EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: fabiodl on August 21, 2022, 10:45:40 am
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Is this a ceramic cap or a varistor? I have only the pic so I cannot measure.
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I guess it's a 390pF ceramic cap.
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Trace separations and widths look inadequate for mains power, so the odds of it being a varistor are pretty low before you even consider the markings.
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Yep, it's on a 5V rail so I was 90% confident it's a cap, but even that it makes little sense.It's on the output of a LS04 on a totally logic circuit. I guess they wanted to slow down the edge.
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390 pF make a lot more sense than a 390 V varistor. Sometimes LS logic was also used for some more analog functions. Some delay of noise filtering is possible, but also just an oscillator.
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Yep, it's on a 5V rail so I was 90% confident it's a cap, but even that it makes little sense.It's on the output of a LS04 on a totally logic circuit. I guess they wanted to slow down the edge.
It looks like that sort of bodgery. I'd bet there's a nasty asynch race condition somewhere upstream of it. Typical 'design' process is late one Friday evening, with the deadline vanishing in the rear-view mirror:
Tech: "Eureka! It actually works now if I probe it here with a x1 scope probe!" ???
Engineer: "Oh S--t!, we've got a glitch. Its probably a race condition, but I'll need a couple of days to track it down and fix it. May need a board re-spin." |O
Boss: "Hell NO! The customer is already screaming, so its got to be done tonight. Redesign denied." :horse:
Engineer: Try bodging in a 39pF cap. If it fixes it, we'll go up an order of magnitude so it *STAYS* fixed." :popcorn:
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That's been used as an alternative to shipping with a scope probe attached many times! :D