Excessive Ripple?

[Magyver]

I went to Advance Auto to get a quick check done on my alternator. The gauge was fluctuating quite a bit but not consistently so I thought the stepper might be bad. I had to the stepper for the tach and speedo anyway so I'd replaced them all. The gauge settled down a lot but I still wondered so when I had the alternator out when changing the tstat I brought it to be tested. It improved when i disabled the DRL per the instructions here. Ironically they didn't have the proper bracket for the machine and couldn't jury rig it well enough for a good test. The guy said he could test in the vehicle. I thought it didn't work as well but whatever. I put everything back together and went back, got this result. Alt seems to be OK but what the heck is ripple?

 

[AtlWrk]

In an ideal world your alternator would produce a perfectly constant 14.4V. In reality, the voltage oscillates a little bit around this 14.4V mark by, let's say, +/-10mV (1/100th of a Volt). That +/-10mV swing is called ripple. A small amount of ripple is normal and tolerable.

There are electronic components inside the alternator called diodes that are responsible for keeping the ripple as small as possible.
If one or more diode fails the ripple becomes excessive. This can cause charging issues or stress other electronic components in the vehicle. The alaternator may still work but it is not working well.

 

[CaptainXL]

90mv AC ripple is the limit. You got a bad alternator.

 

[mrphoenix80]

Too much ripple Bad alternator.

Now the alternator produces AC current in the form of a wave from 0v to +14v back to 0v and down to -14v then back to 0v(I will try my best without drawing a pic).Now the alternator produces power in three phases so take 2 more sets of waves and over lay them on your first set but offset them equally. Then the rectifier bridge basicly flips the negitive to a positive. So if you draw it out you would see a bunch of humps. You then should see the pattern similar to your print out. There is a capacitor in the alternator that smooths out the ripple by charging on the up side of the wave then dsicharging to fill the gap left as the wave falls. It is this cap that I think is your issue. It is this voltage ripple you hear in AM radio as a whine. Your modern electronic systems need clean power. A voltage ripple like this will cause issues down the road.

 

[Magyver]

That clears it up nicely, thanks! I wonder if that contributes to the occasional guage light flicker?

 

[jimmyjam]

you guys would get a kick out of my astro van. junkyard alternator puts out 15v on startup and has so much ripple that the headlights almost strobe. but it runs so...

 

[CaptainXL]

That clears it up nicely, thanks! I wonder if that contributes to the occasional guage light flicker?

Most modern electronics are capacitor filtered so the circuit in question always recieves a steady stream of clean DC current because capacitors block AC. It wouldn't suprise me if the gauge lights were not filtered from AC voltage. So I think what your seeing is an actual byproduct of the bad bridge rectifier. But if you want an exact answer Roadie should be able to expound upon the subject. He's an electronics engineer.

On a side note it's a relatively easy job to rebuild the alternator with new brush kit, bearings, voltage regulator and rectifier. So you could do that or just get a rebuilt alt.

 

[Magyver]

I'm glad you mentioned that because I've seen rebuild kits available for about $50 online. Is it a reasonably straightforward operation? I've rebuilt starters for my boat and motorcycle, but not alternators, and don't depend on them like I do my TB. What's your take on rebuild vs. buy reman/new?

 

[CaptainXL]

What's your take on rebuild vs. buy reman/new?

It all depends on how long you want to keep your vehicle and if you want warranty coverage for a long time or not.

Individual replacement parts inside the alternator usually don't come with a warranty. So if you are rebuilding a unit yourself you take on the risk that any of those parts might fail at any moment.

On the other hand a more expensive refurbished or new unit usually comes with a warranty (most likely 1 year).

So if you are confident in your rebuilding abilities, you buy quality parts and you know the rebuild will last at least 1 year then choose the rebuild kit.

2 NTN bearings are gonna cost about $30 and a voltage regulator is around $40 so $50 seems awfully cheap. You could just get by getting a new bridge rectifier and brush assembly. That would only be about $50. Voltage regulators usually dont go bad all that often because they just use a lower 5 amp current to control the field for the rotor.The most common cause of a failed alternator is old, grounded diodes.

Your battery has more than likely taken a hit as well. So get a new one if it tests bad or is nearing the 5 year mark.

 

[jaguarjoe]

Capacitors do not block AC, they block DC. An inductor will impede AC.

I've been around alternators since their debut years ago. I have never seen a capacitor capable of filtering the alt's DC output. A rule of thumb is to use 2000uF of capacitance for every amp of current draw. At 100 amps alt output, you'll need about a 200,000uF cap to be be effective. A cap that size would be as big as the alt. Just like hooters, bigger is better-some folks use 3000uf/amp.

A three winding alternator has each winding spaced by 120 degrees. After rectification by the the diodes the raw waveform will look like scallops (the peaks of each sinewave). State of the art alternators have highly modified bear claw rotors that do not produce sines, they make waveforms very close to a squarewave which is much more efficient.

 

[CaptainXL]

Capacitors do not block AC, they block DC. An inductor will impede AC.

Correct. My apologies. I mean to say that the 3 phase half wave rectified waveform is smoothed out by the capacitor.

View attachment 26460

 

[AtlWrk]

Capacitors do not block AC, they block DC. An inductor will impede AC.

True, but capacitors do filter AC and CaptainXL's original point remains valid. Pretty much all electronics use capacitors (or caps and inductors) to smooth out AC components.

I've been around alternators since their debut years ago. I have never seen a capacitor capable of filtering the alt's DC output. A rule of thumb is to use 2000uF of capacitance for every amp of current draw. At 100 amps alt output, you'll need about a 200,000uF cap to be be effective. A cap that size would be as big as the alt. Just like hooters, bigger is better-some folks use 3000uf/amp.

What you're suggesting here is that a single capacitor filters all of the power generated by the alternator which I don't think anyone was implying. Most of that 100amps goes towards things like lights, battery charging, motors, etc. that don't need any sort of filtering. The electrical components that do (various control modules, radio, etc.) all have capacitors at the local level in their internal power supplies.

Correct. My apologies. I mean to say that the 3 phase half wave rectified waveform is smoothed out by the capacitor.

Careful. That's a single phase full-wave rectified waveform.


The bold black line on the last graph is the output of an alternator: three-phase, full-wave rectified. Fairly smooth with small peaks and valleys--the difference between those peaks and valleys is the ripple. To the OP, when I a diode(s) fails those peaks and valleys get farther apart and you get excessive ripple.

Now, alllllllll the way back to the original problem:
You say the gauge fluctuates a lot but inconsistently and you occasionally have lights flicker. The alternator problem maybe coincidental.
I would also check for loose connections--particularly at the battery, alt and grounds (engine compartment, cluster grounds, etc.) and consider the "Big 3" upgrade if you haven't already.

 

[AtlWrk]

Final thought: a partially failed rectifier will reduce the output power capability of the alternator. The PCM and regulator may be unable compensate for this adequately during certain load changes resulting in the occasional dip in voltage.

Point being: repair/replace the alternator. Youtube has a number of videos that walk through the process to give you an idea of whats involved.

 

[CaptainXL]

Excellent description. And that's correct about my picture not being alternator output at 3 phase. It was the only picture I could find that shows what a capacitor does.

My point was that excessive ripple will impact circuits which do not have capacitors to overcome the dips. Such as lights which are battery fed either directly or via a relay.

 

[meerschm]

I was going to post a helpful correction, but re-read.

 

[jaguarjoe]

True, but capacitors do filter AC and CaptainXL's original point remains valid. Pretty much all electronics use capacitors (or caps and inductors) to smooth out AC components.


Please show me the high school text book which explains how caps block AC. Caps have reactance which allows them to attenuate AC at different frequencies, but they do not block AC. They never did and they never will. A cap that did not pass AC would be a defective cap.



What you're suggesting here is that a single capacitor filters all of the power generated by the alternator which I don't think anyone was implying. Most of that 100amps goes towards things like lights, battery charging, motors, etc. that don't need any sort of filtering. The electrical components that do (various control modules, radio, etc.) all have capacitors at the local level in their internal power supplies.

I am not suggesting anything. You have me confused with someone else- please see post #4. There are no humongous capacitors filtering the alternator output. Never was and never will be. The battery, in its own crude fashion pretty much tries to do that.

 

[CaptainXL]

Im with jaguarjoe. I represented the attenuation of the full wave signal for DC blocking. Which is not the case.

 

[meerschm]

helpful to remember the battery is similar to a capacitor. voltage is a function of the integral of the current. (for those who remember/understand calculus and almost basic physics) (as an aside, you can find various web discussions on concepts to use capacitors to complement/replace batteries in electric or hybrid cars)



the effective reluctance of the battery by itself, as all capacitance, depends on the frequency.
the level of the ripple changes depending on how fast the engine is rotating. (which changes the current output) ( suspect the diagnostic tool used is designed to be used with engine at idle, and at the battery terminals)

for more reading,

Patent US4178546 - Alternator test apparatus and method - Google Patents fun to look at the patent, and for more fun, follow the referenced patents.

quite a few of these detect a defective diode by frequency analysis of the ripple. the more smarts you put into a tester, the less the technician has to understand.


a bit simpler discussion:

http://www.venselenterprises.com/techtipsfromdick_files/updatingyourripplevoltagetutorial.pdf





(and yes, I am an electrical engineer. ask me what time it is, and if you like, we can discuss the accuracies of various time standards and the challenges of time reference systems)

 

[meerschm]

an alternate answer to the original question: (not really relevant to this thread)

Ripple Wine - Sanford and Son | Flickr - Photo Sharing!

 

[Magyver]

Now, alllllllll the way back to the original problem:
You say the gauge fluctuates a lot but inconsistently and you occasionally have lights flicker. The alternator problem maybe coincidental.
I would also check for loose connections--particularly at the battery, alt and grounds (engine compartment, cluster grounds, etc.) and consider the "Big 3" upgrade if you haven't already.

The big 3 upgrade is pending, and I have checked the grounds and battery bolts as best I can.


meerschm

an alternate answer to the original question: (not really relevant to this thread)

Ripple Wine - Sanford and Son | Flickr - Photo Sharing!


Nice. An alternator answer perhaps?


Thanks for the look into a subject that I am mostly ignorant about. Mechanicals I'm good with. Electricals, not so much, so I appreciate the charts and graphs and the glossy 8x10 color photos with the circles and arrows on them
​

 

[CaptainXL]

The big 3 upgrade is pending, and I have checked the grounds and battery bolts as best I can.

Our gauge wires are rated for the amperage of our alternator so this isnt required. All thats required is #4 AWG for less than 5 ft at 150 amps.

Battery wire size chart for battery wire to Alternator. Mimimum battery cable size needed

 

[CaptainXL]

(and yes, I am an electrical engineer. ask me what time it is, and if you like, we can discuss the accuracies of various time standards and the challenges of time reference systems)

Interesting topics. But with all due respect I think you lost most of us three sentences in.lol.

Even though I work in telecom I did manage to get an associates in EET. So I can follow you a bit.

But the question remains, why did you pick a 2003 chevy trailblazer to drive? Shouldnt you be driving a BMW or something a bit more modern? I would think your engineering savvy would demand higher tech. Just curious.

 

[DDonnie]

But the question remains, why did you pick a 2003 chevy trailblazer to drive? Shouldnt you be driving a BMW or something a bit more modern? I would think your engineering savvy would demand higher tech. Just curious.

Because an electrical engineer would know that the fancier the car, the more s**t there is to break.

 

[meerschm]

Interesting topics. But with all due respect I think you lost most of us three sentences in.lol.

Even though I work in telecom I did manage to get an associates in EET. So I can follow you a bit.

But the question remains, why did you pick a 2003 chevy trailblazer to drive? Shouldnt you be driving a BMW or something a bit more modern? I would think your engineering savvy would demand higher tech. Just curious.

so why do I have the TB?

my brother worked for EDS, which was purchased by GM. this qualified me for GM family plan(formerly employee discount) which was 15% off base price and 20% off options. no dicker, no hassle. I also had a GM credit card which used to give you 5% credit on all purchases on a new car. they said 2003 was the last year you could use both together. I also am a veteran/retired Air Force.
by the time they added up all the discounts/rebates, I paid $25k for a $35k sticker. I grew up in Detroit area and have a sweet spot in my heart for American industry and design.

I was going to get something smaller, but my current wife likes to go skiing and asked me to consider a larger, 4wd for playing in the snow. who could go wrong with the truck of the year?

in addition to being an engineer, I like to pay attention to the value of the thing. (lets not say cheap) my TB is getting close to 150k, and my 2009 VW jetta TDI wagon has100k, ( tomorrow is full of oil change, DSG service and fuel filter for the VW TDI)
both of these are computer controlled. I am a big fan of stuff that is designed well and does what it is supposed to. Don't ask me what I think of BMWs. mostly my impression is of they way they are driven around here.






I do have my eye on an electric car. If the Volt was a wagon, it would do. not happy with a few details in the Leaf. the Tesla looks good, if it was $20k cheaper. If I had a spare garage, i would think of making a DIY electric car.

 

[meerschm]

One last observation, if any one is left.

I had my hat handed to me in college after being asked to analyze a circuit, and chose an automotive voltage regulator. Seems my technician level analysis and discussion (I left out my background as Air force enlisted test equipment technician) missed the point, I was thinking in voltage and signals, and the transistors are really current devices.

There is quite a bit of sophisticated engineering, design, analysis, and simulation behind the alternator design and manufacturing. Rotating magneto-electric fields, that vary in frequency, loads and other operating conditions take a lot of math to closely calculate. The materials, processes, and organization to design, manufacture, diagnose, repair and dispose at the end of life are complex, and the result of many years of development. Most discussions have to oversimplify, but that does not mean any less of those who discuss.

Bottom line here, is that they still break and wear out, and how can you tell when it is time to fix or replace the device?

My hat gets tipped to everyone here who takes time to share what they see, share what they have heard, and try to help others.

Good job, and keep it up!!!

 

[Magyver]

I hate it when OPs start a thread, get people all involved in their issues, then disappear without posting the end result. Well that was me.

I did replace the alternator and now the gauge needle points right at 14 all the time. Not that the gauge is the end all be all of measuring devices on these vehicles, but its all we've got to go by.

thanks to all for the input.