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Fuse Keeps Blowing In Power Supply

Posted by JRMN 
Fuse Keeps Blowing In Power Supply
February 06, 2012 08:13AM
I purchased a 12V 30A DC Universal Regulated Switching Power Supply from ebay and everytime I plug it in the wall outlet the fuse blows. It's set to 115 volts and I'm getting between 123 and 128 volts throughout my house. I talked to my cousin who works for the local power company and he said it doesn't matter that it's that high. He also said that if it wasn't all the electronics in house would have fried by now.

This is how I have it wirded:

Does anyone have an idea why the fuse keeps blowing
Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 09:45AM
What rating is the fuse? Switch mode PSUs can take a big surge at start up so a 5A fuse is normally needed even though 360W is only 1.5A.

The other possibility is the PSU is faulty.

Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 10:55AM
The fuse that it came with was F8AL 250V (fast blow), but the power supply PCB says T6.3A 250V (slow blow). So, I replaced the orginal fuse with a T6.3A 250V and it blew out again.

Edited 1 time(s). Last edit at 02/06/2012 10:55AM by JRMN.
Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 11:58AM
If it blows its own fuse with no load it is faulty.

Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 02:19PM
... i have an old 24V/80Amps-powersource from the 'German telecom', that blows my main-fuse if plugged in the one way, but is OK if i turn the wall-plug ...

But with blowing its own fuse there is something wrong inside - maybe a long pin produces a short to the housing?

Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 02:59PM
I just did a continuity test on +V and -V of the power supply and I got continuity. That's not supposed to be the case right?
Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 05:06PM
Not a direct short but it can be quite a low resistance. I just measured a similar one at ~200 ohms.

Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 07:21PM
I get about 250 ohms between +V and -V.

Can someone tell me why -V is connected to ground in this photo?

Photo is from here.
Re: Fuse Keeps Blowing In Power Supply
February 06, 2012 11:09PM
Yellow-green i think its "safety ground". There is null and phase, these 2 give the power from the electricity company. The 3rd, safety ground its supposed to be a direct local ground, a connection to a grounded, as in a local physically burried underground network made with 6 pieces of 2-3 inch diameter metal tubes, about 1-1,5m tall each, ~3+ meters in between, burried like at least 1m underground, and welded together with very thick metal band like 40mm x 5mm diameter, and with everything tinned. Take a full tour of building outside and if there is a 40mm x 5mm metal band that exits from ground, on the wall of the building, thats it. Its supposed to have very low resistance, measured with a dedicated aparatus should be well under 30 ohms (max acceptable for my local standards). Its meant as safety that connects to cases of equipments, etc, i think to provide a faster and lower resistance ground to shorten eventual currents on a better path than it would go through a human body. I think its generally supposed to be different / have no contact to the phases null.

Thats theory. In practice for households its usually missing (i think). Or its connected to the center null of the 3 phases somewhere, which doesnt make it any different. Its mandatory in industry / factories i think like that.

Since -V is connected to gnd, if there is a real local gnd as it should be, it might help to give it a better ground perhaps. Or if the safety gnd is connected to the null of the 3 phases, that one can get noise from other sources around the neighbours etc, then it might very well alter the potential difference in a bad way. Because when power distribution has issues, that can very well and up with noise on the null line.

I wouldnt do that unless i know for sure what my local situation is like. Although under best of circumstances it shouldnt make much difference at all, under worst circumstances i think it can make a difference either way. If there is a local real safety gnd, i guess it can help. If its instead connected to the null, and this one gets noise from other sources, then it can be a bad thing to do. Hope i am not too far off.

Edited 1 time(s). Last edit at 02/06/2012 11:21PM by NoobMan.
Re: Fuse Keeps Blowing In Power Supply
February 07, 2012 01:19AM
Adrian is from the UK, and all UK sockets have a proper safety ground.

In theory the PSU outputs should be isolated from the mains by split bobbin transformer with double insulation and all the required safety approvals. If that failed the outputs could become live. With a cheap PSU from China the approvals are likely missing, or fake, so it is a good idea to ground the output to prevent it becoming live in the event of a fault.

The downside is that you get a ground loop through the USB cable and the PC which can cause USB comms problems.

i have the same problem, except i forgot to change the PSU to 240v as this is the current in australia. the fuse burnt out. so i changed the fuse and switched the PSU to 240v to see if it would work. no good, it just keeps blowing fuses. not sure how to fix this? should i replace the PSU?
Re: Fuse Keeps Blowing In Power Supply
July 04, 2013 05:15AM
I dont know how similer your PSU is to the one I fixed last week for a fellow RepRapper but... I was presented with a very similar looking PSU with a blown 6.3A fuse and the problem turned out to be that if the cage wasn't securly fastned the pins on the underside of the Mains selector switch were able to short against the lip of the cage. (which is tucked under the PCB slightly.)

A bit of capton tape and snipping of long pins sorted this out.
Hope that helps someone
Re: Fuse Keeps Blowing In Power Supply
July 04, 2013 06:40AM
I'm confused,

Why do you have a UK Style Plug,
but have 123 and 128 volts running through your house?

That doesn't make any sense.

UK and that plug are 220VAC 50Hz.
Re: Fuse Keeps Blowing In Power Supply
July 04, 2013 08:46AM
@slymike, I wish I had known that before I sent it back. My build has been put on hold.

@ShadowRam, good observation. Short answer, those aren't my photos.
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