have three "legs" When did diodes go from two legs to three?
Click to expand...


Isn't this the three phase Bosch alternator? 6 diodes, two per phase?
Click to expand...

Take a look at this schematic. https://www.guzzitech.com/guzzi007/schem ... 0T_USA.gif The factory didn't draw the LeMans schematic very well. I see 11 diodes, each with two legs. Six diodes rectify to DC to charge the system, while five are involved with control circuits.
 
I think John's right, there are 11 diodes (from memory).

All the diodes tested OK using the diode tester so given the info posted here (Thanks Howard and others) the rectifier was reinstalled. Battery voltage went up to 13.95 at 4500 rpm and never dropped below about 13.5 with the headlight on at 3000 rpm.

The dropping voltage seems to have been a corroded ground for the regulator. Cleaned it up and voltage stays up.

I would like to see more than 14 volts at the battery, but it has the old mechanical regulator. I've heard the new digital regulators have a higher upper voltage cutout.

Thank you for all the help!
 
The mechanical regulators can be "adjusted" . Remove the cover and bend the back side of the arm down. That increases the gap and resulting voltage output. You have to be careful not to bend too much and go over voltage which will cook the battery. If you can get 14.5 volts without the headlight, that is as good as you should get. You could get the electronic regulator on this page (yes it is BMW, but it is the same Bosch system) for optimum performance.
 
john zibell said:
You could get the electronic regulator on this page (yes it is BMW, but it is the same Bosch system) for optimum performance.
Click to expand...

What page? I didn't see a link. Thanks
 
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