[ARC5] Testing Caps

[Leslie Smith]

  Hello Wayne/others,

  First, it's clear from your observation and expectation about the
  output from your transformer-rectifier-capacitor combination that I
  can't add to the conversation.  I can say I have some computer-grade
  caps sitting on my work-bench that held charge - at 25 volts - for
  months.  I would estimate 2 to 3 months.  I didn't expect that
  result.    I always assumed some leakage occurred in electrolytic
  caps, but it was clearly less than micro-amps in this case.   I have
  seen polycarbonate capacitors connected to a hi-Z opamp hold charge
  for weeks.
I would be interested to know if the "junker" 8200uF capacitor or the
rectifier diodes caused the problem. (i.e. voltage drop under load.)
My guess:  The 8200uF capacitor value was well down from the spec.  With
a 100 ohm load (300mA) the ripple should be R equ 5 * I / C or not much.
In USA the constant "5" will be a little less, because the mains
operates at 60HZ, not 50Hz.


   Best 73s  de Les Smith
   vk2bcu at operamail.com



On Wed, Jul 25, 2018, at 00:05, Robert  Eleazer wrote:
> I decided to try out a transformer pulled out of a junked AM/FM stereo
> to see if it would make a good 28VDC power supply.  It has a nice big
> 64VCT secondary winding.  I hooked up a couple of 3 amp 1000 PIV
> diodes and a 8200 mfd cap out of another junked piece of gear.>  
> I was astonished to see I got only 29 VDC under no load.  Putting a
> 100 ohm power resistor as the load dropped it to 28.3 VDC.>  
> Today I used two new diodes and an unused 6400 mfd with the
> transformer and got the expected 44VDC.>  
> Now the strange part is that the junker 8200 mfd cap I charged up to
> 50 VDC and found it would still have 43 VDC on it days later.  But
> with the new 6400 mfd cap when disconnected from the circuit  the
> voltage drops right off immediately.>  
> I have assumed that the length of time a cap would hold a charge is an
> indicator of its usefulness - but maybe not?> 
> 
> Wayne
> WB5WSV     
> 
>  
> Virus-free. www.avg.com[1]
> 
> ________________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>  
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html


Links:

  1. http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180725/4f0b03cc/attachment.html>