Beginner Question - Are these two caps interchangeable? - Page 1
Forum section: electronics / projects

author: cdonate
author_rank: Contributor
timestamp: November 01, 2015, 10:46:30 pm
content: Hello, I'm starting with PCB development and I need some help.I'm trying to make a design a little bit more compact. This is the power supply of the system. It takes 220V AC and converts to 3.3V DC with a buck converter.At some point on the circuit I have this CAPhttp://br.mouser.com/ProductDetail/United-Chemi-Con-UCC/ELXZ500ELL101MH12D/?qs=%2fha2pyFaduja%2fRJ16DmbUEcBWCoqtKswETiCKebw9aAqfs2UgrMw9A%3d%3d(ELXZ500ELL101MH12D) and I would like to change tohttp://br.mouser.com/ProductDetail/United-Chemi-Con-UCC/EMVE500ADA101MHA0G/?qs=9l4X0aOfMddzvH8UrKmeSw%3d%3d(EMVE500ADA101MHA0G).My question is, can I do that or this is a very noob thing to do? If I can't, is there another option?The schematic is attached. At this point I would like to change the C3 Cap.Any help is much appreciated.Cheers!

author: DanielS
author_rank: Frequent Contributor
timestamp: November 01, 2015, 11:04:16 pm
content: Powering a low voltage circuit from a buck regulator powered by direct line voltage sounds like a recipe for disaster. Unless you are 300% certain there is absolutely no chance anyone may ever be in contact with the output from this power adapter, you really should be considering some form of isolated topology. Flybacks are quite common for powers under 100W.

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2015, 11:11:34 pm
content: You need to understand and define each components failure mechanisms - and how those will damage the equipment and an someone touching the equipment. You also need to follow all the design rules necessary to ensure it will be legal to sell the equipment. Making something too small will be both illegal and lethal. But maybe you don't care.

author: cdonate
author_rank: Contributor
timestamp: November 01, 2015, 11:14:24 pm
content: DanielS, thank you for the reply.I'll research about Flyback power supply, it is good to know this is no the best solution.The system is going to be self contained, with no usb or any other connection. I'm using the schematic from the manufacturer, and it is only going to be a proof of concept.I really don't want to change too much, the idea now is to make it smaller. I'll think about improving the circuit later on.About the Cap, any input?

author: cdonate
author_rank: Contributor
timestamp: November 01, 2015, 11:20:16 pm
content: tggzzz, I get your concern. I'm now only looking to make a smaller version of the circuit, and I'm only thinking about changing the caps. They take most of the space.I don't want to change any specs, just a different package.It is going to be a proof of concept, far from any consumer application at this moment.With all that in mind, can I do it? Is is technically sound?

author: bson
author_rank: Unknown
timestamp: November 01, 2015, 11:26:03 pm
content: The cap should be fine, although the ripple current spec is pretty modest so it might run a little warm depending on load. If you use a standard footprint it'll be easy enough to swap out.On a different note, if POWER_SUPPLY is 220VAC where's the fuse? If it were me I'd add a 1-3A fast blow fuse right before the 36V zener specifically for this 5V supply.

author: Niklas
author_rank: Frequent Contributor
timestamp: November 01, 2015, 11:29:36 pm
content: How much current do you expect to deliver to the 3.3V output? The R-C-Zener type of supply is typically used for low cost, low current applications. The Buck regulator will draw current all the way up to the peak ripple current, i.e. higher than the average output current.

author: cdonate
author_rank: Contributor
timestamp: November 01, 2015, 11:39:41 pm
content: Yes, bson, I'll definitely do that. The fuse is set before the POWER_SUPPLY, I have yet to design this part.Thanks AcHmed99, I'll keep that in mind. Since this is just a proof of concept, lifetime and running a bit hotter is not really an issue. I thought it was ok to do it, I just wanted to be sure, shipping to Brazil takes a long time, I didn't want to have to re-do the board just because I didn't take 10 min to ask the question.Thanks everyone. All very helpful tips. I'll be back here before long.Cheers!

author: cdonate
author_rank: Contributor
timestamp: November 01, 2015, 11:46:58 pm
content: Niklas, the IC, a CS5490, will draw 3.9 mA, 12.9mW. The converter is rated for 500 mA. It is a bit overkill but more stuff will be added in the future.

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2015, 11:55:31 pm
content: Quote from: cdonate on November 01, 2015, 11:20:16 pmIf it is too small to be safe, what concept does that demonstrate?Can you do it? Wrong question - after all, you can cross the road without looking!

author: cdonate
author_rank: Contributor
timestamp: November 02, 2015, 12:00:52 am
content: Quote from: tggzzz on November 01, 2015, 11:55:31 pmWhy would smaller be equal to less safe? The specs will be the same, the system will be self contained, and will be inside a case. I'm not trying to change all parts, I just want to change the capacitors, they take 30% of the whole board. I get the concern, but I'm not going to do anything crazy.

author: tggzzz
author_rank: Super Contributor
timestamp: November 02, 2015, 01:09:45 am
content: Quote from: cdonate on November 02, 2015, 12:00:52 amI do not know you, nor your equipment, nor your application. Hence the key word in my point: "if".I do know that it is potentially much more dangerous for a conductor to be 1µm from 240V than 1m from 240V. Sometimes safety is ensured by milling a slot in a PCB; if you don't understand why, then you might unwittingly "do something crazy" by mistake.

author: Niklas
author_rank: Frequent Contributor
timestamp: November 02, 2015, 08:10:33 am
content: Quote from: cdonate on November 01, 2015, 11:46:58 pmThe Buck converter might be rated 500 mA, but what about the R-C-Zener? At 50 Hz, C1 will have a nominal impedance of about 9500 ohms. Will it be able to provide the current needed?