Alternator Ripple, Smoothing Capacitor
 

2007 R320 CDI

This has been one of those projects where simply nothing goes right.

I took the car to the best Indy shop in my small town, well known for Mercedes work. They said the alternator and both (main & aux) batteries were bad and needed to be replaced. Quoted $305 for the alternator, a total of $1100 for the job.

I told them to go ahead. They called me back a few hours later, said the price on the alternator was wrong, it was $450. Ok. They called back later and said they couldn't get one at that price, it would be $650.

I said screw that, take out the alternator and give it to me, I'll have a 30-year alternator rebuild shop fix it. They said they did not recommend that, but let me have it.

The alt shop said the voltage regulator was bad, ordered a new one. When it came in, it was bad from the factory, so they ordered another. When I picked it up, they said the regulator wasn't "exactly" the right one, but it should be OK. My Indy put it back in the car.

I took it to AutoZone for an electrical test, said the main battery was bad and the alternator has excessive voltage ripple. I ordered a new battery from Interstate $300, and said it would be 11 days before it got here. Eleven days go by, I call them to pick up the battery. They say its on back order, don't know when, maybe 1-3 weeks. I go get my money back.

Go to NAPA order the battery for next day delivery for $200. That is today.

This morning I decide to study up on voltage ripple because the Indy told me in the beginning that the R320CDI is very sensitive and the electronics can be damaged with a bad alternator, and rebuilts are rarely correct.

Yup, voltage ripple is a serious issue for electronics. Its near certain that the installed rebuilt alternator will need to come out and I don't even have the new batteries installed yet. When its running again, in a day or two, I'm taking it back to the alt shop to see if they will stand behind their work and make this right.

The only possible jury-rig solution is to install a "smoothing capacitor" to tone down the ripple. Some VWs have one of these caps factory installed near the fuse box, but there doesn't seem to be much other info.

Does anyone know something about smoothing capacitors?

Wow, I am really sorry you are dealing with people like I usually encounter... in a small way it helps me feel less alone to know I am not the only one.

Electricity is one of my weakest points... but I have spent a lot of time working with electronic stuff in order to make it my weakest point...

There are electrical engineers on the forum who will stop by at some point..

But I think I can say that a capacitor installed at the fuse box will not harm anything and might help.... but I doubt it since the cause of the ripple and what is affected... are not in that circuit .... if you put one there just check to see what size and type.. it might need to be an electrolytic to last and do any good...

but why you are having a problem with something simple like a working full wave bridge rectifier I really can not imagine.... they are really simple and there are billions of them out their working just fine all the time...

If the four one way diodes are working then the bridge should bring the electricity to smooth DC current....

Getting the proper voltage regulator is probably going to take care of this problems, after all it regulates the voltage and should eliminate the ripple.

It sounds like you are getting hosed by your mechanic. The diodes (6 of them) does the full wave rectification. There will be small ripples which won't hurt anything. The voltage regulator regulates charging voltage and does not remove ripples. Put a DVM on the lowest AC scale and measure charging voltage. If you see a few milli volts AC you do not have any problems. If you see 1 volt AC or higher , one or more of the 6 diodes is bad. Adding a capacitor will not help. The bad diode (s) need to be replaced.

I know the capacitor size has to be "tuned" to the frequency and amplitude of the ripple but I'm not able to tell you exactly how. There are a lot of factors, ripple voltage/%, frequency, load.

I would be interested to see a formula myself.

A cap in a DC line will not transmit any power. A cap in parrallel with an impedance may mitigate a surge, but may have an undesireable resonance. An inductor in line may filter out AC noise as well.

Seems if it were mine I'd make certain the alternator was in perfect condition before attempting to restart it. I've had bad luck several time with alternator shops putting in whatever relglators the thought may work well they didn't and in one case burnt several wires in my harness of a 123 617. For 50 bucks you should be able to order the right regulator as a start and have the alternator tested. Changing out several thousand dollars worth of computers is a frightening thought now that you know ripple is present.

Some vechiles do have factory installed capacitors but more so since the batteries are located far outside of the engine compartment and assist with load changes more so than ripple control.

Here is an interesting read:

http://www.pvv.org/~syljua/merc/TooSeptST07.pdf

I don't think a capacitor will help at all.

The battery itself acts as a huge capacitor, if you are getting ripple with the battery connected then there is a problem with the alternator.

If the battery was bad when this diagnosis was made this may have had something to do with it.

In this google reference... one can count the four diodes needed for something to be called a full wave bridge rectifier...
I supposed more could be added... but a diode is only a one way valve...and it only takes four of them to deal with an ac sine wave to make it dc.. ..... unless placed backwards and used as a clipper of overvoltage (spikes typically)....

https://www.google.com/search?q=full+wave+rectifier&biw=911&bih=438&tbm=isch&tbo=u&source=univ&sa=X&ei=y2ytVOabJcSzggTWtYGA Dw&sqi=2&ved=0CB0QsAQ

In 120VAC applications to get full wave rectification you use four diodes as you aware. Two for the positive and two for the negative aspects of the AC wave.

Car alternators contain three-phases of AC that get rectified to DC so you need an extra 2 to take care of the 3rd phase.

(I believe)

http://en.wikipedia.org/wiki/Rectifier#Three-phase_bridge_rectifier

http://sub.allaboutcircuits.com/images/05019.png

Center tapped AC transformers need 2 diodes for full wave rectification, non center tap need 4 diodes. I think the alternator is wired more like a center tapped transformer, 2 diodes for full wave on ea of the 3 phases.

That sounds right. Its been a while since I've looked at or dealt with much power related items. Mostly software these days.

Thanks..... that makes sense .... and deep down I actually have a visual image of six of them...and the three taps from schematics... but my ac theory jumped in on the definition of full wave bridge... from my Forrest Mims books....

The main larger problem is people owning cars with ' modules'... if a car has no computer then pulsed or variable dc voltage is just fine...

True. One would hope that each of these modules have some kind of input filter/stabilization/regulation circuit on them as well as protections.. but you never really know.