1. Home
  2. Electronics Design
  3. Cap value for timing circuit
  4. Page 2

Cap value for timing circuit

John Fields wrote in news: snipped-for-privacy@4ax.com:

So what IS the story, exactly? Right now you've focussed on the results of a load (the resistor), so you've said things that might be taken to discredit the rest of us who specified that either peak has an absolute value of voltage across a capacitor that when unloaded, must be considered for its safety. While a half-wave rectified form is a more complex wave whose RMS value needs a different calculation, in this case it's the separation between peaks that matters.

Many PSU capacitor failures seems to be a result of people underspecifying the working voltage while assuming they'll be 'safely' loaded to prevent the peak from dictating terms more than the RMS. It's not a safe assumption.

Reply to
Lostgallifreyan
Loading thread data ...

--
Sorry for the confusion, and you\'re right, an unloaded cap will charge
to 1.414 times the RMS value of the unrectified sine wave, no matter
whether the output of the rectifier is full-wave or half-wave.

Chances are that a loaded one will also, since most power supplies are
designed to have the cap charge fully and then discharge, between peaks,
to yield an acceptable ripple.
Reply to
John Fields

No worries, mate! I'm learned(er).

Wow. That's probably why the original 150v cap is no good after 40 years. It should have been spec'd higher. Is 160 good enough for this application?

Thanks.

Reply to
DaveC

DaveC wrote in news: snipped-for-privacy@news.eternal-september.org:

Doubtful, note the post from Hal Murray, about how much line voltage can vary. (Even at exactly 110V which it likely never is for long, you're pushing to to within 5V of a dangerous limit and trusting capacitor voltage tolerance is a Very Bad Idea, unless you're going to stress test samples in multiple copies of a design).

You might get by for a while if the cap is run as cool as possible, but bear in mind that the last one failed, and you now have a very good reason to suspect it failed from being run too close to maximum levels. So leave a generous margin this time.

Reply to
Lostgallifreyan 

--
Dunno... 

Can you post a schematic?
Reply to
John Fields

Old age isn't a mechanism, but is a cause for caps to go bad old age = drying, drying = dieing Frank

loss

than

voltage

be

around

Reply to
Frank S

"Frank S" wrote in news:hbgaft$14h4$ snipped-for-privacy@adenine.netfront.net:

I agree. Was just saying that length of time is like the length of a piece of string. Like someone else here said, some electrolytic caps work fine in gear tens of years old. Valve/tube gear even, where you have enough heat to accerate drying. It sort of suggests that looking at lengths of time is missing plenty, there's a lot of difference between electrolytics, they're not at all consistent.

Reply to
Lostgallifreyan

And there's how hard the user pushes them as regards the voltage and temperature ratings.

Reply to
whisky-dave

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.