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AC current in the DC #5369601 03/07/20 07:51 AM
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Technoid Offline OP
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One of the things that most would over look in a charging system is a failure of an alternator diode. You might have an electrical symptom where components are working strange. When a diode fails it will either short or open. Most of the time it shorts or leaks AC. When this happens you get AC intrusion into the charging system. This can damage electronic components or make them operate wrong. This AC intrusion is in a frequency of 1000- 3000 htz. Your alternator is just that. An AC generator before the diode clusters. To properly check you need a good brand of digital vom. Or if you have an analog meter like a Simpson 260 you need to add a diode to the ground probe as a DC blocker. Any AC in the DC will read by about half of it's actual voltage. If you find AC in your charging system you need to replace the alternator as soon as possible. A weak charge is also an indication of a bad diode or just a bad regulator.

Re: AC current in the DC [Re: Technoid] #5369616 03/07/20 08:09 AM
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atikovi Offline
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You lost me at, leaks AC. How does electricity leak?

Re: AC current in the DC [Re: atikovi] #5369623 03/07/20 08:18 AM
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mightymousetech Offline
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Originally Posted by atikovi
You lost me at, leaks AC. How does electricity leak?


A failed rectifying diode.


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Re: AC current in the DC [Re: atikovi] #5369636 03/07/20 08:28 AM
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Originally Posted by atikovi
You lost me at, leaks AC. How does electricity leak?


Current can absolutely leak if given a path, no different than a pipe can leak if there’s a tiny issue in a solder joint or pipe wall.

Leak isn’t the right term for the ac component of an anticipated dc waveform.

One has to start by realizing that three phases rectified to dc is intrinsically horrible dc power. It’s not a flat, continuous voltage, it’s horribly bumpy. Even fancy multi-pulse power electronic rectifiers can only start to emulate true dc. It needs to be “filtered”.

The fewer diodes, the lumper the rectified dc looks. Lose one diode in an alternator, that sc comes straight through, positive and negative parts of the waveform, and makes it even more bumpy and lopsided.

Re: AC current in the DC [Re: Technoid] #5369641 03/07/20 08:33 AM
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atikovi Offline
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When a pipe leaks you see water leaking on to the ground. How does that relate to electricity?

Re: AC current in the DC [Re: Technoid] #5369675 03/07/20 09:09 AM
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Mr Nice Offline
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Even brand new high end medical equipment has leakage current measured in uA.
It’s perfectly normal.

Re: AC current in the DC [Re: Technoid] #5369690 03/07/20 09:32 AM
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Chris Meutsch Offline
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I think Bon Scott was certainly the superior singer compared to Brian Johnson.

Oh wait a minute, this is about electricity? Disregard.


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Re: AC current in the DC [Re: Chris Meutsch] #5369693 03/07/20 09:33 AM
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spasm3 Online Happy
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Originally Posted by Chris Meutsch
I think Bon Scott was certainly the superior singer compared to Brian Johnson.




LOL And i agree! Although i like Brian Johnson too, but of the two, Bon.


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Re: AC current in the DC [Re: Technoid] #5369698 03/07/20 09:39 AM
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irad Offline
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OP is correct. Google alternator ripple current for more info. A multimeter set to millivolts AC will aid in diagnosing an alternator problem.

Re: AC current in the DC [Re: Technoid] #5369708 03/07/20 09:47 AM
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bullwinkle Online Confused
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I think I would almost consider it contamination when an AC ripple occurs in DC, and it can play havoc with anything electronic, even the battery (which basically acts as a big capacitor and smooths out the "bumpiness" caused by the full wave rectifier in the alternator when the engine is running).


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Re: AC current in the DC [Re: atikovi] #5369784 03/07/20 11:19 AM
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Skippy722 Offline
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Originally Posted by atikovi
When a pipe leaks you see water leaking on to the ground. How does that relate to electricity?


Think of the wires and those little traces on printed circuit boards as a network of water or hydraulic pipes, only instead of transferring water or hydraulic fluid they’re moving electricity. The various components like resistors, diodes, and whatnot are like pressure regulators, check valves, et cetera.

When a pipe bursts or a valve fails, you get fluid where you don’t want it. Same thing happens in electronics, only instead of it physically falling onto the ground, it’s going to some other component that it shouldn’t be.

Last edited by Skippy722; 03/07/20 11:21 AM.

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Re: AC current in the DC [Re: atikovi] #5369790 03/07/20 11:29 AM
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Originally Posted by atikovi
When a pipe leaks you see water leaking on to the ground. How does that relate to electricity?


Stray current paths dissipate as heat. And can damage components.

Pressure and flow are analogous to voltage and current in many ways.

The only difference is that water in a pipe WILL flow out onto the ground. Electricity will have the potential to do so, but wont go unless it has a path.

To the OP's point, AC ripple on the DC bus will cause over voltage which degrades capacitors and other elements in electronics. Ripple that goes positive and negative inside a battery will cause heating. Heat causes degradation in chemical batteries.

Re: AC current in the DC [Re: Technoid] #5369827 03/07/20 12:05 PM
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Basic Diode Operation and rectification:

https://wiki.analog.com/university/courses/electronics/text/chapter-6

A healthy diode in an AC circuit only conducts current when the polarity of the AC waveform is of the proper polarity. A "leaky" diode on the other hand will allow AC current to pass no matter the polarity of the AC waveform.

Here is a PDF file that explains automotive alternators:

https://www.microcharge.de/downloads/Understanding_Generator_Ripple_Waveforms.pdf

To properly view the alternator output waveforms you need an oscilloscope.

Last edited by MolaKule; 03/07/20 12:07 PM.

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Re: AC current in the DC [Re: Technoid] #5369850 03/07/20 12:33 PM
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AC current is forever variable, the diode redirects it positive. Direct current.

None of this really matters, you don’t repair diodes and rebuild alternators anymore...you hook up a little hand held charging system tester and it tells you if the rectifying bridge passes, charging system output, battery voltage, damaged cells ...the whole test takes three minutes.

Last edited by Railrust; 03/07/20 12:38 PM.

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Re: AC current in the DC [Re: Technoid] #5369869 03/07/20 01:19 PM
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When I first started working on electronic devices for automotive applications, somebody gave me a good paper that started off saying something like, "When you connect to a car's battery, you are plugging into the power supply from [heck]."
Load dump, reverse battery, double battery...all kinds of crazy stuff to worry about, and even normal operation is pretty ugly in itself. A messed up alternator makes things even worse, the battery actually makes for a darned good filtering element if it is working (sort of like a big capacitor) but it can only do so much. Load dump is just the removal of the battery from the alternator while it is charging, the current in the coils cannot change instantaneously to respond to the loss of load and a big voltage spike results (V=L*dI/dt). Having a loose battery connection can make you get load dumps over and over.
Luckily, most of the automotive stuff I worked on worked off a regulator from the battery, but I did have some parts that had to deal with this hellish stuff. Add in the fact that every customer had their own definition of load dump with differing peak voltages and attack/decay rates, or maybe multiple definitions...ugh.

Last edited by Virtus_Probi; 03/07/20 01:19 PM. Reason: BAD word

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