PDA

View Full Version : The great Capacitor thread



per
December 15th, 2008, 08:02 AM
As of I know, there is no computer that does not contain capacitors. So there is problably capacitors in every computer. Some times, old capacitors fail, and may explode or catch fire, and sometimes, manufacters just don't bother soldering on all of the capacitors. But what are they for, those small metallic containers that hold a small curent?

Things I'm unsure on, and would like to know more about:

1: There has been a coupple of reports about capacitors catching fire. But when do they usually do?
a. Durning powerup.
b. Durning runtime.
c. Durning shutdown (most likely not).
d. When exposed to wrong voltage/temperature levels.
e: Other (please explain).

2: What are they actually used for in most causes?
a: Filtering, to make sure there is no noise in the current.
b: Preventing current from going the wrong direction (on shutdown/powerup).
c: Charging up a high current for use with devices that usually uses high currents.
d: Backup power for some device.
e: Other (again, please explain).

3: Why does some manufacters not solder in all the capacitors in a PCB?
a: Capacitors might be expensive (for reduced manufacting price).
b: To make the device's lifetime shorter (have to buy a new one more often).
c: To freak out people like me.
d: They really don't care.
e: Other (yes, please explain)

4: As of the above statements, why does the devices still work?
a: Because some capacitors are left.
b: Because they really don't (random faults from time to time).
c: Because no capacitor are really needed (capacitors might be there just to make the device look fancy).
d: To make the device sell.
e: Other (Explain if you can)

Chuck(G)
December 15th, 2008, 08:40 AM
1: There has been a coupple of reports about capacitors catching fire. But when do they usually do?
a. Durning powerup.
b. Durning runtime.
c. Durning shutdown (most likely not).
d. When exposed to wrong voltage/temperature levels.
e: Other (please explain).

All of the above, except perhaps (c). I've never had a cap catch fire, however. Explode, yes, but not catch and maintain a flame.


2: What are they actually used for in most causes?
a: Filtering, to make sure there is no noise in the current.
b: Preventing current from going the wrong direction (on shutdown/powerup).
c: Charging up a high current for use with devices that usually uses high currents.
d: Backup power for some device.
e: Other (again, please explain).

(a) and (e), being "decoupling", "memory", "timing/filtering" and "coupling". (d) and (c) in a way. In a digital application with single-ended (as opposed to differential) signals, when a device changes a logic level it's usually accompanied by a change in supply current demand. This tends to put "spikes" on the supply rails. Small (e.g. 0.1 uF) decoupling capacitors serve to smooth these spikes and prevent them from affecting other devices.

Some devices use RC time constants for filtering or as the timing element for oscillators. Capacitors can also be used to overcome differences in DC levels when an AC signal needs to be passed from one device to another.

Memory--all that DRAM in your machine is nothing more than a whole bunch of capacitors, each storing a charge for a 1 or 0.

(d) Backup power applications are usually only found on more recent (i.e. not "vintage") devices. In particular, "supercapacitors' having very high capacitances (measured in Farads) but low voltage ratings are used to tide clock and memory circuits over when batteries are being changed.

(c) Let's call this one "energy storage", which is really the idea behind a filter capacitor--it accumulates charge during voltage peaks and gives up charge when the voltage drops. Also used in boost converters and voltage multiplier circuits.

If you want a detailed answer, why not look this one up in Wikipedia?


3: Why does some manufacters not solder in all the capacitors in a PCB?
a: Capacitors might be expensive (for reduced manufacting price).
b: To make the device's lifetime shorter (have to buy a new one more often).
c: To freak out people like me.
d: They really don't care.
e: Other (yes, please explain)

(a) sort of. It's the DuMont principle. Use whatever is necessary but no more, a few pennies here and there add up to real money over a million units. Sometimes an optional circuit is left off--there's no need for any of the components to be there.


4: As of the above statements, why does the devices still work?
a: Because some capacitors are left.
b: Because they really don't (random faults from time to time).
c: Because no capacitor are really needed (capacitors might be there just to make the device look fancy).
d: To make the device sell.
e: Other (Explain if you can)

Non sequitur, really. A sufficient number of capacitors is needed to make a device work. The trick to production engineering is to find that value. Since it's not unusual for electrolytic capacitors to vary by 20 percent from the marked value, any design must take this variation into account.

You didn't discuss "Chinese capacitor disease". This is the property exhibited by a number of motherboards made around 1992 that used devices obtained from a Taiwanese manufacturer who used an electrolyte of improper formulation, causing massive failures in the field. If I get a mobo in from that time and it registers a short, I don't even try to deal with it. Replacing a single cap would buy only a limited amount of time.

per
December 15th, 2008, 08:56 AM
You didn't discuss "Chinese capacitor disease". This is the property exhibited by a number of motherboards made around 1992 that used devices obtained from a Taiwanese manufacturer who used an electrolyte of improper formulation, causing massive failures in the field. If I get a mobo in from that time and it registers a short, I don't even try to deal with it. Replacing a single cap would buy only a limited amount of time.

I have a MoBo from about that time period, but it doesn't contain any electrolytes. I Guess I'm lucky then :) .

Anyway, that is the only MoBo form the early 90's I got, so I actually didn't know that.

I couldn't barley find anything but formulas on Wikipedia :rolleyes: .

Kaypro
December 15th, 2008, 09:07 AM
Try looking up ĻCapacitor PlagueĻon wikipedia. Thereīs a nice article on this phenomenon. The Caps in some computers are worse than others. A good example of this are Appleīs Machines from around 1986-1994 (excluding Analog board troubles, the the ][ series). The caps on the motherboards in some models (like the SE) are notoriously bad for leaking and the like. Iīve recapped a 2 SE motherboards, and itīs usually the same every time. Itīs an easy fix, if you catch it before it fries something.

I have had Caps go in other computers though. Quite the shock when a machine goes "POP!", and stops working...

per
December 15th, 2008, 09:18 AM
I have had Caps go in other computers though. Quite the shock when a machine goes "POP!", and stops working...

That was one thing I forgot to ask about:
Does it make a loud noise?
Does it smoke?
What should you do if it happen?
Is it dangerous (for the device AND envroniment)?

compu_85
December 15th, 2008, 09:31 AM
That was one thing I forgot to ask about:
Does it make a loud noise?
Does it smoke?
What should you do if it happen?
Is it dangerous (for the device AND envroniment)?

Sound? Yes, they either go POP or BANG depending on how big the cap is.
Does it smoke? Yes, som.
What should you do? Unplug the system and air out your room.
Is it dangerous? Probably.

-Jason

Kaypro
December 15th, 2008, 09:35 AM
They sometimes make a loud noise. Not so much a BANG! but more of a POP!

They do emit a bit of smoke, though the amount depends on the size of the Cap, and how badly it blew.

If this happens, Turn the machine OFF Immediately! Then wait for the smoke (if thereīs a significant amount) to clear. Open the machine up and find the cap that died. Write the Voltage and capacitance down on a piece of paper (Also make sure to note which leg on the Cap is negative. Installing caps backwards is bad). Then procede to replace the cap, with one of the Same Voltage and Capacitance (Well, close to the original).

It can be dangerous for the device if the itīs left powered on. The smoke is potentially poisonous, so watch out for that (not really a problem, but not good to inhale). Itīs potentially bad for the environment if you through the Blown cap away...

Chuck(G)
December 15th, 2008, 09:57 AM
I maintain that you have not experienced capacitor terror until you've had an old Spraque "Vitamin Q" paper-in-oil capacitor explode on you. Not just the noise (which rivals that of an M80), but the steel shrapnel from the case...

Interestingly these old 1950's capacitors seem to be highly sought after by the tube audiophile community.

Druid6900
December 15th, 2008, 11:29 AM
Yeah, I keep one on me to use as a portable defibrillator in case someone's heart stops when I hand them the bill :-)

rebeltaz
December 15th, 2008, 11:49 AM
I have a MoBo from about that time period, but it doesn't contain any electrolytes.

It doesn't have ANY electrolytics? I can't recall any computer (or very few circuits of any type for that matter) that I've ever seen without any electrolytics.

Kaypro -> You say that the caps are an easy fix. How do you get the caps off the multilayer boards without screwing up the board? I've managed to recap a couple of boards but I wound up having to use sidecutters to cut the cap can off then use needlenose pliars to pull each leg as I heat it from below. Major PITA if you ask me.

BTW - the caps that have failed on me in the 1992 era boards didn't pop, leak or anything like that. When my computer froze continuously and I had eliminated everything software related, I decided to check hardware. As I glance over the board I saw the tell-tale sign that I see on so many pieces of equipment I work on - the top vent on all of the caps around the CPU had puffed out.

just my 2Ē worth...

per
December 15th, 2008, 12:00 PM
It doesn't have ANY electrolytics? I can't recall any computer (or very few circuits of any type for that matter) that I've ever seen without any electrolytics.

That's right. It only got the ceramic type ones. But there are some electrolytes escapely on the soundcard and various of other cards installed in the system.

Kaypro
December 15th, 2008, 12:57 PM
Kaypro -> You say that the caps are an easy fix. How do you get the caps off the multilayer boards without screwing up the board? I've managed to recap a couple of boards but I wound up having to use sidecutters to cut the cap can off then use needlenose pliars to pull each leg as I heat it from below. Major PITA if you ask me.



I never meant to imply it was easy, and youīre right about multilayer boards being a PITA. However, itīs not impossible, just more difficult (ok, some of the more densely packed ones can be almost impossible..). I didnīt want to go over the methods of removing them, as thatīd be a lot of writing, I was just giving the usual steps taken.

patscc
December 15th, 2008, 01:08 PM
Per said... That's right. It only got the ceramic type ones
Not even SMD caps ?
patscc

per
December 15th, 2008, 01:11 PM
Not even SMD caps ?
patscc

Maybe. I'll take a closer look next weekend.

patscc
December 15th, 2008, 01:13 PM
I usually try to isolate the pws from the delicate circuitry and power it up using a variable transformer (for a linear pws at least) and a dummy load and check ripple before I even let it anywhere near the board. Sometimes, if you're motivated, you can either use a in-circuit cap tester, or a multimeter to check dc resistance (make sure the multimeter's ohm function supplies less than 0.6 volts) of suspect board caps before applying power.
Sometimes I'll hook in a lower-rated fast blow fuse into the B+ lines to catch sudden disastrous failures as quickly as possible rather than waiting for some overrated slow-blo main fuse to blo.

For sheer thrill, though, I don't think anything can beat imploding crt's.

patscc

Terry Yager
December 15th, 2008, 04:29 PM
BTW - the caps that have failed on me in the 1992 era boards didn't pop, leak or anything like that. When my computer froze continuously and I had eliminated everything software related, I decided to check hardware. As I glance over the board I saw the tell-tale sign that I see on so many pieces of equipment I work on - the top vent on all of the caps around the CPU had puffed out.

just my 2Ē worth...

Yes, it's that top vent that allows it to go *poof* rather than ***POW!!!***

--T

Chuck(G)
December 15th, 2008, 07:13 PM
I never meant to imply it was easy, and youīre right about multilayer boards being a PITA. However, itīs not impossible, just more difficult (ok, some of the more densely packed ones can be almost impossible..). I didnīt want to go over the methods of removing them, as thatīd be a lot of writing, I was just giving the usual steps taken.

On some boards, with radial lead electrolytics, I've simply pulled or cut the cap apart (easier than it sounds) and soldered to the wire stubs left on the board. Not pretty, but it gets the job done. The same trick can work with axial-lead resistors.

But usually, I don't bother unless it's an unusual piece of gear. Into the oven and shake everything loose when the solder melts. :)

patscc
December 15th, 2008, 07:47 PM
Chuck(G) said...Into the oven and shake everything loose when the solder melts
Remind me never to stop by for cookies and milk.
patscc

Chuck(G)
December 15th, 2008, 07:51 PM
Remind me never to stop by for cookies and milk.
patscc

Hah--no, this oven is in my shop with a dedicated temperature controller. I can soak a board until the solder *just* liquefies without burning (much of) anything. Through-hole components might require a little tugging, but all of the SMT just sloughs right off.

Beats using a charcoal fire like some of the Chinese peasants do. (http://archives.truthaboutchina.com/2008/11/following-the-trail-of-toxic-e.html)

patscc
December 15th, 2008, 07:57 PM
What temperature, by the way ?
patscc

Chuck(G)
December 15th, 2008, 08:04 PM
For 63-37 tin-lead solder, 370F works pretty well. About the same temp for some nice oatmeal cookies. :)

rebeltaz
December 15th, 2008, 08:58 PM
For 63-37 tin-lead solder, 370F works pretty well. About the same temp for some nice oatmeal cookies. :)

And the time in the oven doesn't harm any of the component? :eek:

I'll have to try that on my stockpile of surplus boards!

Ole Juul
December 15th, 2008, 09:29 PM
And the time in the oven doesn't harm any of the component? :eek:

It depends on whether you like your chips runny or not.

Chuck(G)
December 15th, 2008, 10:16 PM
I don't salvage the low-value components (i.e. capacitors, resistors, etc.) but the heat doesn't seem to bother ICs at all. It's probably no worse than a charcoal fire or a hot-air rework tool--and maybe better as there are fewer thermal stresses in an evenly-heated board.

The weapon of choice in depopulating old PCBs used to be a propane torch. Turn the board component side down, wave the torch over the top side and wait for the ICs to fall off. That was pretty crude and very smelly and probably dangerous to your health.

To rework a board with SMT, I use a homebrew version of Quik-Chip; Cerrobend 158 applied as a powder and heated to less than 200F with a spotlamp applied to the reverse side. Even fine-pitch TQFPs pop right off without damaging the board traces at all. Clean, flux and solder the new chip in normally.

Sorry for the off-topic rambling... :oops:

Ole Juul
December 15th, 2008, 11:04 PM
>Chuck(G): "Sorry for the off-topic rambling..."

Actually, I find it quite on topic. Electronics seems to be a "dated" hobby and the average, so called, nerd nowadays couldn't solder his way out of a paper bag - so your ramblings are quite refreshing!

Back to caps, specifically. It is worth noting when dealing with temperatures, that they usually come in temperature ranges. Something like 80C or 100C. That is quite low, so one can expect them to fail in many places where heat accumulates. Also, there is a nice overview of cap types and their uses at radio electronics: http://www.radio-electronics.com/info/data/capacitor/capacitor_types.php
The charts there are also good for deciding on which type to use. That, of course, also depends on what you can get your mitts on.