Are your Capacitors Installed Backwards - Page 1
Forum section: electronics / projects

author: Bryan
author_rank: Frequent Contributor
timestamp: May 18, 2015, 08:12:02 pm
content: Interesting video.https://youtu.be/BnR_DLd1PDI

author: Thor-Arne
author_rank: Unknown
timestamp: May 19, 2015, 10:51:25 am
content: 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?

author: smjcuk
author_rank: Frequent Contributor
timestamp: May 19, 2015, 11:05:58 am
content: That's why I like electrolytics. They're honest. At least if you install them backwards they complain loudly!

author: akis
author_rank: Frequent Contributor
timestamp: May 19, 2015, 11:52:01 am
content: In addition I read recently that MLCCs lose their capacitance as the voltage increases sometimes to a fraction of nominal.

author: dom0
author_rank: Super Contributor
timestamp: May 19, 2015, 12:05:10 pm
content: Quote from: akis on May 19, 2015, 11:52:01 amThis is (afaik) a property of all ceramic capacitors, but mainly affects those with high dielectric constant dielectrics.

author: Fungus
author_rank: Super Contributor
timestamp: May 19, 2015, 12:08:12 pm
content: Quote from: Thor-Arne on May 19, 2015, 10:51:25 amI 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 looksscary(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?

author: paulie
author_rank: Frequent Contributor
timestamp: May 19, 2015, 12:26:54 pm
content: 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.

author: dom0
author_rank: Super Contributor
timestamp: May 19, 2015, 12:31:52 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: May 19, 2015, 01:26:10 pm
content: 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.

author: Fungus
author_rank: Super Contributor
timestamp: May 19, 2015, 01:37:53 pm
content: Quote from: dom0 on May 19, 2015, 12:31:52 pmYes.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).Quote from: dom0 on May 19, 2015, 12:31:52 pmYep. 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.

author: Cliff Matthews
author_rank: Unknown
timestamp: May 19, 2015, 01:52:20 pm
content: Thanks! I enjoyed your video. I have lots of those orange-dipped caps.

author: Kjelt
author_rank: Super Contributor
timestamp: May 19, 2015, 02:11:51 pm
content: 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?

author: Len
author_rank: Frequent Contributor
timestamp: May 19, 2015, 02:59:01 pm
content: Quote from: Fungus on May 19, 2015, 01:37:53 pmPaul'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 watchanyof the video?

author: Muxr
author_rank: Super Contributor
timestamp: May 19, 2015, 03:14:27 pm
content: 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.

author: German_EE
author_rank: Super Contributor
timestamp: May 19, 2015, 04:09:32 pm
content: 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.

author: Fungus
author_rank: Super Contributor
timestamp: May 19, 2015, 05:13:16 pm
content: Quote from: Len on May 19, 2015, 02:59:01 pmWhere did I say he's talking about power supplies?

author: Thor-Arne
author_rank: Unknown
timestamp: May 19, 2015, 05:30:49 pm
content: 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.

author: Len
author_rank: Frequent Contributor
timestamp: May 19, 2015, 06:14:16 pm
content: Quote from: Fungus on May 19, 2015, 05:13:16 pmYou said that's the only place that such capacitors are used "in real life", which is clearly not the case.

author: SeanB
author_rank: Super Contributor
timestamp: May 19, 2015, 06:23:07 pm
content: 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.

author: Thor-Arne
author_rank: Unknown
timestamp: May 19, 2015, 06:36:05 pm
content: 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.

author: Kjelt
author_rank: Super Contributor
timestamp: May 19, 2015, 08:22:28 pm
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 05, 2015, 04:10:23 pm
content: Clever EEs back in the day would orient capacitors so as to use the grounded outer foil as a shield betweeninput & 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.

author: Pentium100
author_rank: Frequent Contributor
timestamp: June 06, 2015, 08:05:34 pm
content: 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.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: June 07, 2015, 01:45:24 pm
content: 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?

author: mikerj
author_rank: Super Contributor
timestamp: June 07, 2015, 01:52:18 pm
content: Quote from: Dave Turner on June 07, 2015, 01:45:24 pmI was wondering this as well, a battery driven circuit seems like overkill.

author: N2IXK
author_rank: Frequent Contributor
timestamp: June 07, 2015, 02:12:23 pm
content: Quote from: TimFox on May 19, 2015, 01:26:10 pmThe issue isn't safety or insulation, it is shielding.When used in high impedance circuitry (think tube amplifiers), you want the outer foil to be connected to the node with lower impedance to ground. In this way, the outer foil can act as a shield around the inner foil, which is connected to the higher impedance point. For a typical coupling capacitor between 2 tubes, the outer foil goes to the plate of the first stage, with the inner foil connecting to the grid of the next stage.Quite surprising to see that those "orange drop" caps aren't always properly marked. Why bother putting the band on at all if it doesn't indicate anything?

author: david77
author_rank: Frequent Contributor
timestamp: June 08, 2015, 12:46:16 am
content: His videos are usually rather good, this one is too.But his solution seems very complicated for something that could be easily achieved using a DPDT switch.I guess it shows how to solve a particular problem, not KISS though.