Poundland 45

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pre65
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#151 Re: Poundland 45

Post by pre65 »

Cressy Snr wrote: Wed Dec 22, 2021 5:00 pm
pre65 wrote: Wed Dec 22, 2021 4:55 pm Last time I earthed a capacitor mounting stud I did not realise it (the stud) was connected to one of the terminals. :oops:

I assume that is not the case with your one.
Dunno Phil. Why would that be a problem?
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#152 Re: Poundland 45

Post by Cressy Snr »

The only thing I can think of where the case is connected to one of the terminals is in the case of an electrolytic cap, which usually does this as a matter of course. Earthing the stud on one of those giant electrolytic caps I got via NickDS might conceivably cause an almighty hum if an earth loop was developed by this action.
On the other hand, connecting one of the terminals to the metal case on a non polarised MKP/film/oil filled type motor run cap would be disastrous if someone got it ‘wrong.’ The High Voltage would be shorted out when the device was switched on. The bang would be pretty loud, so I would assume that a metal cased, non electrolytic would have no connection to its case from any of its terminals.
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#153 Re: Poundland 45

Post by Mike H »

pre65 wrote: Wed Dec 22, 2021 5:11 pm Can't remember now. The door on that portion of my memory has seized shut.
I think I remember something about this, you soldered something to the terminal not realising that it was also connected to the stud which was earthed? Magic smoke was released? :D
 
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#154 Re: Poundland 45

Post by Mike H »

Or, was the stud used to support something else? A bracket or a heatsink? But still not right ...
 
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#155 Re: Poundland 45

Post by Nick »

The problem would be if the negative terminal of the electrolytic is not at the same potential as your case. Say you are stacking two of them for extra voltage handling, or you have a split rail supply where the -ve of one cap is at - some volts (that's when I found the feature. Also as mentioned if you want a small resistor between case and signal ground. Or you have a CMC choke after the cap and the other side is grounded so you will short out half the choke.
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#156 Re: Poundland 45

Post by pre65 »

Nick wrote: Wed Dec 22, 2021 7:16 pm The problem would be if the negative terminal of the electrolytic is not at the same potential as your case. Say you are stacking two of them for extra voltage handling,
That sounds familiar. :)
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#157 Re: Poundland 45

Post by Thermionic Idler »

I think that's the same motor run type as I used in my 300B - if so, then it's only the case that's connected to the stud - I have mine arranged all along the back, bolted to the chassis rear panel:

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#158 Re: Poundland 45

Post by Nick »

What you need to consider is that a motor run cap uses a film or paper based dialectic with the plates isolated from the case. But a electrolytic uses a liquid electrolytic, and so the conductive electrolyte is contained in the can so there is a connection between one of the cap plates and the can.
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#159 Re: Poundland 45

Post by Mike H »

pre65 wrote: Wed Dec 22, 2021 7:22 pm
Nick wrote: Wed Dec 22, 2021 7:16 pm The problem would be if the negative terminal of the electrolytic is not at the same potential as your case. Say you are stacking two of them for extra voltage handling,
That sounds familiar. :)
Yes I think that was it! :thumbright:
 
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#160 Re: Poundland 45

Post by Paul Barker »

Slightly unrelated but pio and polyprops have the can on one Leed, which you ought to put to the lowest impedance end, so for coupling, that’s the source, which ought to be lowest impedance. This can then shields the cap from rf such as interstate transformer power transformer choke and general rf around which creates hum. In the past the polarity was ground or can, to a line, but in modern factory purposes caps aren’t hand placed in the production line so if there is a mark, it is likely to be wrong. You have to connect the scope up, the can end is the end with the lowest amplitude RF. if you can’t find any RF put you’re fingers on the cap, find that on the scope, test it both ways, lowest amplitude is correct orientation. I.e. where the ground Leed of the scope ends up after the discovery from the test, is the source or ground. Now a line that you know is true with a sharpie would save you testing it again.
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#161 Re: Poundland 45

Post by Nick »

I found wrapping a cap in tape and then wrapping copper film around it and soldering a lead to the film is the best way to shield a cap from RF if you want to do more. That way you can connect the shield directly to ground, I am not convinced if the anode of a driving valve or the grid of the following valve will have anywhere enough of a leakage path to ground at RF to make a difference.

After all at RF, the cap (if a coupling cap size) will make both ends look like a short circuit between them so there will not be that much if any difference between the two leads.
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