Cap or not to cap

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• 07-23-2013, 11:58 AM
OldSpark
deleted 25July13 20:00Z
• 07-23-2013, 12:27 PM
settra
Are you saying that the impedance of ZL etc changes with varying current? (Or varying voltage?) . of course?

[image]Attachment 69808[/image]
this says exactly that. otherwise how do you think that the speaker works at all?

i have seen batteries explode in ship (many car batteries together), just because the amperes gone a bit higher than the batteries could hold (ofc bigger than a cars cranking).

solve the diferential equation (Exponantial) , and you can see , that, at bass drops , the current of the capacitor , wont have change enough to make diference. bear in mind that, exponential, drop faster in the beginning, and slower at the end. 12volts, are arledy near the end.

the capacitor in the alternator, filters the DC, because , to the AC, the capacitor is a short circuit, so all the AC, goes directly from the CAP, to the ground, and only a very small amount is left in the DC line.
http://en.wikipedia.org/wiki/Low-pass_filter

as for your last statement. where exactly do you get that ? a batterie is far from a capacitor. they do not even work in the same way! if you really believe they are the same, just remember the old photographic machines, that to use their flash, they had large capacitors, that where being discharged fast enough for the flash (although they had perfectly good large batteries (huge capacitors? ) to discharge)

it swamps AC better than a cap? tell that to my sound system, that with two batteries, still has noise as fak.
• 07-23-2013, 01:41 PM
Quote:

Originally Posted by OldSpark

And a battery won't explode. Do you know of any that have exploded when supplying cranking or winching & amp currents of hundreds of Amps? No even a 0.8AH battery will explode in that situation. However large caps are liable to explode when initially charged (eg, from 0V) if they are not current limited.

I had a battery explode on me. It was a standard automotive battery used with a small Generator.The generator used a standard automotive V8 converted to NG to turn a generator. During cranking this battery literally exploded hitting me in the leg from 6' away and left marks through thick pants. Good thing the brunt of it hit my boots and it was low to the floor. Unknown to me the generator had not been run in months and the battery was old. These generators have a battery charger on the battery at all times. It is likely the battery had bad plates or something but the engine cranked a few times before the battery exploded. And let me tell you they are loud.

After this they removed the batteries from all of the generators at these type stations and disabled the generators so no one else would have issues. I was lucky no pieces hit me in the face or other places. They were not needed any more but I was new to the job and my trainer for the day seldom worked those stations.

So don't say that they don't explode during cranking...
• 07-23-2013, 02:47 PM
OldSpark
deleted 25July13 20:00Z
• 07-23-2013, 02:59 PM
Quote:

Originally Posted by OldSpark

Alternators may have a SMALL capacitor to remove RFI, not to filter DC. (Their regulators may also have caps for proper operation.)
A battery is effectively a HUGE capacitor and swamps AC/ripple far better than a cap. (But there can be frequency dependencies.)

Note that in cars prior to the electronics that have been in cars now for decades if you pulled the battery out of a running car you were not likely to cause damage to anything in the car. It was not a good idea but I know people that did this if their battery died. To protect the alternator they would borrow a battery to start the car, pull the battery and then drive to an auto parts store or garage instead of just going and buying a new battery... Not sure if it actually did protect the alternator but their reasoning was that if the battery was dead to the point it only showed 9 volts that it would burn up the alternator before they could get to the store. I was like 19 and knew nothing about cars.

In ANY recent vehicle that uses electronics the battery is an important protective portion of the system. It filters out a lot of the noise in the system and protects the electronics in the system. Some car user manuals state in them to never run the vehicle without a battery properly installed as it can cause damage to electronic components.
• 07-23-2013, 03:18 PM
OldSpark
deleted 25July13 20:00Z
• 07-23-2013, 03:23 PM
Quote:

Originally Posted by OldSpark
So, your sound system is next to your battery and it has heaps of noise? And that is from the DC supply?
Is that with the engine running?
And it is not due to other connections or ground loops? (It shouldn't have a power ground loops because it's so close to the battery.)

Unless you totally isolate a battery from the vehicle ground you can always induce a ground loop. If you hook your equipment directly to a dedicated battery it is less likely but still possible if it is grounded to the vehicle. You are more likely to pick up RF interference but if you ground to a bad location it can induce interference similar to a ground loop. Always ground to frame or sub frame members. Grounding to sheet metal that is attached to other pieces with spot welds and putty is not a good ground. If you ground a dedicated battery to sheet metal and not a good ground source you can easily induce ground loop type interference when the battery is being charged by the alternator.

As an example, a friend of mine was having noise problems in his car and he couldn't figure it out. He started out with his amplifiers in the trunk and he got noise. He ended up running the power for the head unit back to the trunk to get the same location and he still had noise with everything using the same distribution blocks etc. He then purchased a battery and mounted it in the trunk thinking it would take care of it. (Standard car battery with no venting...) He explained what he had done and asked if I had any suggestions. I went over and saw that he simply drilled a hole in the floor pan of his trunk and put a bolt and washer through the floor to mount his ground to. We proceeded to use an analog VOM and looked at the voltage. While the car was running it was all over the place. I originally assumed he had a good ground but looked at where he placed it. It was in the floor pan and not near a frame member. The trunk was tack welded and they used body seam filler to close up the edges. We ended up temporarily hooking it to a seat belt bolt and the voltage was rock steady and the noise was gone. He ended up finding a solid sub frame member under the car and hooked to that. I mentioned how the hydrogen gas that the battery produced when charging could eventually cause his trunk to explode if he didn't put it in a sealed box that was vented to the outside.

He then mentioned that he only bought the battery to solve the noise issue but also noticed the system was cleaner and louder with the secondary battery. I suggested it was likely a bad ground point all along so he tried it without the battery in place with the new grounding location and you could not tell the difference. He kept the battery disconnected and made a more permanent solution for the connection without the battery in place and never had an issue with it again.
• 07-23-2013, 03:38 PM
DracoPCGuy
Wow the discussion is intense here right? I have a quickie to add here...Can anyone recommend a good place to get an alternator for a decent price...? I have looked around but I am just curious were you all get your stuff.
• 07-23-2013, 04:20 PM
rray
Alternators cost too much, starting at \$400 for the 150amp range even remanufactured ones.

I would look for a late model caddy junkyard pull and try to find a suitable bracket.

Sent from my SCH-I535 using Tapatalk 2
• 07-23-2013, 05:46 PM
settra
For gods sake OldSpark.
http://en.wikipedia.org/wiki/Electrical_impedance

THE IMPENDENCE OF INDUCTORS ICREASES AS FREQUENCY INCREASES (look at it. its there). god. you use the phrase " they lack basic electric knowledge" . and yet you do not know such basic staff like that? i do not misunderstand anything. i STUDY phisics. i know the proof for those formulae and how they work....

and my formulae do not have volt on them, Because frequency is the way volts in AC change. and by god my formulae is not Ohms law.
do i need more proof here?
and what do caps have to do with that? yes a cap also has impendence, and yes, for a theoritical "short circuit" you would need a capacitor infinite small, but for a real time effect in the noise, you dont need to go to infinite.

The fixed velue of the Speaker, is not impendence, but resistance, and it has to do with the maximum current allowed to pass through it.

as for the capacitors voltage, i suppose that since you did not mentioned it, you aggre with me now?

and yes the capacitor is there to filter the DC line. Have you ever heard of Bypass filtering? they do not need to be big or, anything, because only the AC passes through them.
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