Electronics > Beginners
500mA fuses blow at 1A, am I testing them incorrectly?
(1/5) > >>
John_doe:
Greetings, electronics experts.
I usually buy my fuses at a local store but this time I couldn't find the fuses I was looking for anywhere locally, so I ordered them from eBay.
They are 5mm x 20mm glass fuses, the description says "Type: Quick Blow Glass Tube Fuses , Rated Current: 0.5A(Amp)".
I like to test everything that I buy from eBay (if I can) and so I connected one of these fuses in the simplest circuit I could: DC power supply with adjustable current connected to the fuse in series with a low resistance high power resistor.
I start cranking up the current and nothing happens when I get over 500mA. I even tried staying at 900mA for a few minutes and still nothing, but the second I get to around 1A the fuse blows. I tried connecting a DMM in series to see if maybe the PSU current sensing is bad but the DMM shows the same current as the PSU.
I tried 4 of these fuses and they all behaved the same.
The fuses are labeled "250V 0.5AL" on the metal part.

This is the first time I ever tested a fuse's current rating so before I go complaining to the seller I just wanted to make sure that I'm not doing something wrong, or maybe I have a misconception as to how fuses work.
So... Did I get 1A fuses mislabeled as 0.5A ?
Psi:
That's 100% normal, it takes twice its rated current to immediately blow a fuse.

A 500mA fuse will handle 500mA pretty much 24/7, though it might blow after a year or two since you are running it right on the edge of its rating.
A 500mA fuse would blow at 750mA but it might take hours.


A 500mA fuse doesn't magically know when you reach 501mA. Its all based on heat and getting the fuse wire hot enough to melt and that takes time. The fuse manufacture doesn't know what ambient temp is or how well the fuse is heatsinked to its holder, so there has to be a massive amount of added error margin in fuse design.

The general rule of thumb in the industry is that a fuse blows at twice its rating.  So your 500mA fuse blowing right away when you apply 1A is normal
mcinque:
I experienced similar results with a fuse kit purchased (cheap) to a fair. I strongly believe that that fuses came from a bad stock. Since I experienced wrong values also in capacitors, resistors purchased from fairs, I concluded that that market isn't reliable. Since that experience, I never purchased anymore any component from a not reliable source, expecially protection components.

I don't think you're testing incorrectly, they are probably cheap chinese junk coming from the vast ebay junk market.
mcinque:

--- Quote from: Psi on April 22, 2018, 12:28:21 pm ---That's 100% normal, it takes twice its rated current to immediately blow a fuse.

--- End quote ---

According to a datasheet (http://www.littelfuse.com/products/fuses/cartridge-fuses/5x20mm-fuses.aspx), a Fast acting 500mA 5x20 should blow within a second when the current raises just 1,5x it's value, but I guess the time/current curve changes between manufacturers. Is this right?
Benta:
+1 to what Psi said.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod