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isolating farad capacitorPrinted From: the12volt.comForum Name: Car Audio Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc. URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=119081 Printed Date: May 03, 2024 at 12:18 AM Topic: isolating farad capacitor Posted By: teknofix Subject: isolating farad capacitor Date Posted: January 03, 2010 at 10:56 PM I am about to install an 8 Farad Capacitor parallel to Battery to improve my Audio system performance I have been reading forums about how to install an Farad Capacitor, everybody is talking about charging the capacitor thro' the 12v bulb an installing as close to the Amplifier. That's all fine. The purpose of the Farad Capacitor is, smooth, bump-up and hold the stored voltage, they are rated up to 24 Volts. Ok. lets say Battery voltage is : +12V before entering the Farad Capacitor, and it is bumped up to +16 Volts by the Capacitor, so there is a 4 Volt difference between Battery and the Capacitor, nobody mentions how to isolate the voltage difference between the batttery and the Capacitor, another words preventing bumped-up voltage by Capacitor going back to battery but intsted only going to amp. I think, This can be done by simply placing a high wattage rating Diode between the battery and Capacitor on the + wire , hence Diode conducts only one way, Voltage stored on the Capacitor will only go to the amplifier, and it will be blocked to go back to battery, but the battery will supply to the Capacitor.Anybody agree or disagree with me or any suggestions how to isolate the mentioned voltage difference ? Teknofix@bellsouth.net Replies: Posted By: whiterob Date Posted: January 03, 2010 at 11:42 PM teknofix wrote: A cap will not "bump up" the voltage. A cap will help to "smooth out" the voltage fluctuations like you mentioned but it will be about the same voltage at the cap as at the battery. It would actually be slightly smaller at the cap because of voltage drop due to the resistance in the wire but that is another story. Also, the voltage when the car is running will be around 13.8-14.4 volts for an average car. This would be about what you would expect to see on the voltmeter the cap has. Posted By: i am an idiot Date Posted: January 04, 2010 at 7:36 AM Now the Capacitors are MAGIC. We should give up.
Posted By: teknofix Date Posted: January 04, 2010 at 6:21 PM Quiz to asked question : What happens if I place Diode between Batt. and Cap. as shown below : ? Batt = + 13.8 V ---------------I>I-------------I I-------------> +V to Amp Diode Farad Cap I know there will be 0.6V drop across the Diode, but what Will be the final Voltage at the Cap + ( using 8 Farad Cap ) ? A = 13.2V B = 14.4 V C = 14.6 V D = None ? I will let you know, after I experiment to find correct answer ( awaiting ordered 8 Farad Cap) Thanks for answers..Happy New Year to All.. Teknofix@bellsouth.net Posted By: i am an idiot Date Posted: January 04, 2010 at 6:38 PM Probably around 13.4 until the diode shorts. Then it will be 13.8.
Posted By: oldspark Date Posted: January 05, 2010 at 4:05 AM Forget the capacitor.
As I have written elsewhere (also dealing with an 8F capacitor), a cheaper small AGM battery will do far better. Caps are for looks only, and to provide certain people with a "non-value-add" income stream. Posted By: haemphyst Date Posted: January 05, 2010 at 8:34 AM teknofix wrote: This is yor first mistake. If you have not addresed the power source in your car (two guesses, and the first seven don't count), you are wasting your time and energy, thinking this is going to "fix" or "help" anything. teknofix wrote: First, it's a capacitor. JUST capacitor. Not a farad capacitor. Farad is the capacitance value. teknofix wrote: Wrong. The purpose of a capacitor when installed in a system with an inadequate charging source (have you figured it out yet?) is to pad the profit margin of the dealer selling it to unsuspecting, uninformed and undereducated consumers. teknofix wrote: Wrong again. The capacitor cannot output more voltage than it is connected to. If you are connecting it to a 12V source, that is the voltage you will read across it's terminals. teknofix wrote: I disagree. First, diodes are rated by current @ a given voltage, not "wattage". Second, knowing this now, any diode of sufficient current rating to pull amplifier current demands through, will be HUGE, and will probably require you to mount it securely to the chassis of the car, (as the heatsink) in literally a ½" hole. I might also mention to you the expense of said diode! (Expect $100 or more...!) teknofix wrote: Nothing. The cap doesn't go in series as you have indicated in your diagram. It's a parallel connected device. teknofix wrote: First, it's .7V drop for silicon diodes, and about .3V for germanium diodes. (Not really sure who told you it was .6V...) So, in answer to your question, AND based on the illustrated voltage of 13.8V, your cap will be charged to 13.1 volts, using the most common diode material - silicon. It will be around 13.5 volts using the less common, and more expensive germanium. Again, using your diagram, what will be the voltage at your amplifier? Zero. Your amp will not even turn on. teknofix wrote: You already HAVE the correct answer: You are $200 poorer (give or take), will see little to absolutely zero benefit for that $200 loss, AND have still not addressed the POWER SOURCE in your car. (Have you guessed it yet?) ------------- It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down." Posted By: tommy... Date Posted: January 05, 2010 at 9:13 AM I know...I know...!!!! He needs a new _____________! Mr .I will send you a schematic as a "How To"...I bet he will send it for ...Oh...Lets say...$149.99...Off the top you have saved $50 bucks and countless hours of trying to explain why it will work in your situation...! I think im going to refer to caps as "anti-spark" from now on...!
------------- M.E.C.P & First-Class Go slow and drink lots of water...Procrastinators' Unite...Tomorrow! Posted By: tommy... Date Posted: January 05, 2010 at 10:18 AM https://bcae1.com/capacitr.htm ......................... https://bcae1.com/chargin2.htm ........................................... Just some basics... Where in your readings did someone recommend adding a cap...?
------------- M.E.C.P & First-Class Go slow and drink lots of water...Procrastinators' Unite...Tomorrow! Posted By: haemphyst Date Posted: January 05, 2010 at 10:18 AM haemphyst wrote: Third: The capacitor cannot output more voltage than it is connected to. If you are connecting it to a 12V source, that is the voltage you will read across it's terminals. tommy... wrote: Probably car audio dot com, online car stereo dot com or woofers etc dot com... Or... Wait for it... BEST BUY!!!! YEA!!!! I love those guys. So amsrt! You know how those guys are. And to the OP... Have you guessed what you are missing yet? All the hints dropped in my first post to you haven't helped at all? ------------- It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down." Posted By: oldspark Date Posted: January 05, 2010 at 7:48 PM Geepers, the alleged experts on this site seem so anti-cap.
Why is that? Perhaps they some vested interest in the truth, or NOT ripping people off? Again - I like this site. But I still have to get used to people agreeing WITH me....! Now a few short pedantic comments: - voltage can appear to "increase" with a capacitor, but that is a fault of the measuring system (ie, averaging and not true-RMS measurements etc). - as a capacitor is like a (parallel voltage) smoothing device (as an inductor is a series current "smoother"), and as it tends to decrease the dip depth, it also decreases the peaks. A diode feeding a (parallel) capacitor is like a sample-&-hold circuit (the diode prevents discharge back into the source) which used to hold the peak voltage (which could be well over 16V for a 12V vehicle) for heading by a hi-impedance voltmeter. But a load is NOT a high impedance - a voltmeter may be over 10MR (Mega-Ohms); a headlight may be 2R, and an audio amp under 0.1R. Do the math - the capacitor hold-up time is similar to its time constant or proportional to t=RC (=2x8F = 16 secs for headlight; 0.8 secs for an amp etc - not that t=RC is the formula used; I'm just demonstrating the relatively short hold up time, and rebuffing related pro-cap claims). - lamp of resistive charging of a cap is good to see since it indicates a low internal cap resistance. A discharged cap has infinite inrush current. Luckily internal resistance usually limits it to less than infinity. But the current can be so high as to destroy the capacitor or other circuits, hence why some require pre-charging. (Huge caps of several Farads that don't require precharging are either of robust construction and don't care about other circuitry, else they have relatively high internal resistance. I have long questioned how the latter is removed once in operation!) In a previous 8F discussion, I pointed out how a cheap $35 battery had many times the hold-up time of an 8F capacitor. Alas it stopped short of dynamic (AC) analysis, but that too shows the battery's superiority (low cap & battery ESRs become negligible wrt to load impedances for high-power SMPT amps etc, and alternator sub-transient response (albethe alternator too far from the amp for that benefit). Keeping in mind that most with such HUGE amps will utilise a second battery to ensure preservation of the cranking battery, the above factors are solved by an additional large capacity AGM battery mounted with the amps. The original vehicle battery remains.... The AGM battery is only connected whilst the alternator is charging.... The AGM's surface charge provides more "Farads" than the largest capacitors.... And when cap would have discharged to the battery voltage (say 12.7V or less) and therefore been useless/negligible, the AGM takes over as a "generator" of electricity providing thousands of equivalent "Farads" of energy. Alas I have only scraped the surface - there is lots more. But for now.... Posted By: custombass Date Posted: January 05, 2010 at 8:33 PM For roughly $300, you could have right around a 220A fix. Caps really do nothing except provide one more physical connection between amplifier and battery. For a cap to be efficient, it needs to be WAY larger than 8F. Word of advice, get a high output alternator. It's the 220A fix I mentioned at the beginning of my reply. I used 220 amps as an example. Your application might warrant a different size alternator. Ohms law is a great tool to use for figuring out what size alternator you will need. This is the ONLY way you will get a "bump-up." Good luck in your endeavors sir....
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