Capacitor questions for electronics experts

[jton219]

I have an old blown 200watt car amp i want to repair and hook up to a small subwoofer. The amp is a class D amp. The cause of failure is a blown capacitor. I don't know very much about components. The rating on the blown cap is 1000uf, 80v, 105C. The cap that is blown is very close to the power-in wire connector from the battery.

Im looking online for a replacement and cant find anything exactly the same. Closest i found was a 1000uf 100v buts it only 85c temp rated. i've found many 1000uf caps with lower than the original voltage value..ie 50v, 63v, 25v, ect

Question 1, why is the original rated at 80v when a Car's alternator only puts out 12v DC. is this overkill or does that 80v actually help with something?

Q2, i also found there are specific DC caps.. since a car is DC current, do i have to buy a DC cap?? im really confused. the original cap doesn't say anything about being DC. i would think all caps are DC because even on home audio equip using 120v AC, the internal power supply turn that into DC right??

[little_scrapper]

Capacitors can be used for either AC and DC. Caps with polarity like electrolytics (DC) have a positive and negative and you best be hooking them up correctly or they will (maybe) explode! The 1000microfarad, 100V should be fine in the fact the higher voltage rating is better but.....Yea the 85c rating makes me wonder how hot it will get inside the amp, but 85c is 185F. I know electronics but I will not say I know how hot it gets inside of an amp. 185F is pretty hot and my kneejerk reaction is that it wont get that hot inside but I could be wrong.

The 80V rating is what its rated to withstand. If you run that cap on a 100V system you are overclocking your capacitor by 25%

Any chance you have the remnants of that cap you can show pictures of.

[Oklahoma Wolf]

Question 1, why is the original rated at 80v when a Car's alternator only puts out 12v DC. is this overkill or does that 80v actually help with something?


Because the amp's circuitry operates at a voltage greater than 12V.

What I would be more concerned about is what caused the capacitor to fail... what happened exactly when the amp died?



The 80V rating is what its rated to withstand. If you run that cap on a 100V system you are overclocking your capacitor by 25%

If you apply more voltage to a capacitor than it's rated for, it explodes.

[jton219]

Because the amp's circuitry operates at a voltage greater than 12V.

What I would be more concerned about is what caused the capacitor to fail... what happened exactly when the amp died?

If you apply more voltage to a capacitor than it's rated for, it explodes.

does buying a 100v cap make the components work at 100v?? according to scrapper, i would just have another 20v of headroom before the new cap is at its limits. should i get the 100v cap?

The amp is about 6 years old and i've put over 100k miles on my car in 6 years usually with the music up pretty loud. i did find it strange that the Gain on the amp was set to maximum. i didn't set it there. i believe thats why it blew.

i remember when it died. i was playing some music on the way to a club, heard 2 loud thumps that wasn't part of the song, and then there was no more sub. the fuse was blown, i replaced the fuse thinking id fixed it, still no output and fuse blew again in less than a minute.

here is a picture. both caps in the pic are identical, just one is showing the frontside the other the rear side

[jton219]

when a cap blows..it doesn't blow up most of the time.. that elastic plastic wrapper holds everythign together. u can see the blown lump on the side as well as the cracked plastic from the pressure.

[ZOMGVTEK]

The capacitor is likely still together internally, and has some capacitance, even though its likely 1/2 or less of its rated value, it should not cause the amp to not work. Even if it was missing, the amp should still work.

It takes a lot to do that kind of damage to a electrolytic. Incorrect polarity would do it easy, but seeing as its a 80v cap, its likely it wouldn't even get any voltage on it if you threw -12v on the amp. It would likely be caused from a component failing and throwing a massive voltage spike on the thing. A good 80v cap can handle 100v no problem generally, that damage looks like about 150v, or extreme temps. If the cap decided to vent, its quite likely you have a blown FET and possibly other components as well, and for a 200w amp its not worth repair if you ask me.

Buy a new one, theyre $20-40...

[jton219]

please explain what an FET is. caps themselves only cost like a dollar or so.. im trying to fix it as a hobby and to kill time. also the amp is part of one of those enclosed self powered subs. it was off of an Infinity basslink.

here is a link. http://www.infinitysystems.com/car/products/product_detail.aspx?prod=BASSLINK&cat=BSL&ser=BSL&Language=ENG&Country=US&Region=USA

The amp still turns on, but the servo/clipping light is always on telling me the amp is thinking its working at maximum power. I guess that is the voltage spike your talking about. ive had my uncle who is an electrical engineer check it out and the only thing he saw wrong was the cap..he didnt do any voltage testing though.

[Oklahoma Wolf]

You can replace capacitors with higher voltage ones - doesn't hurt anything, and it won't "upgrade" the circuit to higher voltage. But, in this case, I think the amp's likely fried... it's unlikely replacing just the caps will bring it back to life. You can try though. Also go through it and resolder any suspicious solder joints - that can kill an amp too. And the Infinity Basslink has the amp on the sub... those vibrations can loosen up soldering joints.

I just fixed a Visonik class AB V217SX that acted totally dead by resoldering just about the whole amp. Works fine now :)

FET is short for MOSFET - metal oxide semiconductor field effect transistor. They do the actual amplifying. If one of them is blown, forget about it - easier to shop for a new amp than replace one. Often you can't find exact replacements to buy.

[little_scrapper]

FET = Field Effect Transistor. google it cause an explaination of "it" would be time consumuing. There are just far to many application for this device.

[Vinas]

How old is the amp? Capacitor aging combined with heat is the typical cause of death for most caps. Cap aging happens when used heavily or over an extended period of time (1+ years) a power circuit will slowly lose some of its initial wattage capacity... Check out your mosfets and other components on the AMP. As long as you don't see any brown or dark spots you should be ok. Either way soldering the new cap in place might do the trick. If you do go with the 85C rated cap get some ventilation on there. A small 80mm fan should do the trick.

[jton219]

thanks guys.i put the order in for the 1000uf 100v.. it was 6 bucks for 2 of them shipped. im gonna replace both. ill get back to guys on how it goes.