capacitor

[gbrower]

I get voltage reading on top of aluminum electrolitic capacitor. Does that mean the capacitor is shorted/bad?

[Chris Smith]

If your running a volt meter on a dry cap out of service, it can build up a stray volatge just sitting in a drawer. <p>Short it out and retest it. <p>Use a resistor or light bulb to discharge the cap voltage. <p>
Stray RF in the Air can charge up caps.

[jollyrgr]

More details please....<p>But let me assume a couple things here.<p>You have one lead to ground.<p>The circuit is energized.<p>The capacitor is electrolytic.<p>The part you are touching is the exposed metal can on top of the capacitor.<p>If this is true, you might read some voltage.<p>But give some more details please. For instance, is the capacitor an electrolytic? Where is the other meter lead at? Is this in an energyzed circuit? The exposed metal on top of electrolytic capacitors is connected to one of the leads in many cases.<p>Use Chris' method to short the capacitor out if it is out of circuit. After you confirm there is no voltage on the cap, use an Ohm meter to determine if one of the leads is connected to the case of the capacitor. Likely you will find that it is.

[gbrower]

Yes, it is electrolytic and energized. I was just amazed that it's reading 60VAC and shocked me too.

[Dean Huster]

"VAC" was a rather important bit of information to be posted so late. You've got ripple out the wazoo. With ripple like that, you have an open/dried-up cap or one with high ESR. Replace it. If there are other similar caps in the circuit, it wouldn't hurt to give those a check, too. And if you lay a scope probe on that cap and find that it's a sine wave AC vs. a rectified sine, you may have some rectifier problems, too.<p>Dean

[rshayes]

The can of an electrolytic is usually the neagtive terminal. Normally you won't see a ripple voltage on it unless it is a folter on a negative supply.<p>Your reading of 60 VRMS is about what you would expect on the case of a capacitor used as a filter for a bridge rectifier tied to the power line. This is basically a half wave rectified line.<p>If this is the case, get your fingers and test equipment out of that area until you understand what is happening. This may be a circuit with no line isolation and it is potentially dangerous.<p>Many switching power suplies use a bridge rectifier on the AC line. On a schematic, the negative lead looks like a ground, but it is actually has half wave AC on it. Connecting the ground lead of a scope probe can have spectacular results, since the scope ground is usually tied back to the scope case, which is tied to the green safety ground in the building wiring. If you are lucky, the power supply fuse blows before too much damage occurs.

[bcurrey]

Ummm ...
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>stephen Many switching power suplies use a bridge rectifier on the AC line. On a schematic, the negative lead looks like a ground, but it is actually has half wave AC on it. Connecting the ground lead of a scope probe can have spectacular results, since the scope ground is usually tied back to the scope case, which is tied to the green safety ground in the building wiring. If you are lucky, the power supply fuse blows before too much damage occurs. <hr></blockquote><p>Isn't THAT what ISOLATION TRANSFORMERS are for ?<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Dean Huster "VAC" was a rather important bit of information to be posted so late. <hr></blockquote><p>Yes ... there is STILL a lot of important information needed to formulate an answer that actually means anything.
#1) Application/type of unit the cap is used in?
#2) Function of the cap IN that circuit ... filter? coupling/decoupling ? charge-pump? etc...
#3) Value of cap in uF ?
#4) Max Vdc rate of cap ?
#5) Operating voltages of the circuit it's found in ?
#6) Method used to determine that there IS a voltage there ... meter ..scope ..(wet index finger... ;-P) ...?<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Gary Wayne Yes, it is electrolytic and energized. I was just amazed that it's reading 60VAC and shocked me too. <hr></blockquote><p>A 60Vac reading can OFTEN be caused by the following;
-A) Measuring "across" a circuit ..where PART of the circuit has an " isolated ground "
-B) Stray EMI in the measuring area. Such as having your meter/scope wires near another device that has a transformer in it. ( soldering iron controller ...florescent lamp base ...etc ...)<p>Hope that helps

[rshayes]

That is exactly what isolation transformers are for. But isolation transformers are somewhat rare and fairly expensive these days.<p>Most of the diagrams that I have seen for off-line switching supplies show a straight line across the bottom of the drawing that is obviously a common return. It is the reference point for the primary switching and control circuits. It looks like a ground, but it isn't. It is usually connected to the negative output of a bridge rectifier supplied by the AC line directly. Half the time, it is close to ground. The other half of the time it is a negative half sine wave.<p>Usally, the circuits using this return drive a high frequency (50 to 100 KHz ) transformer. The secondary windings on this transformer are isolated and may be connected to ground without any problem.<p>If you use an isolation transformer with this type of power supply, then it is possible to ground that common return lead. This makes testing much easier and safer.

[Dean Huster]

After following subsequent posts, I suppose that I was assuming (incorrectly?) that when you said "on top" that it was meaning at the terminals and not on a bare can where you might be picking up half the AC line voltage on a high-impedance meter. Yeah, the answer is in the details.<p>Dean

[jollyrgr]

I'm still confused but let me venture a guess here.<p>
AC HOT --|>|------+||---------AC Neutral<p>Now if the metal can of the capacitor in this simple single rectifier circuit is the minus terminal of the cap and you have everything plugged in correctly, you don't get hit by the chassis being hot.<p>AC HOT ------||+---------|<|---AC Neutral<p>Now the minus side of the capacitor, and the metal can/shell is on the "HOT" side of the AC line and you will see positive half cycles above ground potential. This is why polarized plugs and isolation transformers are used. But polarized plugs are defeated and isolation transformers are much more expensive than a diode and cap.<p>When servicing old electronic amplifiers (without isolation transformers) and many tube radios and TVs you will see the entire chassis tied to one side of the AC line. If you plug the plug in one way the chassis is at neutral or "ground" potential. Plug it in the other way and the chassis is at AC HOT potential.<p>Many years ago a TV service person told me about what happened to one lady for whom he serviced a set. She had a portable TV. The set had a single rabbit ear and a second wire for the antenna to go to chassis ground. These wires were to be disconnected when an external roof antenna was connected to the TV. Well the lady connected both the outside antenna and the internal ground/rabbit ear antenna. The plug was in so that the chassis was hot. The roof antenna was grounded to prevent lightning from getting in the house. While some of you are laughing (because you know where this is leading) others are not. It turns out it was winter when the service guy discovered her TV hookup "problem". The ground rod the antenna was connected to had a big area around it where the snow had melted. AC was traveling up the antenna wire and finding a ground back to earth. There was a good current flow because of the snow and thus it melted it.

[rshayes]

With a bridge rectifier the circuit on the positive half cycle is:<p>AC HOT --|>|----+||-----|>|--AC Neutral<p>The can is one diode drop above ground. Not a great problem.<p>On the negative half cycle the circuit is:<p>AC HOT --|<|-----||+----|<|--AC Neutral<p>Now the can is connected to the hot lead of the AC line with only a conducting diode between them.<p>If you swap the hot and neutral leads (ie reverse a two wire plug), you still get the same result. At some time, the can will be connected to a low impedance negative potential relative to the grounded AC lead.<p>The waveform on the can will be a negative half cycle followed by a nera zero potential for one half cycle. The RMS value of the waveform is 120 volts, but if it is only present 50% of the time it will read 60 volts on an average reading meter calibrated in RMS volts. (It would read about 85 volts on a true RMS meter, but these are not as common as average reading meters.)

[Timothy Rasch]

It is normal to have stray voltage on electrolytics on their top.I don't know why. In audio coupling circuits using 4.7uf to 10 uf I have used this stray voltage on top for finding audio path as an aid to troubleshooting by laying signal tracer hot lead on top to find audio in a radio, tuner or cd player,then I checked both sides of cap for the same audio level .If different the cap was replaced. Sometimes electrolytics can affect the bias of a transistor base or an IC input so when you have a bias problem try replacing the electrolytic first, then recheck bias.
Tim Rasch [email protected]

[rshayes]

There isn't anything "stray" about the voltage on the can of an electrolytic capacitor. The can is directly connected to the negative lead of the capacitor.<p>If you look at an axial lead capacitor (one with a lead out each end), you will find that the positive lead goes through a bushing (usually rubber or something similar) and that the negative lead is connected directly to the can (usually by welding).<p>The positive electrode of the capacitor is usually a strip of aluminum foil rolled up with a porous separator. The surface of the foil is usually chemically etched to increase its surface area. The dielectric of the capacitor is a very thin layer of aluminum oxide (usually a few tenths of a micron). The neagative electrode of the capacitor is a conductive liquid or paste that can conform to the rough surface. The electrical connection to this liquid is the capacitor case.