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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Is this capacitor polarized?
I have to replace a small capacitor that had a lead ripped out when
the tv was dropped and the pcb broken. It's a cylinder about a half inch high, a quarter inch in diameter, with two leads at the bottom, and a narrowing near the bottom, like a waist line, but it gets back to full diameter at the bottom. It's black and says on it: extra info TL [in an elongated circle] 50v4.7uF -- CD71 40/085/10 N -- There is lead coming out next to the N and next to the 4.7uF. Does the N mean that that is the negative lead? If not, does that mean the capacitor is non-polarized? BTW, what happens if I use a polarized cap where a non-polarized was intended, or vice versa? Will a reverse voltage puncture the dielectric? Even if it is less than 50 volts, like the rating here? Thanks. If you are inclined to email me for some reason, remove NOPSAM :-) |
#2
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Is this capacitor polarized?
"mm" wrote in message ... I have to replace a small capacitor that had a lead ripped out when the tv was dropped and the pcb broken. It's a cylinder about a half inch high, a quarter inch in diameter, with two leads at the bottom, and a narrowing near the bottom, like a waist line, but it gets back to full diameter at the bottom. It's black and says on it: extra info TL [in an elongated circle] 50v4.7uF -- CD71 40/085/10 N -- There is lead coming out next to the N and next to the 4.7uF. Does the N mean that that is the negative lead? If not, does that mean the capacitor is non-polarized? BTW, what happens if I use a polarized cap where a non-polarized was intended, or vice versa? Will a reverse voltage puncture the dielectric? Even if it is less than 50 volts, like the rating here? Thanks. If you are inclined to email me for some reason, remove NOPSAM :-) A capacitor of this value and voltage rating could be either polarised, or not. Usually, a polarised type is quite clearly marked, but an alternative give-away is the PCB silk screening where it's located. If it is a polarised type, there is a 99% certainty that one of the legs will be marked. Sometimes the board will be marked with a " + " sign, and sometimes, there will be a circle with one side shaded. The shaded side normally indicated the " - " leg of the cap, and corresponds to a stripe on its body. The ESR figures for polarised and non-polarised electrolytics are usually quite different, which could cause a problem if substituting one type for the other, depending on the cap's circuit function. Also, if the cap is being subjected to a high level of AC across it, which is why non-polarised types are usually used, then a polarised type will probably not stand up to it for very long, although in saying that, standard polarised electrolytics are often used in the VFD supply multipliers in hifi's and although they do fail because of the reverse stresses across them, they do last quite a long time. Arfa |
#3
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Is this capacitor polarized?
On Mon, 19 Feb 2007 19:55:37 -0500, mm put
finger to keyboard and composed: I have to replace a small capacitor that had a lead ripped out when the tv was dropped and the pcb broken. It's a cylinder about a half inch high, a quarter inch in diameter, with two leads at the bottom, and a narrowing near the bottom, like a waist line, but it gets back to full diameter at the bottom. It's black and says on it: extra info TL [in an elongated circle] 50v4.7uF -- CD71 40/085/10 N -- All my Googling suggests that "CD71" is a bipolar or non-polarised aluminium electrolytic. Yours seems to be a standard temperature type, ie -40C to +85C. The "10" may refer to the tolerance. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#4
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Is this capacitor polarized?
On Tue, 20 Feb 2007 01:48:34 GMT, "Arfa Daily"
wrote: A capacitor of this value and voltage rating could be either polarised, or not. Usually, a polarised type is quite clearly marked, but an alternative give-away is the PCB silk screening where it's located. If it is a polarised type, there is a 99% certainty that one of the legs will be marked. Sometimes the board will be marked with a " + " sign, and sometimes, there will be a circle with one side shaded. The shaded side normally indicated the " - " leg of the cap, and corresponds to a stripe on its body. The ESR Thanks. OK, there are no polarity markings on the pcboard, so I guess it is not. figures for polarised and non-polarised electrolytics are usually quite different, which could cause a problem if substituting one type for the other, depending on the cap's circuit function. Also, if the cap is being subjected to a high level of AC across it, which is why non-polarised types are usually used, then a polarised type will probably not stand up to it for very long, although in saying that, standard polarised electrolytics are often used in the VFD supply multipliers in hifi's and although they do fail because of the reverse stresses across them, they do last quite a long time. The cap is on the other side of a resistor that is near the flyback, and the other side of the cap goes to the pcb ground. I wish I could say more than "near the flyback" but it's very hard to see. Later today, I'll try to unscrew and move the pcb from the plastic frame it's mounted to. Arfa If you are inclined to email me for some reason, remove NOPSAM :-) |
#5
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Is this capacitor polarized?
On Tue, 20 Feb 2007 15:55:10 +1100, Franc Zabkar
wrote: On Mon, 19 Feb 2007 19:55:37 -0500, mm put finger to keyboard and composed: I have to replace a small capacitor that had a lead ripped out when the tv was dropped and the pcb broken. It's a cylinder about a half inch high, a quarter inch in diameter, with two leads at the bottom, and a narrowing near the bottom, like a waist line, but it gets back to full diameter at the bottom. It's black and says on it: extra info TL [in an elongated circle] 50v4.7uF -- CD71 40/085/10 N -- All my Googling suggests that "CD71" is a bipolar or non-polarised aluminium electrolytic. Yours seems to be a standard temperature type, ie -40C to +85C. The "10" may refer to the tolerance. Thanks. I didnt' realize CD71 would be the part number. Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same appearance as the one I need to replace, that I was going to connect in parallel, with the negatives connected together. If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? - Franc Zabkar If you are inclined to email me for some reason, remove NOPSAM :-) |
#6
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Is this capacitor polarized?
"mm" wrote in message ... On Tue, 20 Feb 2007 01:48:34 GMT, "Arfa Daily" wrote: A capacitor of this value and voltage rating could be either polarised, or not. Usually, a polarised type is quite clearly marked, but an alternative give-away is the PCB silk screening where it's located. If it is a polarised type, there is a 99% certainty that one of the legs will be marked. Sometimes the board will be marked with a " + " sign, and sometimes, there will be a circle with one side shaded. The shaded side normally indicated the " - " leg of the cap, and corresponds to a stripe on its body. The ESR Thanks. OK, there are no polarity markings on the pcboard, so I guess it is not. figures for polarised and non-polarised electrolytics are usually quite different, which could cause a problem if substituting one type for the other, depending on the cap's circuit function. Also, if the cap is being subjected to a high level of AC across it, which is why non-polarised types are usually used, then a polarised type will probably not stand up to it for very long, although in saying that, standard polarised electrolytics are often used in the VFD supply multipliers in hifi's and although they do fail because of the reverse stresses across them, they do last quite a long time. The cap is on the other side of a resistor that is near the flyback, and the other side of the cap goes to the pcb ground. I wish I could say more than "near the flyback" but it's very hard to see. Later today, I'll try to unscrew and move the pcb from the plastic frame it's mounted to. Arfa I guess that the only way that you're ever going to know for sure, is with a set of schematics and a reference number for the cap. No markings on the board would suggest a non-polarised type, but the fact that one leg goes to ground would suggest otherwise. As it's near the flyback, it could be decoupling for a rail, but its value seems a little small for that, and the value of the resistor would have to be pretty low if it was a flyback derived rail. Could be a pulse integrator for use by some other piece of the circuitry, but that also would not require the cap to be non-polarised. Bit of a mystery really. Arfa If you are inclined to email me for some reason, remove NOPSAM :-) |
#7
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Is this capacitor polarized?
"mm" wrote in message ... On Tue, 20 Feb 2007 15:55:10 +1100, Franc Zabkar wrote: On Mon, 19 Feb 2007 19:55:37 -0500, mm put finger to keyboard and composed: I have to replace a small capacitor that had a lead ripped out when the tv was dropped and the pcb broken. It's a cylinder about a half inch high, a quarter inch in diameter, with two leads at the bottom, and a narrowing near the bottom, like a waist line, but it gets back to full diameter at the bottom. It's black and says on it: extra info TL [in an elongated circle] 50v4.7uF -- CD71 40/085/10 N -- All my Googling suggests that "CD71" is a bipolar or non-polarised aluminium electrolytic. Yours seems to be a standard temperature type, ie -40C to +85C. The "10" may refer to the tolerance. Thanks. I didnt' realize CD71 would be the part number. Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same appearance as the one I need to replace, that I was going to connect in parallel, with the negatives connected together. If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? To "make" a non polarised electrolytic, you have to connect two electrolytics of twice the required value - in your case that would be 10uF each - in inverse series. Usually, you connect the two ' - ' legs together leaving the two ' + ' legs to go to the outside world. Each cap need only be half the working voltage of the original non-polarised in theory, but it's not a bad idea to make each one the same as the original as, depending on what exactly is across the cap, you can't guarantee that the voltage will divide equally between them. However, all that said, non-polarised caps are readily available, so unless you are really stuck for obtaining one, it's better to fit what was originally there. Arfa - Franc Zabkar If you are inclined to email me for some reason, remove NOPSAM :-) |
#8
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Is this capacitor polarized?
On Tue, 20 Feb 2007 08:37:33 -0500, mm
wrote: Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same appearance as the one I need to replace, that I was going to connect in parallel, with the negatives connected together. If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? In series, either positive to positive or negative to negative (back to back.) You need to compute the value that results (formulas and calculators abound on the web...) but with just two, you'd end up with half the capacitance so you need a couple of 10 uF caps (each rated at 50 volts). However, series back to back capacitors are not (IMHO) a good solution--better to go out and get the right part instead. |
#9
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Is this capacitor polarized?
On Tue, 20 Feb 2007 14:00:13 GMT, "Arfa Daily"
wrote: If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? To "make" a non polarised electrolytic, you have to connect two electrolytics of twice the required value - in your case that would be 10uF I remember that now from 10 or 20 years ago, although I never knew the details you give below. Thanks to you and Peter. One more question for now. Like I said, the tv must have been droped, even though there is no damage on that part of the case. I'm soldering jumpers across all the broken traces. But my supply of spare caps is small, I guess because I only inherited a few and there are so many possible kinds to have. So it will take me a while to get this one little cap. Can I apply power to the tv without damaging anything if I have nothing where the capacitor under discussion should go? Or a bigger or smaller non-polarised one temporarily? each - in inverse series. Usually, you connect the two ' - ' legs together leaving the two ' + ' legs to go to the outside world. Each cap need only be half the working voltage of the original non-polarised in theory, but it's not a bad idea to make each one the same as the original as, depending on what exactly is across the cap, you can't guarantee that the voltage will divide equally between them. However, all that said, non-polarised caps are readily available, so unless you are really stuck for obtaining one, it's better to fit what was originally there. Arfa - Franc Zabkar If you are inclined to email me for some reason, remove NOPSAM :-) |
#10
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Is this capacitor polarized?
PeterD wrote in news:lg1mt2tc5n34le0lauqcqc31qgl82n50u4
@4ax.com: On Tue, 20 Feb 2007 08:37:33 -0500, mm wrote: Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same appearance as the one I need to replace, that I was going to connect in parallel, with the negatives connected together. If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? In series, either positive to positive or negative to negative (back to back.) You need to compute the value that results (formulas and calculators abound on the web...) but with just two, you'd end up with half the capacitance so you need a couple of 10 uF caps (each rated at 50 volts). However, series back to back capacitors are not (IMHO) a good solution--better to go out and get the right part instead. Should there not also be a couple of diodes, one across each capacitor, to prevent applying reverse voltage to either? I seem to remember that electolytic do NOT like reverse voltage applied. It removed the 'insulating layer' that has been electroplated on one of the plates (that also serves as the dielectric). This tends to lead to high current flow and breakdown. Since there is another capacitor in series, the current will be limited, and the dielectric may reform when polarity is right again, but I would not expect it to be very reliable. http://yarchive.net/electr/electrolytic_caps.html has some interesting information. -- bz 73 de N5BZ k please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. remove ch100-5 to avoid spam trap |
#11
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Is this capacitor polarized?
"mm" wrote in message ... On Tue, 20 Feb 2007 14:00:13 GMT, "Arfa Daily" wrote: If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? To "make" a non polarised electrolytic, you have to connect two electrolytics of twice the required value - in your case that would be 10uF I remember that now from 10 or 20 years ago, although I never knew the details you give below. Thanks to you and Peter. One more question for now. Like I said, the tv must have been droped, even though there is no damage on that part of the case. I'm soldering jumpers across all the broken traces. But my supply of spare caps is small, I guess because I only inherited a few and there are so many possible kinds to have. So it will take me a while to get this one little cap. Can I apply power to the tv without damaging anything if I have nothing where the capacitor under discussion should go? Or a bigger or smaller non-polarised one temporarily? Personally, without knowing the purpose of the cap, I wouldn't go ahead and apply power. If it was in some kind of snubber circuit for instance, the effect of it not being there might prove catastrophic to some following semiconductor Arfa |
#12
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Is this capacitor polarized?
On Wed, 21 Feb 2007 01:17:19 GMT, "Arfa Daily"
wrote: "mm" wrote in message .. . On Tue, 20 Feb 2007 14:00:13 GMT, "Arfa Daily" wrote: If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? To "make" a non polarised electrolytic, you have to connect two electrolytics of twice the required value - in your case that would be 10uF I remember that now from 10 or 20 years ago, although I never knew the details you give below. Thanks to you and Peter. One more question for now. Like I said, the tv must have been droped, even though there is no damage on that part of the case. I'm soldering jumpers across all the broken traces. But my supply of spare caps is small, I guess because I only inherited a few and there are so many possible kinds to have. So it will take me a while to get this one little cap. Can I apply power to the tv without damaging anything if I have nothing where the capacitor under discussion should go? Or a bigger or smaller non-polarised one temporarily? Personally, without knowing the purpose of the cap, I wouldn't go ahead and apply power. If it was in some kind of snubber circuit for instance, the effect of it not being there might prove catastrophic to some following semiconductor Arfa OK. Thanks. If you are inclined to email me for some reason, remove NOPSAM :-) |
#13
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Is this capacitor polarized?
"bz" wrote in message 98.139... PeterD wrote in news:lg1mt2tc5n34le0lauqcqc31qgl82n50u4 @4ax.com: On Tue, 20 Feb 2007 08:37:33 -0500, mm wrote: Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same appearance as the one I need to replace, that I was going to connect in parallel, with the negatives connected together. If instead I connected them in parallel with each negative connected to the other's positive, would that give me the equivalent of a non-polarised cap? In series, either positive to positive or negative to negative (back to back.) You need to compute the value that results (formulas and calculators abound on the web...) but with just two, you'd end up with half the capacitance so you need a couple of 10 uF caps (each rated at 50 volts). However, series back to back capacitors are not (IMHO) a good solution--better to go out and get the right part instead. Should there not also be a couple of diodes, one across each capacitor, to prevent applying reverse voltage to either? In theory I guess, that works, but in practice, I don't think that it would prove to be an issue I seem to remember that electolytic do NOT like reverse voltage applied. It removed the 'insulating layer' that has been electroplated on one of the plates (that also serves as the dielectric). This tends to lead to high current flow and breakdown. Since there is another capacitor in series, the current will be limited, and the dielectric may reform when polarity is right again, but I would not expect it to be very reliable. http://yarchive.net/electr/electrolytic_caps.html has some interesting information. You are right that again, in theory, electrolytics do not like reverse polarity, but I think that the trick to this is the duration / duty factor of the reverse polarity. Electrolytics are subjected to AC across them in many circuit locations, and sometimes, in output stages for instance, the level of reverse voltage that they are subjected to can be quite substantial, but it does not seem to cause any long term problems, except when a cap with a poor ESR or insufficient basic voltage rating is used. One particular hifi that I used to repair a lot of, springs to mind in this regard. Both of it's output coupling caps used to fail open circuit, quite regularly. Once they had been replaced with low ESR 105 degree types with twice the voltage rating, there was never any further trouble. However, even though caps in this sort of position do suffer reverse voltage across them, the average duty factor is 50%, and the durations of the reverse excursions are relatively short, all of which, I think, contributes to the scheme working by and large, reliably. As I said elsewhere, another area where electrolytics are subjected to a severe reverse pounding, is in the voltage multipliers for VFD supplies. In these circuits, they have transformer derived line-power frequency AC of high levels across the multiplier input cap. Sometimes, ultimately, these do fail - it's very common in Aiwa's for instance - leading to a dark display. There also used to be a very popular Sony tape deck that was part of a stacking system, where two normal polarised electrolytics were in the VFD filament supply lines. These used to fail and wreck the display tube. Sony supplied a revised type that was brown with gold writing, but it was still a normal polarised type. However, the revised ones did not re-fail, so there doesn't seem to be any hard and fast rule that designers stick rigidly to in this regard. Arfa |
#14
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Is this capacitor polarized?
On Wed, 21 Feb 2007 01:17:19 GMT, "Arfa Daily"
wrote: Or a bigger or smaller non-polarised one temporarily? Personally, without knowing the purpose of the cap, I wouldn't go ahead and apply power. If it was in some kind of snubber circuit for instance, the effect of it not being there might prove catastrophic to some following semiconductor Well, lucky me. This is one of 46 capacitors that Radio Shack sells, and it's even one that they normally keep in the store, AND according to the webpage, it is in 3 different stores near me. That's a good thing, because I was having a lot of trouble keeping myself from turning the tv on. There is a store I know has every such part, but it's 6 times as far away. Arfa If you are inclined to email me for some reason, remove NOPSAM :-) |
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