Wall wart switching power supply for permanent mounting inside case? - Page 1
Forum section: electronics / projects

author: trashpanda
author_rank: Newbie
timestamp: April 10, 2018, 09:37:01 pm
content: I'm working on a small project to upgrade a lab incubator with a digital thermostat and circulation fan. This is a really small incubator, maybe 36 inches on each side, so it only needs a tiny fan. The digital thermostat I'm using takes mains voltage but all the small fans I can find are 5v-12v.The incubator has a pretty simple design. It has an upper and lower chamber, with everything constructed out of sheet aluminum. The lower space is about 2 inches high, and contains all the electronics, with sheet of aluminum above it forming the base of the main chamber, and then the main chamber above. The heating element is bonded to the bottom side of that dividing aluminum sheet. Everything is at mains voltage. The heating element can get quite hot and lower chamber gets warm because of it, but the overall temperature in the incubator shouldn't exceed about 120ºF (~50ºC), so the lower chamber temperature probably shouldn't exceed that by very much if at all.So, to power a 60cm fan or similar, I was thinking of mounting a 5V switching power supply in that lower chamber. I have a spare wall wart supply as pictured, which I opened up to find a simple single board, taking 110V AC in and putting 5V DC out. If I can find a way to mount it securely inside the case in the lower chamber with the other wiring, would it be safe to mount this as a 'permanent' 5V supply?

author: Ian.M
author_rank: Super Contributor
timestamp: April 10, 2018, 11:06:43 pm
content: Nope. For a hobby project you might get away with it, but its totally unsuitable for a work environment - you've removed the casing that any safety approvals for that PSU relied on, and have absolutely no idea of its maximum ambient temperature rating. Your organisation's health and safety officer will have kittens if they hear about it, and if it is implicated in a lab fire investigation or if anyone receives an electric shock from it, your insurers may deny coverage.A linear supply would be a much better bet for operation at elevated temperatures, while keeping the cost down. A 9V transformer, feeding a bridge rectifier and 1000uF 25V 105 deg C filter cap would get you in the right ballpark to feed a 12V fan. Otherwise, get an industrial 12V SMPSU, either enclosed or open frame, that's actually rated for operation at elevated temperatures.If you need to reduce the fan speed, use a 4 wire PC fan and PWM it with a CMOS 555 timer. See my comments about 555 circuits inhttps://www.eevblog.com/forum/beginners/externally-powered-pwm-for-some-spare-fans-as-a-fume-extrator/

author: trashpanda
author_rank: Newbie
timestamp: April 11, 2018, 02:07:33 am
content: Thanks for the replies. This is for home,not workplace use (I use it for fermenting foods) but obviously I would like to not burn my house down.I’ll look into those options.Sent from my iPhone using Tapatalk

author: CopperCone
author_rank: Super Contributor
timestamp: April 11, 2018, 02:21:33 am
content: lol, if the bottom has a bit of insulation and a fan/airflow, your actually improving the thing because its not enclosed in a tight chamber made of plastic.if anything it should run cooler and last longer. It has no shielding anyway so its not like your loosing a shield.I recommend mounting it with 4 points of contact with screws. You might also wanna add more strain relief if there is gonna be wires coming out of it. Drill small ones on the corners and get standoffs that bolt it to a piece of plastic which holds it off the ground.Be sure to ground the chassis right with properly tensioned screws and star washers.Add a fuse and a piece of Styrofoam to isolate the upper and lower chambers.If you wanna improve it, buy some capacitors from digikey from cornell dublier, panasonic, etc, rated to high temperature for a long time and replace the ones there.when you solder wires to it, make sure you strain relief them, and trim the solder joints so you have good clearance from everything.adding a thermal override to your chamber in the form of a thermostat set for like 65c would be helpful too. you can have it just break the circuit (this will cause thermal oscillaiton and eventual failure) or connect it to a contactor circuit to shut it down if the thermostat is triggered.

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2018, 02:53:40 am
content: I wouldn't use a 5V PSU unless I had a really nice 5V fan in stock. 12V fans are much more readily available and far easier to control the speed of - if you get the 4 wire PC variety, as your speed control circuit only has to provide a PWM logic signal, not chop the full fan motor current. However, assuming you have a 5V fan handy and intend to use that PSU board, before you do so, lets see the underside of the PCB so we can check clearances etc.As you may have gathered, I'm not a great fan of using wallwarts inside equipment,unless you can fit a chassis mount mains socket on a bracket to plug them into, and secure them to the bracket with a couple of cable ties so they cant work their way out of the socket, and also keep their ambient temperature under 40ºC, so they aren't stressed more than they'd be on a hot day with no air conditioning.BODGE AT YOUR OWN RISK!However, if I was bodging it for home use, I'd try to keep the base of the wallwart case, removing the prongs and drilling it for some Nylon bolts to fasten it down. Then, if there's no good place to drill holes to secure the PCB, I'd secure the PCB in the base with a bit of neutral cure (electrical grade) silicone caulk at the edges, taking care not to bridge tracks on the primary side or from primary to secondary. I'd also put a cable tie right round the base and board, over the transformer with a scrap of closed cell foam to pad it where its pushing on the winding, so its still firmly held if the caulk lets go. The mains wiring would be fastened into the base with small cable ties (and extra holes drilled for them in the base) and lead out via two holes drilled the opposite end to the notch for the 5V output cable. The output cable would similarly be cable tied to the base.If you can manage to thermally insulate the heating element from the base electronics chamber, (preferably without using polystyrene in contact with any electrical part or wiring - it interacts badly with the plasticisers in PVC wire insulation, especially at elevated temperatures) consider adding a second fan to draw air through the base to keep the electronics cool.