Re: Hmm!
- From: "Phil Allison" <philallison@xxxxxxxxxx>
- Date: Mon, 28 Apr 2008 12:18:49 +1000
"Trevor Wilson"
**UPDATE!
I checked a bunch of the fuses. The ones which measured high were 'Ralmar'
branded. I checked some branded (Littlefuse) ones and found them to be 5.3
Ohms. A couple of unbranded ones in the bottom of the drawer (probably WES
ones) were 5.5 Ohms. The Ralmar fuses have been placed where they belong
(in the trash). I also checked a Littlefuse M205 type at 4.7 Ohms.
** There IS a manufacturing problem with your 250mA fuses !!!
I found that some of my stock of low current fuses gave absurdly high
resistance readings too - if tested with a DMM or Bobs ESR meter.
After testing at the rated current, with a current limited PSU, the fuse's
resistance value returned to normal.
Theory:
Low current fuses are made with resistance wire, not copper or tin wire as
with higher current types.
Where the wire ends are soldered to the fuse caps, a high resistance joint
can form.
Applying a few volts breaks down whatever corrosion is causing this
resistance.
DMMs etc do not apply enough voltage to have any curative effect.
...... Phil
.
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