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Are your Capacitors Installed Backwards
Posted by
Bryan
on 18 May, 2015 20:12
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#1 Reply
Posted by
Thor-Arne
on 19 May, 2015 10:51
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I'm aware of this, but I'm not sure how significant it is.
Wouldn't the manufacturers mark this properly if it was important?
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#2 Reply
Posted by
smjcuk
on 19 May, 2015 11:05
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That's why I like electrolytics. They're honest. At least if you install them backwards they complain loudly!
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#3 Reply
Posted by
akis
on 19 May, 2015 11:52
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In addition I read recently that MLCCs lose their capacitance as the voltage increases sometimes to a fraction of nominal.
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#4 Reply
Posted by
dom0
on 19 May, 2015 12:05
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In addition I read recently that MLCCs lose their capacitance as the voltage increases sometimes to a fraction of nominal.
This is (afaik) a property of all ceramic capacitors, but mainly affects those with high dielectric constant dielectrics.
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#5 Reply
Posted by
Fungus
on 19 May, 2015 12:08
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I'm aware of this, but I'm not sure how significant it is.
Wouldn't the manufacturers mark this properly if it was important?
I don't think it's as important as the audiophools would have you believe (or capacitors would be marked, as you say).
The signal he's picking up sure looks
scary (woooh!) but it's really in the millivolt range and you wouldn't be using big honking power-supply capacitors on things with millivolt signals and high impedance inputs. You'd be using itty-bitty ceramics for that, right?
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#6 Reply
Posted by
paulie
on 19 May, 2015 12:26
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In addition to concern for installing non-polarized caps backwards also make sure those resistors are not reversed too. Specially important for audio applications use oxygen free copper on the PCB.
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#7 Reply
Posted by
dom0
on 19 May, 2015 12:31
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Fungus: I didn't watch the video (1 hour!), but what you write sounds like the vid is about the orientation of the outer film/foil layer (which is susceptible to external electric fields). Is this correct?
If so, this effect rarely matters, except for very big film capacitors with extremely low applied levels (think sub-nV²/Hz amplifiers) driven by low impedances. As electric fields are easily shielded mitigation is simple in cases where it matters.
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#8 Reply
Posted by
TimFox
on 19 May, 2015 13:26
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Years ago, I learned that paper (then) capacitors had the "outside foil" marked for safety purposes. That lead should be connected to the node with lower DC voltage, since the insulation outside the outside foil was minimal.
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#9 Reply
Posted by
Fungus
on 19 May, 2015 13:37
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Fungus: I didn't watch the video (1 hour!), but what you write sounds like the vid is about the orientation of the outer film/foil layer (which is susceptible to external electric fields). Is this correct?
Yes.
He's an audio guy and in the video he builds a special device to detect the "correct" capacitor orientation.
He starts out with a really old paper covered capacitor which has "outer film" clearly marked on it then goes on to show that modern film capacitors are misleading because don't have this marking. They obviously need a special device to detect the "correct" orientation, which he goes on to build (I fast-forwarded most of the "build" part of the video).
If so, this effect rarely matters, except for very big film capacitors with extremely low applied levels (think sub-nV²/Hz amplifiers) driven by low impedances. As electric fields are easily shielded mitigation is simple in cases where it matters.
Yep. I'm not an expert but about half way through the video I started to suspect it wouldn't make a real difference in places where that size of capacitor is used in real life (ie. power supplies, etc.)
Still: His device will look good next to the one which detects proper speaker cable orientation.
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Thanks! I enjoyed your video. I have lots of those orange-dipped caps.
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#11 Reply
Posted by
Kjelt
on 19 May, 2015 14:11
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Interesting indeed.
Hey is it just me or does it bother others that the guy calls the oscilloscope settings in mHz per division?
I always use time per division not frequency, maybe that was different on the older scopes?
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#12 Reply
Posted by
Len
on 19 May, 2015 14:59
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Fungus: I didn't watch the video (1 hour!), but what you write sounds like the vid is about the orientation of the outer film/foil layer (which is susceptible to external electric fields). Is this correct?
Yes.
He's an audio guy and in the video he builds a special device to detect the "correct" capacitor orientation.
He starts out with a really old paper covered capacitor which has "outer film" clearly marked on it then goes on to show that modern film capacitors are misleading because don't have this marking. They obviously need a special device to detect the "correct" orientation, which he goes on to build (I fast-forwarded most of the "build" part of the video).
If so, this effect rarely matters, except for very big film capacitors with extremely low applied levels (think sub-nV²/Hz amplifiers) driven by low impedances. As electric fields are easily shielded mitigation is simple in cases where it matters.
Yep. I'm not an expert but about half way through the video I started to suspect it wouldn't make a real difference in places where that size of capacitor is used in real life (ie. power supplies, etc.)
Still: His device will look good next to the one which detects proper speaker cable orientation.
Paul's a "radio guy" more than an "audio guy". The capacitors he's talking about are for repairing old tube radios or amplifiers with paper foil capacitors. (Not modern audiophile equipment!) He’s got a lot of experience with tube RF & audio circuits, so if he says the capacitor orientation makes a difference I'm inclined to believe him.
Also, note that he’s talking about tube amplifier circuits not power supplies. Did you watch
any of the video?
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#13 Reply
Posted by
Muxr
on 19 May, 2015 15:14
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I watched some of his other videos too. He does some great work with his homebrew PCBs. It's a good series on how to apply clean fixes and workarounds to the old gear.
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#14 Reply
Posted by
German_EE
on 19 May, 2015 16:09
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Capacitors installed 'the wrong way round' can have an effect in high gain audio circuits. Some radio amateurs avoid the traditional superhet receivers and build units called Direct Conversion Receivers where all of the receiver gain is in the audio stages. This can mean up to 40dB of audio gain so hum pickup from various sources can be a major problem. Making sure that the outside layer of a capacitor is wired to the low impedance source helps.
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#15 Reply
Posted by
Fungus
on 19 May, 2015 17:13
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Also, note that he’s talking about tube amplifier circuits not power supplies. Did you watch any of the video?
Where did I say he's talking about power supplies?
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#16 Reply
Posted by
Thor-Arne
on 19 May, 2015 17:30
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I'm still not convinced that this is significant, at least in most applications.
Perhaps in some high gain amplifiers, but I suspect that the rest of the circuit (pcb traces, resistors, inductors) will have a larger effect than just having the capacitors "installed backwards".
I have no idea about the effect in tube circuits.
For the older paper-type capacitors in HV applications I can understand it, but that is a insulation issue rather than a noise pickup issue.
Still, if this was significant I'm sure the capacitors would be marked.
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#17 Reply
Posted by
Len
on 19 May, 2015 18:14
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Where did I say he's talking about power supplies?
You said that's the only place that such capacitors are used "in real life", which is clearly not the case.
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#18 Reply
Posted by
SeanB
on 19 May, 2015 18:23
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Kind of important in valve circuits, as you have a very close source of hum, the cathode heater. The extra shielding was very useful to reduce this, along with having the heater wires wound as a tight twisted pair right to the base pins.
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#19 Reply
Posted by
Thor-Arne
on 19 May, 2015 18:36
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As I said, I have no idea about tube (valve) circuits, decided to not mess with it many years ago because of the HV involved.
However, I vaguely remember to have seen caps with a extra metal can around as shielding.
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#20 Reply
Posted by
Kjelt
on 19 May, 2015 20:22
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As I suspected there is also a capacitance limit to which you can test this.
With a 10n MKP it is visible, but with a 2u2 MKP nothing.
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#21 Reply
Posted by
vk6zgo
on 05 Jun, 2015 16:10
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Clever EEs back in the day would orient capacitors so as to use the grounded outer foil as a shield between
input & outputs of high gain valve (vacuum tube) circuits.
If you wired the caps the other way round,the shielding was not as effective,& the stage might oscillate.
This was mainly done with early MW TRF radios,& was not common with later Superhet designs,except maybe in those with HF RF stages.
Replacing those older caps with modern much smaller caps might give you problems with instability,even if you did mount them with the outer foil connected to ground.
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#22 Reply
Posted by
Pentium100
on 06 Jun, 2015 20:05
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I had to replace leaky paper caps in a tape recorder. The capacitors had a metal case that was not connected to either pin of the capacitor, but was used as a shield (with a wire wrapped around it and connected to ground. Naturally, the new caps have a plastic case and the circuit started to oscillate. I had to wrap the new caps in aluminium foil and connect that to ground.
Also, good to know about the caps, now I will make sure to install them properly when I am building a tube amp.
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It is specifically mentioned that his interest is in repairing real old school kit where the capacitor orientation was important.
My thoughts are why not just use a double pole double throw toggle switch in a box oriented so that the toggle points to the relevant alligator lead?
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#24 Reply
Posted by
mikerj
on 07 Jun, 2015 13:52
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It is specifically mentioned that his interest is in repairing real old school kit where the capacitor orientation was important.
My thoughts are why not just use a double pole double throw toggle switch in a box oriented so that the toggle points to the relevant alligator lead?
I was wondering this as well, a battery driven circuit seems like overkill.