Basic question...Capacitors and being an idiot

Discussion in 'General Electronics Chat' started by Phsion, Mar 14, 2009.

  1. Phsion

    Thread Starter New Member

    Mar 10, 2009
    6
    0
    Hi All, I was hoping to find some help with Capacitors here...The short question is; If I have a Capacitor(the Aluminum Can SMD style...47uF, 35v) feeding +18v to a circuit with 6 Resistors, and I solder the Capacitor on BACKWARDS, can i blow all 6 Resistors? The long questions is; so i have this device...and i broke it. It has card A, and Card B. Card A gets power from the main power source, and then feeds Card B it's power. On Card A, there are four 47uF Capacitors (Paralleled in groups of 2...one set for +18v, the other for -18v), then the ribbon that feeds Card B. Then on Card B, from what i can tell, the +18v first hits all of the Resistors (22.1Ohm SMD, Thick Film?). Here's what happened...somehow the Capacitors got shorted, and the resistors got hit so bad they blew in half. So I went through the whole thing, replaced all the Capacitors, and replaced all the resistors, and then turned the thing on again...And each resistors blew right back up. So i took a closer look, and realized I had soldered one of the capcitors on backwards, so negative was touching positive and vise versa. What i'm trying to figure out is, if i swap that Capacitor around, whats the chance that i fixed it? I really don't want to replace those resistors if i'm just gonna blow them right back up again. Thanks for the help.

    Oh, and by the way...those resistors arn't in series...there are 6 similar but slightly different circuits, all next to each other, being fed from the same power. The resistors are just the first thing ( i think) in the circuit.
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Usually the capacitor will explode.

    Check the power supply, and the circuit for a short to ground, to find the actual cause of the symptoms you are seeing. :)
     
  3. Phsion

    Thread Starter New Member

    Mar 10, 2009
    6
    0
    Its odd...this is a 32 Channel Audio Mixing Console. The Channels are split into groups of 8, and everything works just fine besides this one card. The original problem was caused by overspray from someone (idiot) using FaderLube to clean the console, so the first time the Capacitors exploded, there was a nice puddle of conductive grease around the Capacitor to short it out. Now there is nothing. The card actually works fine (i can "mix" the white noise that all consoles generate at high gain) but i just can't get any signal into the board itself. The resistors are before each Gain POT, and since they are dead, i would assume that they are the ones breaking the signal path. Anyhow, thanks for the suggestion...i believe that since everything else works, that the power supply is fine...but i will look for a ground short. Also, i'm getting really bloody tired of using a 4mm soldering iron to poke through a 5mm hole...are those desolder tweezer things actually worth it? Do they sell a pair (not the wellers, i can't afford another $280 tool this month) and any type of big chain electronics stores (i.e. Frys Electronics)? Or anyone seen them in the Inland Empire/Southern California area. I would like to work on this now, instead of 7-10 business days
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Unfortunately, it's highly likely you've destroyed the cap that you installed backwards. :(

    Polarized electrolytic caps do not take well at all to having the wrong polarity across themselves.

    People often don't know that electrolytic (and tantalum) capacitors degrade over time, but often can be "re-formed" by slowly charging them via a resistor to their rated voltage, which causes the dielectric to lower it's leakage current.

    The acceptable leakage current for an electrolytic capacitor can be calculated by:
    Max leakage current(uA) = Capacitance(uF) x working voltage x 0.15
    In your cap's case:
    Max leakage current = 47uF x 35 x 0.15 = 246.75uA, or 0.24675mA.

    You could use an adjustable bench power supply to charge a cap up to 35v slowly through a 10k Ohm resistor. Once the voltage across the resistor dropped below 2.4675V and the capacitor voltage reached 35v, the capacitor would have an acceptable leakage rate.
     
  5. Phsion

    Thread Starter New Member

    Mar 10, 2009
    6
    0
    Okay, then i have a new question...i can replace the capacitor, no problem. And also, i can replace the Resistors (bloody pain in the ass...it should be law, if you have something really really tiny, you can't surround it by things 100 times as high, and 100 times as wide...im not kidding, the resistors are surrounded by POTs and switches, usually separated by about the width of the resistor) but i'd like to know, in your opinion, are the Caps blowing out the resistors? Also, i have you basic bench multimeter...its a nice one, but i still got it at home depot. It can test Capacitors...but will it tell me its leaky? I test a cap, it gives me the rated farads...so im all good?

    I know no one can tell me the capacitors are blowing out the resistors for sure, but i have little expirence with electronics like this. I just looking for what an expirenced tech would "expect"...

    Oh ya...i have no "bench". I fix large stuff...usually made in the 70s...I got my $120 Ideal multimeter, a weller WES51d, and a 4mm solder tip...and some hopes and dreams. Oh, and lots of spare parts :)
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If the caps are shorted, and they are being charged via the resistors, sure.

    A DMM might tell you if a cap has a dead short, but won't tell you the leakage at the rated voltage. You need a power supply capable of sufficient voltage and a series resistor to do that.

    You need to find out where the short(s) are. Trace out the circuit. Then divide and conquer. Use Ohm's Law to determine if the resistance readings you're getting are reasonable.
     
  7. Screamtruth

    Member

    Apr 17, 2006
    10
    0
    Too add to the "stupid questions" list, why do you need a capacitor with leakage current? Or are you just finding what the acceptable rate should be?
    (FYI, this is new to me as well!)

    Semper Fi Marine.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Check resistance (both ohm and on diode check) from both sides of resistors to ground, then power, while the unit is off, and you know the caps are good. If the value is far below any of the resistors, the problem is elsewhere.

    To test leakage without a bench supply, some multimeters have an "insulation resistance" range. On a Fluke, switch to resistance manual range, then nanoSiemens (nS, 1nS = 1GigΩ, 1000nS=1MΩ). This range is useful for measuring resistances > 40MΩ, reverse diode leakage, as well as capacitor leakage. If measuring a capacitor and get a high, steady value instead of overload, you are seeing leakage. Compare with a known good cap of exact same type/value to be sure.

    On the back of mine is a chart for uF/second on different resistance ranges:
    400Ω = 2,600 μF/sec
    4kΩ = 300 μF/sec
    40kΩ = 30 μF/sec
    400kΩ = 3 μF/sec
    4MΩ = 0.3 μF/sec

    A similar chart can be made for yours with a stopwatch and spare time. The values above are actually about 29 rather than 30 on my meter, but close enough for a fast check without having to move the dial. If you have a "conductance" range, you can roughly test for leakage, it's more appararent when the rate the value changes is different for a similar sized cap, even on resistance ranges. :)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's not a stupid question at all. ;)

    ALL capacitors have "leakage" through their dielectric. Electrically, it acts as a high-value resistor in parallel with the capacitance value, and slowly drains the charge within the capacitor. Problems occur when this resistance value decreases over time; more and more power is dissipated within the capacitor instead of performing useful work. This heats the capacitor; in particular the liquid electrolyte. This causes the electrolyte to boil, forcefully rupturing the capacitor's case. Modern aluminum electrolytics have their cases designed with weak areas, so that the pressure won't build too high before rupturing. The old caps could go out with a real "BANG!". Tantalum caps are also vulnerable to leakage current; they can go bad just by sitting around for half a year.

    The formula I posted above was derived from tables that were generated some 50+ years ago. The formula is much easier to use than having to deal with tables.

    The original 1957 document that I found the tables in is named ReformerNo1.pdf - you might find it with a Google search. I won't post a direct link here.

    Failing that, search for "Electrolytic capacitor reforming"; you'll get plenty of hits.
    S/F
     
  10. Screamtruth

    Member

    Apr 17, 2006
    10
    0
    Bringing back all that old MCCES stuff that I forgot, years ago.


    Ended up being a TACP chief anyways..........so I lost all of that useful info.
    Semper gumby and thanks!
     
Loading...