Identifying a fusible resistor  - Page 1
Forum section: electronics / beginners

author: nonius
author_rank: Newbie
timestamp: April 05, 2020, 01:51:03 pm
content: Hi Everyone.I need some help with the identification of what I suspect is a fusible resistor.I got one closeup picture you you can see what it is. See picture below.This component is used on a Dreamcast console (about 20 years old).It is in series on the 5V power rail, providing some protection from faulty devices that may be plugged in on the gamepad ports.The component has a blue body, is marked as F1 on the PCB, and has a color coding similar to that of resistors. See:orange 3white 9gray *0.01gray 10%green ?Measured value: 0.46 ohmI've searched on the Digikey catalogue, but I couldn't find anything similar to this.Do you have any idea?Thank you.

author: Moriambar
author_rank: Unknown
timestamp: April 05, 2020, 02:20:00 pm
content: I think it should be read as afive bandresistor, unless fusible resistors have different colourings. IEorange=3white=9grey=8silver=*0.01green= ±0.5%so it should be (3.98±0.5%)Idk whether it's something special though

author: wraper
author_rank: Unknown
timestamp: April 05, 2020, 02:24:03 pm
content: It's 4 band resistor. And both bands are silver. 5th band has nothing to do with resistance. Just MFG specific marking of resistor type. 0.39 Ohm 10%.

author: madires
author_rank: Super Contributor
timestamp: April 05, 2020, 02:46:55 pm
content: wraper is spot on. Today seems to be the International Fusible Resistor Day.

author: nonius
author_rank: Newbie
timestamp: April 05, 2020, 03:25:10 pm
content: Hi.What led me to believe this component was some kind of special resistor was the designation F1 (instead of R1, for example) and it's special symbol on the silk screen (like a little fuse, with and arched trace between its terminals). And also that some Dreamcast owners reported that these had become faulty somehow and replaced them with some kind of polyswitch.I raised this question, because I don't have access to any first party schematics and because I couldn't find any kind of convincing rationale for the above.So in the end you all agree that this is just an normal resistor.Thank you again.

author: madires
author_rank: Super Contributor
timestamp: April 05, 2020, 03:43:23 pm
content: Not quite, it's a fusible resistor. A polyfuse could be a great replacement since it's resettable.

author: wraper
author_rank: Unknown
timestamp: April 05, 2020, 04:10:06 pm
content: Quote from: madires on April 05, 2020, 03:43:23 pmNope it won't be a good replacement in general. Polyfuse does not break circuit completely, is slower and usually have very low voltage and relatively low maximum current rating. So great consideration should be done before doing such substitute.

author: Ian.M
author_rank: Super Contributor
timestamp: April 05, 2020, 04:28:36 pm
content: A measured value of 0.46 ohms indicates that there's almost certainly absolutely nothing wrong with it. Its near impossible to get an accurate measurement of fractional ohm resistances with any two wire meter, and even if you removed it and did a four wire measurement with a bench meter with that facility, the acceptable +/-10% tolerance range for it would be 0.351 to 0.429 ohms.Its reading approximately 0.3 ohms higher than its upper tolerance limit, which if measured by a two wire meter, is either due to contact resistance, and/or due to other accuracy limitations of the meter.TLDR: it isn't broke, and unless you've got another unit that it has blown in, you don't need one!P.S. it is *NOT* just an ordinary resistor. Fusible resistors are designed and constructed to rapidly go open circuit on serious overload, without catching on fire. An ordinary resistor may well survive even a x10 overload, and if it doesn't frequently catches fire.

author: bob91343
author_rank: Super Contributor
timestamp: April 05, 2020, 06:02:15 pm
content: A clue is the powder finish rather than gloss. It appears it's standard to paint fusible (or flameproof) resistors with that finish. Exactly how much current or dissipation causes it to open is something to investigate.

author: wraper
author_rank: Unknown
timestamp: April 05, 2020, 06:31:05 pm
content: Quote from: bob91343 on April 05, 2020, 06:02:15 pmIt's completely standard for non fusible resistors too. Particularly for those which offer large power rating in small package as they are rated to work at higher temperature compared to enamel coated resistors.