Cap or Resistor? - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: February 18, 2014, 12:44:26 pm
content: I know virtually nothing about SMD components. Now, I had one such component, which I think is a Cap, die and release the magic smoke!!!So, my question is, is this a Cap or a Resistor? And if so, how do I find the value of said cap or resistor?The one on the left is the bad, the one on the right the good.By looking at the image and according to the pcb trace, it looks like the cap/resistor is across the +/- terminals of the through hole component (which is the VOUT +/-). I could be wrong. If this is the case, what kind of cap value might work? The VOUT +/- can be as high as 46V (and around 3A). I use it at 1A @ 36V.This is a picture of the whole unit (two units). The unit with the broken cap still works, although I don't know if this will last. I don't know how vital the cap/resistor is. This is a DC to DC 10-32V to 10-46V 150W Boost constant current power supply Regulator.

author: made2hack
author_rank: Frequent Contributor
timestamp: February 18, 2014, 12:45:32 pm
content: Should I still use the regulator with the busted cap? Or might this somewhere down the line Fail in a glorious display of magic smoke?And can I replace it with some other cap? Even though it might be through hole?

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 18, 2014, 12:50:02 pm
content: it is a cap.you shouldn't use it with the busted cap.desolder it.to find the value, desolder the good one and measure with an LCR meter (although a good multimeter would probably do)the value probably doesn't matter though.

author: klr5205
author_rank: Regular Contributor
timestamp: February 18, 2014, 12:57:55 pm
content: Judging from the sheer thickness of the traces its connected too I wouldtendto think that is just a decoupling cap, in which case the value isn't very critical as peter said. Running without the cap will probably hurt performance, in the form of worse output ripple or noise. A 1uF cap might be an appropriate replacement.You could always desolder the good cap on the other board and measure it to get a good idea of the value.

author: made2hack
author_rank: Frequent Contributor
timestamp: February 18, 2014, 01:56:25 pm
content: Does the voltage matter? Should it be a 50V ceramic 1uF SMD? Can I get a through hole and connect to both leads of the VOUT?Thanks

author: klr5205
author_rank: Regular Contributor
timestamp: February 18, 2014, 03:48:03 pm
content: 50V is cutting it very close for an output that can be 46V... although if that is a 0805 you might not find anything with a higher voltage rating in that package size.In that case you certainly could solder a through hole cap with a 100V rating or so to Vout as long as it doesn't affect how the board is mounted (if it even is mounted). Remember that the voltage rating of the cap is just the voltage it can withstand before failing. Its usually good to choose a cap rated at least 30-50% higher than the max voltage you expect it to see. Higher is better as long as you're not wasting money on a part that's way overrated for your application.

author: tszaboo
author_rank: Super Contributor
timestamp: February 18, 2014, 04:37:39 pm
content: Quote from: klr5205 on February 18, 2014, 03:48:03 pmThere are 0,47 and 1uF capacitors with 100V rating and 0805 package. Example: TDK C2012X7S2A105K125AB . This one is X7S, not X7R but it should not be a problem. I would go with this one, unless the one on the working board is much smaller.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 18, 2014, 05:53:01 pm
content: I'd go for 100V rated for sure, running the cap with that much DC bias, and that close to its rating is bound to cause it to fry. Look at your unit as an example. Also, whilst running the unit without the cap may reduce it's performance, running it with a shorted or low resistance accross the output isn't terribly good either. From the looks of it, you could probably get 4x 1uf 100v, scrape off a bit of the solder mask and stack them at the back. If the one cap cooked itself, it not only probably wasn't up to the voltage or DC bias, but the ripple current would kill it too, a few in parallel would help this a lot.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 18, 2014, 10:53:44 pm
content: It will be easier to replace the SMD cap than try to bodge a through hole cap on there.SMD really isn't has hard as people think it is, certainly not simple parts like this. Try it.