Choosing a power supply for harsh conditions - Page 1
Forum section: electronics / projects

author: miceuz
author_rank: Frequent Contributor
timestamp: April 11, 2014, 06:10:12 am
content: Hi all,I'm designing an LED installation for Burning Man festival this year and I got into argument with my friend who is doing mechanics. He wants to close a power supply into a hermetic box and says me to spec it up. I say it does not matter if you put a more powerfull supply - if it does not have air flow it will heat up and die anyway. Am I right?To be more specific - my current consumption is 4A max, I'm thinking of putting a 8A power supply. The supply will be closed into a hermetic wooden box. Will, say, 16A power supply die later in the same conditions?As I understand it, supply has to dissipate the power and it does not matter if supply can put out more amps it still needs to dissipate the same power. So I should choose a more efficient supply, not necessary more output-capable?

author: nowlan
author_rank: Frequent Contributor
timestamp: April 11, 2014, 06:25:16 am
content: My first reaction is to suggest a meanwell led driver. They are ip67 rated, sealed switching ps.http://www.meanwell.com/product/led/LED.html

author: miguelvp
author_rank: Super Contributor
timestamp: April 11, 2014, 06:29:40 am
content: Quote from: miceuz on April 11, 2014, 06:10:12 amAs long as it lasts for the full event and it doesn't burn before the effigy, you'll be alright!

author: ejeffrey
author_rank: Super Contributor
timestamp: April 11, 2014, 06:51:05 am
content: Yes. All you do is calculate the heat dissipated inside the box, and the thermal conductivity of the box walls. That plus the ambient temperature gives you the equilibrium temperature inside the box. If that is above the operating temperature of the power supply you are screwed.Your thinking is correct that speccing a beefier power supply will not really help. Big power supplies have big heat sinks to better dissipate heat to the environment, but if that environment is the inside of an insulated box it doesn't help. Where buying an oversize supply will help is if you have a high ambient temperature where you have less temperature delta to work with.So yeah, you need the most efficient supply you can get. You also want to really examine if there is any way you can get better thermal conductivity out of the box. Can you have a heat sink protrude through the wall of the box and then caulk it shut? Can you make some or all of the box out of metal? If you are really desperate and don't need to run it for long you could consider a big heat reservoir inside the box: e.g., a block of ice that will last for as long as required.

author: Fsck
author_rank: Super Contributor
timestamp: April 11, 2014, 06:59:15 am
content: ditto the above, but remember that power supply output power and lifetime derate w.r.t temperature.

author: miceuz
author_rank: Frequent Contributor
timestamp: April 11, 2014, 01:01:45 pm
content: Thanks to all! IP67 is the important keyword here, no 5V options on ebay though...

author: wraper
author_rank: Unknown
timestamp: April 11, 2014, 01:11:10 pm
content: I would say it also matters how effective is power supply. There are usually higher power losses if it is operated close to maximum. Therefore more heating. Also think about electronic parts safe operation area. It derates with temperature. Therefore more beefier capacitor/transistor/diode is safer to operate at given load and temperature than those with smaller specs.

author: nowlan
author_rank: Frequent Contributor
timestamp: April 11, 2014, 01:30:27 pm
content: I was under the impression power supplies were/had better efficiency the closer to their rating.

author: ruffy91
author_rank: Regular Contributor
timestamp: April 11, 2014, 01:57:45 pm
content: Yes, most power supplies have their max. efficiency at 100% of the specified load. They still generate more heat at 100% then at 50%.A 16A Power supply at 8A will most likely generate more heat than a 8A power supply at 8A (except it's a chinese 3A supply that's labeled as 8A)

author: wraper
author_rank: Unknown
timestamp: April 11, 2014, 03:10:10 pm
content: Quote from: ruffy91 on April 11, 2014, 01:57:45 pmNo, most power supplies usually have the best efficiency at about 40-70% load. The simple reason is because manufacturers tend to make cheap, without significant margin. They are often pushing them to the limits when efficiency start to significantly drop. Like this:

author: ejeffrey
author_rank: Super Contributor
timestamp: April 11, 2014, 04:33:23 pm
content: Quote from: miceuz on April 11, 2014, 01:01:45 pmIf that is all that matters, just use a die cast aluminum box with a watertight gasket. Obviously you will need to make sure that any input/outputs are sealed equally well, which is the hard part. You will still need to make sure that the heat is transfered to the enclosure. How you do that depends on the type of PSU and the amount of heat you need to dissipate.