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  • Capacitor damage threshold?

    Hello gurus,
    In my seemingly never-ending work of trying to complete my first ground up design, I think I was really close and just making minute adjustments, and then this:
    Would the ever so slight and fast touch of the iron on this tweeter cap be the cause of going from the first measurement to the second? I WAS SO CLOSE TO FINISHED!

    I did test the cap and it does hold voltage, but I'm not sure how to test if the actual capacitance value has changed.

  • #2
    The cap has metal and dielectric layers rolled up like a Ho Ho, very thin. If you damaged the layers the cap is affected.

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    • #3
      If your FR rose (hump around 2k) and that's a series cap on a tweeter (or mid) covering that range, it means the resistance (impedance at 2kHz) has dropped.
      Do you have the ability to do an impedance sweep (on the cap)? If you can post it, we can tell if it's normal or not.

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      • #4
        Originally posted by Chris Roemer View Post
        If your FR rose (hump around 2k) and that's a series cap on a tweeter (or mid) covering that range, it means the resistance (impedance at 2kHz) has dropped.
        Do you have the ability to do an impedance sweep (on the cap)? If you can post it, we can tell if it's normal or not.
        I have an impedence jig but haven't used it in a while! I could give it a go this afternoon

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        • #5
          Kind of looks like you need to improve your soldering skills, especially if you are in the habit of cooking and killing capacitors. May I recommend that you use wires with alligator clips for temporary connections and maybe you select a mechanical means of electrically connecting crossover components, i.e. barrier strips.


          Click image for larger version  Name:	mess.jpg Views:	1 Size:	227.4 KB ID:	1393158

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          • #6
            Additional info;
            I found when doing the coil-test rig that the steel in the conventional barrier strips really affected inductance values heavily. I ended up using Euro-style strips as they minimally affect the inductance likely due to less/smaller size of the ferrous metal contact, and/or the alloy used for the metal contacts.

            You could always attach an alligator clip between the solder junction and the body of the cap to act as a heatsink during the soldering process.

            Later,
            Wolf
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            • #7
              I THINK the op is showing us that he accidentally bumped the iron tip to the outside of the cap body (right there in the middle) causing the outside to slightly melt. I sure wouldn't think that that would do any internal damage to a cap, not a pp cap anyway.

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              • #8
                A good trick is to heat sink the components - that is attach aligator clips next to the body so it draws / spreads the heat and shares the thermal load with the component (resistor / capacitor)

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                • #9
                  Originally posted by AEIOU View Post
                  Kind of looks like you need to improve your soldering skills, especially if you are in the habit of cooking and killing capacitors. May I recommend that you use wires with alligator clips for temporary connections and maybe you select a mechanical means of electrically connecting crossover components, i.e. barrier strips.


                  Click image for larger version Name:	mess.jpg Views:	1 Size:	227.4 KB ID:	1393158

                  Personally - prefer strip connectors. Less likely if you have to move your setup a component / wire goes stray and creates a short. Allows thicker gauge wiring too

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                  • #10
                    Originally posted by Chris Roemer View Post
                    I THINK the op is showing us that he accidentally bumped the iron tip to the outside of the cap body (right there in the middle) causing the outside to slightly melt. I sure wouldn't think that that would do any internal damage to a cap, not a pp cap anyway.
                    The first part was my take as well. Unless I missed it I'm surprised no one mentioned in addition to practice, using a variable wattage solder station in combination with practice is the best approach to learn IMO.

                    I'm talking about this aspect (training / practice) because to melt a cap like that would take either extended contact (not paying attention) or one hell of a hot tip (no-no on solder guns for component work at these tiny tolerances, unless you're awesome and even then it's still very dangerous. A 100W/250W+ gun can boil a component in a couple seconds, heat sink or not).

                    Digital solder stations with spring holder, cleaning pad and finite control for tip wattage can be found in abundance and hella good prices these days.

                    The one I used in the USAF DASD lab in Cheyenne Mnt. wound up being the same one used during tech school down in Biloxi (Keesler and the 81st Training Wing). We used to use rolled braided brass to wick solder, not just a solder sucker, so I recommend those too if you want to be able to clean up messes and/or de-solder.

                    At the time those things were very expensive, but an equivalent (except even better now) can be had today for damn near nothing in relative terms. Ours had the classic sponge you wet a bit to wipe the tip. Today I instead use a brass filed tip cleaner (looks like a open side garbage can miniaturized) and tip tinner.

                    The combination of all of that isn't too expensive if you'll be doing even moderate to rare soldering over spans of years. It'll all last an extremely long time under those conditions.

                    So that'd be my recommendation. Get a solder station, not just a locked wattage stick. Look up the specs on whatever solder you're using to know its melting point and other important characteristics.

                    You want to use the absolute minimum heat necessary to melt the solder and achieve good adherence to the target. Doing that will help prevent damage (a lot) including accidental extended contact, or at-least give you more time to realize you're touching something you didn't mean too before you boil it.
                    Feel free to rip my assumptions apart when wrong, or fix if close.

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                    For woofer(s) with large Xmax vs Sd, all PR(s) with Xmax at-least double all woofer(s) Xmax will work.
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                    • #11
                      Originally posted by Chris Roemer View Post
                      I THINK the op is showing us that he accidentally bumped the iron tip to the outside of the cap body (right there in the middle) causing the outside to slightly melt. I sure wouldn't think that that would do any internal damage to a cap, not a pp cap anyway.
                      This is what happened - just a momentary bit of inattention and the stem of the iron made direct contact with the outside of the cap. I wasn't able to get the impedence rig set up to test (work got in the way). However, I did get a few bags of the plastic euro-style connectors so next crossover will be using those.

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                      • #12
                        This is a Z-sweep of 7.4uF. Before I had DATS (which measures caps directly) I'd take a sweep w/my WT3.
                        A formula to calc uF would be to measure the resistance at a given freq. (I'd use 100Hz) For 100Hz you take 1592 and divide by the ohms to get uF.
                        This sweep reads 215n(ohms) @ 100Hz: 1592 / 215 = 7.4(uF).

                        For 6.8uF you should see an impedance a bit higher, like 235n @ 100Hz (OR: 23.5 @ 1k, or 2350n @ 10Hz - for example).

                        - the BLUE line is the impedance plot (red is phase angle)


                        Click image for larger version  Name:	7pt4uF.jpg Views:	1 Size:	200.0 KB ID:	1393259

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                        • #13
                          [QUOTE=Chris Roemer;n1393258]This is a Z-sweep of 7.4uF. Before I had DATS (which measures caps directly) I'd take a sweep w/my WT3.
                          A formula to calc uF would be to measure the resistance at a given freq. (I'd use 100Hz) For 100Hz you take 1592 and divide by the ohms to get uF.
                          This sweep reads 215n(ohms) @ 100Hz: 1592 / 215 = 7.4(uF).

                          Ok thanks Chris - I wouldn't have known at all how to calculate this.

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                          • #14
                            Full formula: Xc is the impedance of a cap at a particular frequency, capacitive reactance.

                            Xc = 1 / (2 * pi * frequency * capacitance)

                            Solved for C = 1 / (2 * pi * frequency * Xc)

                            Chris's value of 1592 solves for using 100Hz, and converting straight to uF by taking out the 10^-6 power.
                            Electronics engineer, woofer enthusiast, and musician.
                            Wogg Music

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                            • #15
                              Yeah, I got that out of my 2nd (1st "modern") speakerbuilding book (by David Weems).

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