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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th May 2020, 7:21 pm   #1
BrianAllen
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Default Capacitor Polarity

Good evening all, which is positive/negative on the attached caps.

Why would they not make things easier and just put + and - on the caps?
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Old 29th May 2020, 7:23 pm   #2
Dave Moll
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Default Re: Capacitor Polarity

+ve on the left, -ve on the right in the upper one. N.B. apart from any other clues, the minus signs in the circles have an arrow pointing to the right.

On the same basis, the -ve in the lower one is toward the viewer.
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Old 29th May 2020, 7:25 pm   #3
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Default Re: Capacitor Polarity

Thanks Dave; just checking with you all.

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Old 29th May 2020, 7:35 pm   #4
Graham G3ZVT
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Default Re: Capacitor Polarity

Does someone know what prompted the change from highlighting the +ve end to the -ve end?

The current scheme always seems a bit contra-intuitive to me
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Old 29th May 2020, 8:31 pm   #5
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Default Re: Capacitor Polarity

I agree Graham. Back in the day the +ve often had a red spot on the tag.
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Old 29th May 2020, 8:47 pm   #6
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Default Re: Capacitor Polarity

Isn't the body/case always negative on an electrolytic?
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Old 29th May 2020, 8:57 pm   #7
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Default Re: Capacitor Polarity

Case doesn't always have to be negative on aluminium electrolytics, also some get formed the wrong way round. They are symmetrical before forming.

Convention is to mark the -ve pin on aluminium parts, but the +ve pin on tantalums. God only knows why.

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Old 29th May 2020, 9:13 pm   #8
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Default Re: Capacitor Polarity

Sorry I meant on an axial leaded one
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Old 29th May 2020, 10:14 pm   #9
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Default Re: Capacitor Polarity

I heard it was to do with component insertion machines and they optically recognise the stripe and most PCB's are marked with a shaded area for the negative terminal.
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Old 30th May 2020, 12:14 am   #10
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Default Re: Capacitor Polarity

Those Illinois Capacitors are not very good at high voltage !! Yes thats off topic but I change them regularly. They blow the end seals out, AND with very much lower voltage than written on the can.

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Old 30th May 2020, 7:31 am   #11
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Default Re: Capacitor Polarity

I saw this problem with a brand new-old-stock Tektronix FET probe I picked up for not very much. I opened the power supply to change the mains transformer tapping, and the main (axial) smoothing cap seal had failed.

Turned out that over a period of time Tek had that problem as a result of the de-fluxing agent they used on the boards. It compromised the rubber seal on the + end of board-mounted electrolytics.

Perhaps that was what saw off those Illinois caps, Joe.

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Old 30th May 2020, 11:30 am   #12
BrianAllen
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Default Re: Capacitor Polarity

Thanks Joe, will keep an eye on that one; luckily I only needed to purchase one of them.

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Old 30th May 2020, 2:29 pm   #13
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Default Re: Capacitor Polarity

Quote:
Originally Posted by rambo1152 View Post
Does someone know what prompted the change from highlighting the +ve end to the -ve end?

The current scheme always seems a bit contra-intuitive to me
For cheapness AFAIK. Marking the connections directly adds another step to the manufacturing process.
Printing the details on the heat shrink sleeve adds no cost, it must be printed and applied in any case.
This pre-dated the general use of machine vision and automatic assembly.
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Old 31st May 2020, 7:38 pm   #14
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Default Re: Capacitor Polarity

I find that a good double check is that the positive lead is usually the longer one on radial caps
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Old 31st May 2020, 8:04 pm   #15
duncanlowe
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Default Re: Capacitor Polarity

Apparently some years ago RS sent out a batch of capacitors, where the sleeve had been put on the wrong way round. Allegedly amongst other things there were a few gnnnnnn bang incidents at the army school of electronics. As told to me by my first boss who had been an instructor there. In case anyone doesn't recognise it gnnnnnn is the sound of amps (as described by him). I recognise that sound.
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Old 31st May 2020, 8:29 pm   #16
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Default Re: Capacitor Polarity

There was a thread here a while back where someone queried the polarity indication on an RS-supplied Sprague HV electrolytic, and it turned out to be from another reversed-sleeve batch.
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