home improvement turns into science fair projects?

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terkio
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Re: home improvement turns into science fair projects?

Post by terkio » Sat Jun 20, 2020 7:45 am

The reactance of a 22uF at 60 Hz is 120 Ohm.
So when plugged on 120V mains the current through the cap is 1A.
I would not dare to do that without protection heavy goggles and gloves despite it should not blow up.
Theory tells, it undergoes a 1A rms current, charges and discharges 60 times per second at 120V dc and - 120V dc.
It doesn't heat because voltage and current are in quadrature ( not exactly because of delta ).
During a 60 Hz cycle, it receives energy, then gives it back, so the net received energy is zero.
Of course, this is well known but boggles my mind thinking about such a large ripple current that would quickly ruin power supply caps.

Plugging a 22uF 400V AC cap on a 240V mains socket looks scary to me. Instinctively I do not trust theory, I do not feel, it is safe to do that.

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JR.
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Re: home improvement turns into science fair projects?

Post by JR. » Sat Jun 20, 2020 8:34 am

terkio wrote:
Sat Jun 20, 2020 7:45 am
The reactance of a 22uF at 60 Hz is 120 Ohm.
Don't forget that the cap is also in series with the blower.
So when plugged on 120V mains the current through the cap is 1A.
I would not dare to do that without protection heavy goggles and gloves despite it should not blow up.
Theory tells, it undergoes a 1A rms current, charges and discharges 60 times per second at 120V dc and - 120V dc.
It doesn't heat because voltage and current are in quadrature ( not exactly because of delta ).
During a 60 Hz cycle, it receives energy, then gives it back, so the net received energy is zero.
Of course, this is well known but boggles my mind thinking about such a large ripple current that would quickly ruin power supply caps.
The cap does not charge and discharge every cycle, but passes the current through without much (any?) terminal voltage across it.
Plugging a 22uF 400V AC cap on a 240V mains socket looks scary to me. Instinctively I do not trust theory, I do not feel, it is safe to do that.
So don't do that.... I recall over 50 years ago as a junior technician at MIT Instrumentation Lab, some other technicians there dared me to plug a small electrolytic cap into an outlet... Of course it blew up sounding like a rifle shot. :oops: Something you only do once. Coincidentally on that same job, working on an DC to DC switching PS for a navy project we blew up a big film capacitor that was damping switching spikes across a transformer. I was away from bench when that guy blew up and my bench was covered with a blanket of tiny pieces of film (looked a little like snow). :lol:

JR

PS: FWIW I've been using these film caps to scrub off blower speed for probably a year or more already.
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mediatechnology
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Re: home improvement turns into science fair projects?

Post by mediatechnology » Sun Jun 21, 2020 5:54 am

I did the same thing for one of my PCs case fans - ran the 12V fan on 5V.
https://ka-electronics.com

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