Why did my power supply fuse blew out on my Anet A8?

Question

So yesterday I got my power supply in the mail and I thought, let's check it out!

I put all the wires in correctly (as shown in this YouTube video, Anet A8 power supply unit fuse blown, I just searched really quickly, this person has the same problem as me) and "BANG", it blew.

Luckily the only thing broken on it is the fuse, for which I can get a new one, but my main question is .. why? why did it blow out? Did I wire it up wrong? The 12 V output goes to a RAMPS 1.4 board which is working correctly.

It worked the first time I plugged it in; the little green light went on. The second time a nice "big bang". When I tested it the first time, I didn't add the 12 V wires to the RAMPS board yet. When I did, the fuse of the PSU blew out.

The board is a RepRap RAMPS 1.4, which works fine when connected via USB to laptop.

Upon closer inspection I found a burned out resistor in the circuit of the power supply, maybe this caused the bang?

BIG update!

I opened it up and flipped the board, and noticed that there was a big ol' blob of tin connected to one of the pins I connected my wires to. I am very sure that this blob shouldn't connect to that pin at all, hence shorting my board.

Is this the cause?

Here is a link to the webpage I bought it from: Anet® 12 V 20 A 240 W Power Supply Dual-input Centralized Power Monitoring For 3D Printer

Here is how I wired it up at the PSU side:

At the RAMPS side:

This shows the internals of the PSU, it clearly shows burn marks:

Answer 1

You need to closely look at how to connect to the output connectors. In this case it appears that you have wired it correctly according to the stamped in markings above the connectors. You have chosen not to connect the earth, this is strongly advised.

Did you check if the power switch matches the voltage of your country? If you are in Europe the switch should be set to 230 V, not 115 V (which is used in America).

E.g. for the following power supply you see such a switch on the side:

If a PSU is connected without load and works, and short circuits when externals are connected there are a few options:

• there is a short in the connected printer board,
• the load that the board takes is too large for the PSU,
• the PSU cannot deliver the load,
• the PSU was faulty to begin with.

Answer 2

Unless you know what you're doing, do not remove the covers from a PSU. The components can store a lethal charge long after power is removed, even if the fuse is blown.

My guess is something loose (now vapourised) in the PSU. Electronics have a tendancy of failing either quite soon after manufacture, or lasting reasonably well. This is described with the Bathtub Curve.

Looking at the failure, there was a significant short circuit somewhere - quite likely within the PSU itself. This doesn't mean you can be sure that the RAMPS board survived, but it has a reasonable chance of not being harmed.

There is no significant chance of the PSU failing like that simply due to an excessive load in its output - unless it was overloaded for a significant period of time. Generally these PSUs will have some degree of overload protection even if they are not explicitly designed to survive an overload.