Unknown capacitor.

Discussion in 'Datasheets, Manuals and Component Identification' started by Tha fios agaibh, Nov 9, 2018.

  1. Tha fios agaibh

    Tha fios agaibh

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    mysterycap.jpg Fixing a 120v electric paper punch for a friend. I believe the problem is that square blue capacitor C2. Its the first component after the line fuse, and is parallel with a 1meg resistor. It only reads 678nf (out of ckt) and is only passing about 3v on to what I believe is a smt bridge rectifier. (labeled MEH3)

    Any idea what this cap is?
    Google let me down this time, but the numbers look like N113554923 652
    Thanks.
     
    Tha fios agaibh, Nov 9, 2018
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  2. Tha fios agaibh

    davenn Moderator

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    well it's labelled 652 = 0.0065 uF = 6.5nF ( not your 678nF) ...

    I think you missed a decimal point 6.78nF would be with in tolerance range of a 6.5nF with somewhere around 10 - 20% tolerance rating


    well it's a capacitor ... so it wont be passing voltage on to anything ;) .... capacitors have a gap between plates


    show us more of the component side of the board as well as the solder side

    Dave
     
    Last edited: Nov 9, 2018
    davenn, Nov 9, 2018
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  3. Tha fios agaibh

    Harald Kapp Moderator Moderator

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    To narrow down the search: The logo seems to be EPCOS (now TDK).
    Can you make a better photo (crisp lettering)?
     
    Harald Kapp, Nov 9, 2018
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    davenn Moderator

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    It's a typical mylar/polyprop. 250V rated cap ... nothing too special
    And I have already stated the actual value he should be looking for :)
     
    davenn, Nov 9, 2018
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  5. Tha fios agaibh

    Harald Kapp Moderator Moderator

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    I'm terribly sorry, that must have been while I was searching for the Logo :(
     
    Harald Kapp, Nov 9, 2018
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    hevans1944 Hop - AC8NS

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    Jeez.... is this another one of those power supplies that uses capacitive reactance to drop line voltage to a lower value?

    The cap in the photo "looks" okay. Does it have any measurable conductance when measured "out of circuit" and removed from the board? I doubt that its capacitance could have changed without physical evidence of a catastrophic failure. Mylar and polypropylene dielectrics sandwiched between aluminum foil electrodes are pretty damned stable after the capacitor is "potted" in its plastic case. I would look elsewhere for any power supply problem associated with this electric paper punch... like maybe some shorted turns (smell test) on whatever solenoid drives the punch or punches? Any chance of reverse-engineering a schematic for your sick puppy?

    SMT bridge rectfier? That cheesy looking board appears to made for through-hole, not surface-mount, components. Please provide a picture showing more details as Dave asked in his post #2.
     
    hevans1944, Nov 9, 2018
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  7. Tha fios agaibh

    Tha fios agaibh

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    2018-11-09 14.53.23.jpg Heres the bridge on the back. Only 3v on Ac side, nothing on DC side. Just took a brief look at this last night, but I remember nearly all of the incoming120vac was dropped across this Cap and parallel 1m resistor.
    More pics comming.
     
    Tha fios agaibh, Nov 9, 2018
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  8. Tha fios agaibh

    Tha fios agaibh

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    2018-11-09 15.21.42.jpg
     
    Tha fios agaibh, Nov 9, 2018
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    Tha fios agaibh

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    Thanks Dave, I wish they would just mark the actual value since there's room for it. Forgive the capacitor passing voltage comment. I was thinking of its Xc resistance dropping line voltage as hevans1944 mentioned.
     
    Tha fios agaibh, Nov 9, 2018
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    Ylli

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    Sounds to me like you have excess current draw. I'd start by using an ohmmeter and checking the diode bridge. Should be OK doing it in circuit (power off of course). If the diode bridge looks good, then check the 100 uF 50v cap for leakage or short.
     
    Ylli, Nov 9, 2018
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    Tha fios agaibh

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    Yes, exactly. I'll take another look at it when I get home and sketch something up if I have time. The cap was suspect because I read ~120v across resistor parallel to it.
     
    Tha fios agaibh, Nov 9, 2018
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    Bluejets

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    Almost every mains driven timer, daylight switch etc. etc. seem to employ this arrangement these days.
    In my experience there is zero visible evidence of failure, just a weak reading on the cap ( out of circuit).
    Managed to get many up and operational again during holiday season or after hours when no wholesalers can be found.
    First one I struck (hospital generator/mains c/o timer) I was lucky enough to use the same value/type on the cdi ignition systems I used to make here for miniature engines.
    Now I keep a variety of values spare.
     
    Bluejets, Nov 9, 2018
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    hevans1944 Hop - AC8NS

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    I suppose it could be internally open. Maybe substitute a known "good" one?

    Lots of itsy bitsy teeny weeny 3-terminal devices on the "bottom" side of the board, but what is controlling power to the punch solenoid(s)? Would that be the black 3-terminal thingy at the top left (opposite U2)? And what is the 6-terminal IC device on the "top" side of the board? An optical isolator? Sounds like some reverse engineering is in order. Enjoy!
     
    hevans1944, Nov 9, 2018
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    Tha fios agaibh

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    The teeny devices all read .7 vd one direction so they appear to be healthy transistors.
    Black 3 term thingy is a triac to turn on motor driven punch.
    Yes, the 6 terminal IC is an opto isolator.
    Of course with no voltage rail nothing will come alive.

    Thanks, I'll keep you posted when I get back to tinkering.
     
    Tha fios agaibh, Nov 9, 2018
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    Tha fios agaibh

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    Here's what I found. Cap C2 does in fact read 680nf (.68uf) but should be 6.5nf as Dave pointed out. So yes, it appears to be the problem. 2018-11-09 20.52.14.jpg
     
    Tha fios agaibh, Nov 10, 2018
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    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Capacitors rarely increase in value as the fail :) I suspect that there was an error transcribing the part number.

    I would be interested in what is to the right of the circuit you show. Is there a zener diode?
     
    (*steve*), Nov 10, 2018
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    Tha fios agaibh

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    Steve! You are the man! Yes, there's a zener thats shorted ahead of it.
    So I guess the shorted DC side was enough for the AC side of the bridge to collapse also.
    Here's the 652 giving a rare capacitance reading. 2018-11-09 22.19.20.jpg
     
    Tha fios agaibh, Nov 10, 2018
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    kellys_eye

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    That capacitor looks to be marked u68 = μ68 = 680nF and therefore correct on the meter. 680nF is consistent with the value necessary to drop the volts and is a standard value - unlike 6.5nF.

    Where they use zeners to regulate voltage they often under rate them (dissipation-wise) and they fail easily when the relays are NOT energised (as the zener, rather than the relay, dissipates the excess voltage).

    Fit a new zener but one with a higher power rating. The capacitor is fine.
     
    kellys_eye, Nov 10, 2018
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  19. Tha fios agaibh

    Tha fios agaibh

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    That black thing U2 (3rd picture) is a photo switch that senses when the punch carriage returns. Its got a led on one side with a 3.3k limiting resistor, I don't know the specs, but perhaps we can figure out the ps/zener voltage based on that sensor.

    Alas, I've got other projects to tackle this weekend so I may not respond for a while.

    Thanks (as always) for everyones input.
     
    Tha fios agaibh, Nov 10, 2018
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  20. Tha fios agaibh

    Tha fios agaibh

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    Perhaps its good, but It definitely says 652 on the right hand side.
    Question is, what voltage?
     
    Tha fios agaibh, Nov 10, 2018
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