Fig.1

Fig.2

Greetings, all! I've got this DC supply with variable positive and negative outputs (Fig.1), and a portable USB electric fan (Fig.2) with the wires stripped. I attached the fan wires, as well as the leads of a multimeter, to the positive output with alligator clips and switched on the power supply. It was at the minimum position of the knob (equivalent to about 6V) and the fan powered up as expected, but without touching any setting the fan gradually increased in speed as the voltage level also increased according the the DMM. Soon after the LED indicator turned off and the fan stopped. Foolishly, I replicated the setup with the negative output, reversing the leads of course, and the same results happened. I also noticed a slight burning smell, but didn't find smoke. My main question here is which component inside the power supply was damaged? Could it most likely be one of the capacitors? I probed the mylar capacitors and the readings made it seem like they're shot, but is that all? I have a background in electronics but here I'm asking for the insight of some more experienced tinkerers. I didn't think an electric motor would have an effect on a variable supply.

Presented in Fig.3 to Fig.5 below are the circuit board assembly as well as the schematic, and Fig.6 shows the electric fan inner workings, in case that's any help in determining what went wrong.

Fig.3

Fig.4

Fig.5

Fig.6

 

Most likely option: the fan short circuited the DC supply and activate some defence or burned something.

Second option: the fan drew too much current which activated some defense or burned something.

Third option: maybe only the fan burned.

It would be good if you can post a good picture of that barcode and label in the first photo.

I like repairing electronics, so I will try to help you.

 

The left transistor on the second to last photo seems gone. Can you measure it?

 

What are the output voltage/current/power ratings of the DC supply?
What is the current/power rating of the fan?

 

Thanks for the replies, I appreciate it.

1) The fan is still working when I put the leads to the constant 5V output, so no doubt there.

2) What I've got here is a project that I assembled in the Electronics workshop about 4 years ago. That barcode is for the folding table that the power supply is on. Back then they didn't give us any ratings on power or current output of the device. The variable voltage outputs can go as high as 15V, but I'm sure that when I plugged in the fan it was at minimum setting. As for the fan it's just a cheap product with no schematics either. The wire used to terminate in a USB plug so I presume it runs on 5V.

3) The 7805 ICs for the variable outputs aren't working anymore. That is, I get a zero value when I measure the output pin. The input pin reads 30V for both the positive and negative output circuits of the supply. I checked the 7805 in the constant 5V supply circuit as a sort of control and it works as expected: about 16V at the input leg and approx 5V at the output. My hand slipped while doing that and there some sparks but nothing fried. Then I tried checking the voltage across some of the capacitors in the variable supply circuits. Saw fireworks there too so it 's likely that capacitors are what burned out.