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Old 27-06-2014, 17:08   #16
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Re: Did I burn out a circut

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Does anybody have a comment on this this note I received

Direct Current Generators to produce a direct flow to batteries. The problem with them is that they are not a efficient as the newer Alternators. The alternators on the other hand do produce Alternating Current that needs to be modified to Direct Current before being sent to the batteries. I understand there are three diodes that are designed to cut off the half of the current that "flows backwards". When the engine ignition is turned off before the alternator a back flash of current can destroy some or all of the diodes. IF this happens they can be replaced quite easily. I have found a local expert who became so used to my problem that his price for repair was significantly reduced. The diodes themselves are worth very little, -it is the time to take the unit apart and test it that costs.

The alternator itself is in good shape. My friend actually, on one occasion improved its output by adding, I believe, some permanent magnets to its casing. Someone in the business can actually do the diode replacement in less than an hour.


It is probable that at least one of the diodes is dead and should be replaced. Only someone who knows the business and has a test bench can actually tell you the condition of the output.

As you said the engine will continue to start as long as there is power in the startiing battery. If you have power at your mooring, the battery charger can be left on to keep the battery up to power. If you were to spend too much time at a mooring, you may be without starting capability.
The diodes referenced are in a configuration called a "bridge rectifier". How many diodes depends upon the design. There is no current "flowing backwards"....AC current alternates between + and - (its a wave form). The bridge rectifier modifies the phases of the AC current wave form so that DC current results as output. Think of it as chopping off the +/- parts of the wave and redirecting just those parts to output as DC current (+ on one lead and - on the other).

See http://en.wikipedia.org/wiki/Diode_bridge.

Damage to these diodes occurs when the output of the bridge rectifier has no where to go. Such as disconnecting the B+ "battery +" lead (normally by a switch). This could happen on some ignition switch configurations but is not typical.

Diodes are easy to test. Most multimeters (even cheap ones) have a diode test mode.

Bridge rectifiers are realatively easy to replace. Not a bad spare to carry, especially if you have a history of this issue on your boat.

No harm of course done by shutting down with the ignition on, but in most installations accidentally turning it off won't do any harm.
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Old 29-06-2014, 19:09   #17
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Re: Did I burn out a circut

So much to learn. Thanks all.
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Old 29-06-2014, 20:30   #18
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Re: Did I burn out a circut

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Just removing field voltage will cause it to produce no output and will not blow the diodes. Something else is going on there.
+1. Removing the field voltage is not what causes a problem, it just shuts off the alternator output. Disconnecting the alternator output from the battery while there is still a field voltage is what can blow diodes.
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Old 30-06-2014, 00:33   #19
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Re: Did I burn out a circut

Some alternators will retain a residual magnetism within the rotor, if it lacks avalanche diodes, then yes you will blow the rectifier.

And if the rotor circuit lacks a snubber then you will blow the regulator.

Lloyd

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+1. Removing the field voltage is not what causes a problem, it just shuts off the alternator output. Disconnecting the alternator output from the battery while there is still a field voltage is what can blow diodes.
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Old 30-06-2014, 00:35   #20
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Re: Did I burn out a circut

As well


(edit to add because I cannot edit a post after the fact for some reason)


Lloyd
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Some alternators will retain a residual magnetism within the rotor, if it lacks avalanche diodes, then yes you will blow the rectifier.

And if the rotor circuit lacks a snubber then you will blow the regulator.

Lloyd
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Old 30-06-2014, 06:04   #21
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Re: Did I burn out a circut

Can you burn out one of the three diodes? Will having only one of three cause damage or just be less efficient in recharging?
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Old 30-06-2014, 07:50   #22
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Re: Did I burn out a circut

Yes, it's possible to have 1 diode fail but more likely to have 2 fail together. A bad diode generally causes the alternator to have very low or no output.
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Old 30-06-2014, 10:54   #23
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Re: Did I burn out a circut

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Can you burn out one of the three diodes? Will having only one of three cause damage or just be less efficient in recharging?

Just FYI: a very basic bridge rectifier circuit requires 4 diodes.
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Old 30-06-2014, 23:11   #24
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Did I burn out a circut

Alternators are almost always 3 phase. Most alternators use 6 diodes and often there are 2 or 3 diodes in one package. So you may only find 2 or 3 devices inside.

Diodes fail if the voltage output by the alternator exceeds the diode avalanche voltage capability. This usually happens when the battery is disconnected. If the battery is always connected then removing field voltage will not damage the diodes. Turning the key off cannot hurt the rectifiers as long as the battery remains connected to the alternator output.

There used to be a good online book for free called The 12V Doctor Alternator's Handbook. Try searching for it.
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Old 01-07-2014, 01:21   #25
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Re: Did I burn out a circut

Here is a typical alternator diode test procedure. Note this is not for the faint of heart and it's not like "changing a fuse."

- Note you have to unsolder the diode pack from the stator windings
- Note the B+ versus the B- test requires the polarity of the meter to be reversed - this is because basically the diode is allowing current in one direction and you are testing the opposite polarities.

My alternator has a 3rd Diode pack called the magnetizing diode. I suppose if you were stubborn you could disassemble the bridge and replace diodes but Volvo says replace the diode pack.
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