Broken fusible resitor? - Page 1
Forum section: electronics / repair

author: Thyreen
author_rank: Newbie
timestamp: October 17, 2020, 09:04:12 pm
content: My 240v led light stopped working. When measuring continuity I probably found the problem in a blown fusible resistor (F1).Can anyone confirm this indeed such a resistor, and am I correct it is 11 ohm and ?? watts. I think one color hides behind the metal cap?The light iself has a maximum rating of 170ma.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 17, 2020, 09:38:20 pm
content: This is the fuse. It could have burned out from an external factor - a surge in voltage, or due to a circuit malfunction.

author: bob91343
author_rank: Super Contributor
timestamp: October 17, 2020, 09:46:11 pm
content: Not really enough info, for me at least. What are the colors? That is a very tiny resistor, maybe 1/4 Watt or less.So if you are drawing 170 mA it's probably protecting at somewhere near 250 mA. For 1/4 W dissipation that would mean a voltage of 1 V across it and a resistance of 4 Ohms. Maybe 11 Ohms seems high but maybe not. Then that would mean with 170 mA a drop of 1.9 V or so and an operating power of maybe 320 mW. Not impossible. It just seems a lot for such a tiny part but with a 1/2 W rating it comes pretty close to reasonable.

author: KevinA
author_rank: Regular Contributor
timestamp: October 17, 2020, 10:51:56 pm
content: 100mA Pico fuse

author: Thyreen
author_rank: Newbie
timestamp: October 17, 2020, 11:23:52 pm
content: As far as I can see the colors are from right to left. Brown Black Black Gold

author: james_s
author_rank: Super Contributor
timestamp: October 18, 2020, 12:29:55 am
content: The question is, why did it fail? Fuses rarely fail for no reason, usually they are blown by an overload due to another failed part.

author: aqibi2000
author_rank: Regular Contributor
timestamp: October 18, 2020, 04:17:17 pm
content: Crude reactive ac voltage divider circuit from mains AC to power the LEDs.My bet would be on a that electrolytic.Worst case scenario is to be left with a shorted LED.

author: Thyreen
author_rank: Newbie
timestamp: October 18, 2020, 08:20:30 pm
content: Well I have tried to apply power by bypassing the fuse and the light works. So apparently no harm to other devices. At least when it burns for a short amount of time.In line there are also 3 CYT100A's Constant current led drivers max 60 mA each. Attached an extra image of the whole circuit.To try it out would this component be any good?:NFR2500004708JA500Through Hole Resistor, Fusible, 4.7 ohm, NFR25 Series, 330 mW, ± 5%, Axial Leaded, 250 VIt would give a voltage drop of 846mV and power dissipation of 152 mW.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 18, 2020, 08:29:52 pm
content: There is no divider. The resistor is used as a fuse and for the release of high pulses closed by the varistor. After the resistor, a diode bridge and a capacitor are installed. The LEDs are connected in series with a total voltage of 230V. This is a very primitive scheme.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 18, 2020, 08:33:35 pm
content: It turns out that there is a current limiter. Most likely, the resistor, which is used here as a fuse, was killed by the varistor during a power surge in the supply line. You can install a fuse instead.