Safe enclosure for A/C power? Also heat sinking? - Page 1
Forum section: electronics / projects

author: rea5245
author_rank: Frequent Contributor
timestamp: October 11, 2012, 12:52:56 am
content: The short version of the question is: is it safe to have A/C power (120 V) inside an aluminum enclosure? Should I have the ground wire go to the aluminum case? Or should I just make sure that none of the A/C wires touch the case (sounds failure-prone)?A secondary question: can a black aluminum case be used as a heat sink for a solid state relay?The long version is this: I'm making a sous vide controller. I'm using an LCD display that normally fits in a PC's disk drive bay (a Crystalfontz drive bay kit), so I bought an aluminum external drive enclosure to hold the project. But A/C power is going into the enclosure, so I'm worried about safety.The second part is, I started this project using a mechanical relay, but I was worried about its lifetime. (It's rated for a minimum "mechanical life" of 10^6, which is great, but a minimum "electrical life" of 10^5. I don't understand the difference, but 100,000 doesn't sound enough to me, given that a PID algorithm can turn the power off and on a lot and it will be running for hours at a time, and I hope it will work for years to come.)So I've switched to an SSD. Now I need to worry about heat. Putting a heat sink in this case (a Sabrent EC-ST5B) is going to be tight, if it's possible at all. Could I use the case itself as the heat sink? The aluminum is black. It's cool to the touch, but I don't know how heat-conductive the black coating is.If an aluminum case is unsafe, or if this case is too small, my plan B is to buy a bigger aluminum or plastic enclosure, use a real heat sink on the SSR, put an electrical junction box inside, ground that box and keep the high voltage stuff inside it. But it won't look as attractive.Any suggestions?Thank you,Bob

author: senso
author_rank: Frequent Contributor
timestamp: October 11, 2012, 01:35:16 am
content: You should use a TRIAC and a zero crossing detector, so you can apply "pwm" to your heater and have a much better controlo over the temperature.

author: rea5245
author_rank: Frequent Contributor
timestamp: October 11, 2012, 02:14:20 am
content: senso, isn't that essentially what the solid state relay is? I know it's a zero-crossing SSR.

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2012, 04:58:44 am
content: Ground the case, and attach the ssr to it as well, use heatsink compound between ssr and case. What ssr are you using ( current rating) and what power is the load?

author: Smokey
author_rank: Super Contributor
timestamp: October 11, 2012, 06:00:54 am
content: All jokes about the recent dangers of pc power supplies aside, whenever I need a metal case with AC input, I use a gutted ATX power supply case. It already has the AC input connector and usually a nice power switch. Plenty of room for a project. I have an atx supply grave yard for scavenging high voltage caps and cases. You can take a look at how they do grounding too which is almost always from the ground terminal on the connector to a standoff in the case.As for the ssr, check the data sheet as far as isolation, but most likely you will end up doing what seanb said and ground the case with thermal paste under the ssr depending on how much heat it makes.

author: rea5245
author_rank: Frequent Contributor
timestamp: October 11, 2012, 11:56:07 am
content: SeanB, the SSR is a Crydom D2425, rated 240 V at 25 A. My load is 120 V at ~12 A. So this is one aspect of the design I'm not worried about.Being able to use my current case as both the ground and the heat sink solves my problems. Thank you.Smokey, the idea of gutting a power supply for its case never occurred to me, and now I wish I hadn't thrown away most of my old power supplies a few weeks ago. But for this project, I'd still prefer to stick with the external drive enclosure. The wife will be using it in the kitchen and it has just the right opening for my LCD display.But I'll have to remember that for the future.- Bob

author: ptricks
author_rank: Frequent Contributor
timestamp: October 11, 2012, 12:04:43 pm
content: One essential part that always seems to get overlooked , the fuse. Make sure you have a fuse on the AC input hot wire. I like those screw in fuse holders that can be accessed from outside the case.http://www.bgmicro.com/FUS1008.aspx