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Author Topic: How to know what 'type' of caps to use??  (Read 2714 times)
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Colorado
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You could use tantalum here but they are expensive and a fire hazard.
Woah, hold the phone.  What would cause that?  Over voltage?
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Manchester (England England)
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No not over voltage!
When a tantalum cap fails it fails short circuit where as most caps fail open circuit. So a tantalum cap used as a supply decoupled causes a short on the power rails. That often leads to a fire. That is why you should not use them in this situation. Just ask UL.
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Colorado
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Sweet, instant "whoosh!"

And yet, so many regulators suggest using one on the input phase ...
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Manchester (England England)
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They might and if the regulator has a current limit in it then it will probably be OK but you will never get it past UL.
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Colorado
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Just out of curiosity, what happens, or what could happen, if the regulator asks for one, and I put a regular ceramic in place?

For that matter, what would happen if one replaces a polarized cap for a non-polarized one?  I've always followed the regulator's datasheets, so now I'm curious.
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It depends on the circuit, if you have a polarised one subject to AC you damage it. If you put a non polarised one in place of a polarised one then in theory nothing. In practice you might not be able to get the value or you might have to use a more lossy dielectric to get the value.

A ceramic cap will not have the ESR of a tant so it would not handle heave ripple currents as well.
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Fort Lauderdale, FL
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Batteries have positive and negative terminals, but I've also never heard of them referred to as the anode and cathode terminals.
I never called them "anode terminal" or "cathode terminal".  I simply stated "anode" and "cathode."  You added the world terminal. (Which might explain our disagreement.  The positive terminal is connected to the anode of the capacitor.  The negative terminal is connected to the cathode.  To me that's saying the same thing.)

As for more articles and datasheets:
http://en.wikipedia.org/wiki/Anode
"In a recharging battery, or an electrolytic cell, the anode is the positive terminal"

http://www.vishay.com/docs/28396/140crh.pdf  
"Black mark or “-” sign indicating the cathode (the anode is identified by bevelled edges)"

http://data.energizer.com/PDFs/lithiumcoin_xsection.pdf
See picture.

Lastly, check out the origin of the word anode:
http://dictionary.reference.com/browse/anode

Edit: Added anode definition link

« Last Edit: October 31, 2011, 06:07:44 pm by James C4S » Logged

Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.c

Fort Lauderdale, FL
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Just out of curiosity, what happens, or what could happen, if the regulator asks for one, and I put a regular ceramic in place?
Most circuits don't care if the capacitor is polarized or not.  As mike points out, the exception to this case is when dealing with AC.  Anytime a capacitor can be reverse biased (negative voltage applied), it needs to be non-polar.

Circuits like regulators are more interested in matching Capacitance and ESR than a specific type of capacitor.  (However the C and ESR usually define, as a secondary rule, what type of cap is used.)

Assuming no reverse-bias occurs, "what could happen" is that the regulator doesn't work correctly if the ESR is too high (or, sometimes, too low.)
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There are many different types of Capacitors due to a few key factors.  

  • How the cap is used (Voltage/Current)
  • How much it costs
  • Space considerations

Regarding how used: in tuned circuit versus bypass usage.  Or high current/high capacity [car stereo] versus low current/high capacity super cap, etc.  In some cases, a tantalum would be preferred because they "stay on value" and don't lose electrolyte over time the way electrolytic's can. In the case of tuned circuits, some more expensive Mica or Polyester types are closer tolerance to marked values than an equivalent ceramic.

This leads to item 2... cost.  The same value capacitor, lets say 100µF  can cost pennies from a vendor (electrolytic) and the same value capacitor can cost $50.00 from a vendor that caters to Audiophile's.  Google BLACK GATE CAPACITORS. The affordable 100µF will probably work just fine in a hobbyist robotics project for many many years. And then there are the "magical properties" that capacitors are given by some vendors... you pay for that too.

Sometimes you pay more for miniaturization SMD can cost more than traditional Thru-Hole parts...  or in the case of car stereo cap (IE Rockford Fosgate 1F Capacitor) You pay for the size... ( and then toss in some more for the brand name )

In general... a VALUE of capacitor is usually what to focus on first...  then cost (how much are you willing to spend)...  and then it's the application then dictates what kind makes the most sense.  (Electrolytic versus Tantalum) (Ceramic versus Mica/Polyester etc) (voltage ratings/ESR)

Nearly all capacitors have a voltage rating. You can nearly always use a value higher voltage rating than you need and it will work.  Applying too much voltage or reverse voltage on polarized types (especially to certain types, like tantalum and electrolytic) and you can experience catastrophic failure (explosion or fire)

So rule of thumb.  Go affordable unless otherwise specified, don't operate too close to rated voltage (stay well below is preferred  IE: 5V Circuit, buy 15V or higher capacitors).  

Lessons to advance your knowledge: Try to understand how the capacitor works in the circuit or why it's there.
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I'm getting ready to buy 'bits' I don't have and that will include assortments of caps, etc. I only intend to be dealing with up to 12V DC, TTL kind of electric. If I had to decide right now then I'd probably buy an assortment of ceramic caps. Perhaps I should read more.

Would really small value caps between a PWM line and ground flatten the pulses all nice? Would a larger value work just as well or perhaps smaller then larger? I seem to remember using caps after the transformer in home built linear PS, to flatten the ripples.  If it worked there....

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Manchester (England England)
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I would also include some electrolytics of 25V or higher working at 10uF 47uF and 100uF.

As well as 1uF and lots of 0.1uF ceramics.

As to PWM filtering it is a bit more complex that that, see this page for a discussion about a filter at the end:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html
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Velleman sells some affordable starter packs for ceramic and  electrolytic types.  Might be a good idea.
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I've been looking in the Yourduino shop, assuming that one S&H charge goes for a whole order.
I do have little to medium size bags of some parts but missing many I need and/or want.

I kind of like the Starter Kit: Electronics Components + Displays
http://arduino-direct.com/sunshop/index.php?l=product_detail&p=109

I want to add 1 ea of the bags of caps they sell, the 20-ea assortment of 5% resistors, the 100x220 ohm resistors, an led assortment and maybe a few $ of those 4-lead RGB leds because whew, they have possibilities!

I have many mixed wall warts but at a 12V at their prices couldn't hurt.

I also want at least 2 ATTiny85's and Mouser has the best price so maybe they have better deals on caps, etc.

I -definitely- want at least 2 of these: Micro SD Card Adaptor -- assuming it won't take a lot to use!
http://www.pjrc.com/store/sd_adaptor.html
If I wait, they might be gone!
I'm going to agonize about a Teensy++ but not buy one yet. Same with a USnooBie though there's a real draw for one of those too.

I need a multimeter as I can't find my real one, haven't seen it in a long time. I made the mistake of buying a Centon at Harbor Freight (dunno where my brain was, probably out looking for me or just stayed home) and OMG put the thing to test resistance, touched the leads together and it gives me positive values! WTF? What a POS! There's a trimmer inside the case and it makes no difference as far as a zero. But then, there is a trimmer inside the case, and an LCD, a 9V battery clip, etc. So my money isn't -totally- wasted.

I was looking at one of these Velleman # DVM850BL as it looks like it will do and I have another purchase to make at Allelectronics, and they do charge only once for S&H.
http://www.allelectronics.com/make-a-store/item/DVM-850BL/3-1/2-DIGIT-LCD-MULTIMETER-W/BACKLIGHT/1.html

I'll probably think of some other bits in the next day or two but that list stretches the hobby budget already.






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I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

Manchester (England England)
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Quote
put the thing to test resistance, touched the leads together and it gives me positive values! WTF? What a POS!
Lead wires and plug / socket contacts have resistance as well. Was it the right trimmer? If it made no difference it probably wasn't.
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I only saw one trimmer in there, none on the outside. Touch the leads together and the thing hunts around from double-digits down to about 3-8 ohms if I press the leads together and hold them real steady.

I've owned meters before and never saw anything this bad.

And I got the name wrong, it's Cen-Tech. Here's the miscreant and the price has dropped but it's still not worth it:
http://www.harborfreight.com/7-function-digital-multimeter-90899.html

It's going to meet my soldering iron, solder sucker and cutters some day. Till then it's in the junk parts bin.

There's a lot of junk there but also some good deals here and there. Got my "helping hands" magnifier with gator clips arms there for $3. The magnifier is actual glass and doesn't distort. The base is heavy cast iron. I couldn't do much with the UNO without the thing, can't see close enough to get a jumper in the right hole with just my glasses.
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I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

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