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July 24, 2017

3D Printer Power Supplies

Having built a dozen 3D printers, you get used to looking at parts that are top notch, some marginal, and some that you wonder why they were ever produced.

But there’s also some parts that you just take for granted. Case in point, the typical 12V, 20 or 30A (200-360 watt) power supplies.

Normal cost is in the $25 – $40 range (USD or CDN). Paying a premium for one, double or triple the cost, does not promise any higher quality than the budget ones in my experience.

The Tin Can

The typical tin can power supply takes 110-220 and drops it down to 12V (or whatever working voltage you need) that has a trim pot for fine adjustment. The majority of them look much like the one in the photo.

12V 30A Supply

The variation of these are ones that have a mesh covering and have no fan. Relying on convection to keep the wee beastie cool. For most of these types, 20A is usually the limit of their current rating, but I’ve seen a few 30A ones.

12V 30A Mesh

There’s a soldered fuse inside because using proper fuse clips would add cost and make no mistake about it, these things are cheaply made.

Cooling

The mesh covered ones are self cooling. Or as I found, when you fry one, they remain permanently cool. As you’ll eventually see this may not be a bad thing either.

The tin can versions all sport a fan. Considering the PCB’s in the ones I’ve looked at from various suppliers are all oddly similar, the fan voltages range from 12 to 16V. Or perhaps whatever is in the parts bins for that days assembly.

Paying a premium for one doesn’t ensure better quality but occasionally you’ll find a temperature controlled relay so the fan doesn’t run all the time. I blogged about this previously

For the MOSFET’s that do all the current handling, cooling is either them clamped to the case itself, or with an aluminum bar between the MOSFET’s and the case. Usually with some silicon material for insulation and goop for heat transfer.

IMG 1155

It’s not pretty, but it’s at least marginally functional.

Quality

Like most end users, I just wire them up and away I go. However, since I wrote the blog about adding a relay for fan control, I’ve modified half a dozen power supplies and that means I have to take them apart to do it.

What I found in TOO many is damned scary. Seriously. If you have one of these power supplies, my advice is disconnect it, take it apart and INSPECT THE THING! Yes, do it. If you get a new one before you put it into service, take it apart, INSPECT IT!

And don’t worry about the warranty sticker covering one of the screws, it’s a tail light warranty at best. I.e. when they can’t see your tail lights the warranty is up.

Disassembly

There will be a maximum of SIX screws on the top piece holding the fan. The “warranty” sticker might cover one. Don’t be surprised if some screws are missing, or stripped. I’ve found both. Standard M3 screws are what they use.

IMG 1151

Once you get the top screws off, you’ll see something like this, if not exactly the same if you’re disassembling a 12V 30A supply.

IMG 1153

Unplug the fan connector from the PCB and set it aside.

Remove the four or six screws holding the MOSFET’s to the case from the one side and end. Be careful not to rip the insulting gasket. If your supply has a metal bar between the MOSFETs and the case, wiggle it a little and slide it up out of the case. Set all those parts aside aside.

There are five holes in the PCB for holding it on to the case. I’ve yet to find a supply that uses five screws. Screws cost money, You’ll no doubt find four screws (one in each corner, one rarely in the centre) so remove them. Set them aside.

The whole PCB should now lift almost straight up and out of the case. And the fun begins.

Inspection

It doesn’t matter if you know zero about electronics or not. You’re not troubleshooting or doing component level repair.

What you’re looking for is potential hazards that will be pretty obvious once you know what to look for…

First, check the solder side of the PCB. You don’t want any untrimmed leads hanging down more than 1/8″ under the circuit board. On some of the supplies there is a plastic shield under the PCB, but I’ve also found a few that have nothing. If you see a long lead, don’t bend it over, trim it off. Use nail clippers if you don’t have wire nippers.

A reasonably good power supply PCB will look like below. All shiny, no dull solder joints, no leads poking out.

Good Supply

On the other hand, you can find something that looks like this one. See those BIG wire jumpers? That tells you SOMETHING is amiss with the PCB foil traces. Take a photo or two, put it back together, tell the seller it’s junk and don’t use it.

Not So Good

Moving on, flip the circuit board over so you’re looking at the component side. What you’re looking for is two things. Components that are bent over, shorting against other components and solder splashes.

I’ve only found one capacitor that was laying on top of a nearby resistor, which would have caused the supply to leak smoke when I powered it up.

On the other hand, solder splashes? LOTS..and LOTS. The trouble with solder is that it can stay fixed to where it settles, but at some point, heat, maybe a jar to the supply when you’re moving or transporting it and now it’s a shorting strip looking for a place to cause havoc.

Take a look at this photo…big thingie with coloured bands is a power resistor. But! What’s that “glint” of silver on the top of it? Solder splash. Get some tweezers and take it off. In some cases, you can flick it off with a finger nail. Just don’t flick it back on the circuit board.

Power Supply Solder Glob

Or look at this example on the last supply I checked…under the heavy jumper wires there is a piece of solder splash. Just. Sitting. There. I removed it with a pair of tweezers and it wasn’t attached to anything.

After which I thought I was done. Right….not! Take a look at the “400.”… that dot after the 400 silkscreen isn’t a dot, it’s a ball of solder. One touch and it moved.

Power Supply Glob 2

Tilt the board and look under any components you can. Loose solder loves to pin itself up against other component leads.

Summary

If you currently have a power supply in use and are under the impression that since you’ve been using it for “x” hours, it must be fine. Don’t assume, stop, check it.

The one I showed with the big jumpers on the PCB was one I’d used for over 400 hours. It worked fine. But every now and then the printer would do something odd and I blamed the firmware. So I took it apart. One of those jumpers wasn’t properly soldered. No, I didn’t fix it. I tossed it out.

I doubt these power supplies go through anything more than the briefest cursory glance for quality control. No doubt more about how fast they can be assembled, than can be they be assembled correctly. The old, time is money, problem.

Sadly, the end users, you and I, will end up the loser. Thus if you have one of these supplies, do yourself a favour and have a real good look at it. At least you’ll get a little piece of mind when you start one of those 33 hour print jobs.

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