240vac Electronic Fuse - Page 1
Forum section: electronics / beginners

author: DarkZero
author_rank: Regular Contributor
timestamp: January 28, 2020, 07:18:10 am
content: Hello all!every now and then i repair some sort of smps psu and so far i have been using incandescent light bulb as my current limiting when I'm testing it but lot of times i change from one light bulb to another which breaks the seal between the glass and metal part so they explode at times sometimes ruining thing i was working on.is there any reliable AC electronic fuse circuit something like 10mA to 1A ? i had a look in those big 300W variable resistors but their price is just to much for what I'm doing.

author: kripton2035
author_rank: Super Contributor
timestamp: January 28, 2020, 07:26:32 am
content: something like this :http://nutsvolts.texterity.com/nutsvolts/200909?folio=42&pg=42#pg42

author: kripton2035
author_rank: Super Contributor
timestamp: January 28, 2020, 07:27:20 am
content: and the code is here :https://www.nutsvolts.com/uploads/magazine_downloads/refuseable.zip

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2020, 10:26:46 am
content: The useful thing about using a bulb is its positive temperature coefficient. It will pass more current initially before increasing its resistance as required (hence 'dimbulb tester'). You want a current limiter, not a 'fuse'.I would suggest two or three bulb holders and ordinary light switches. That way, you don't need to keep unscrewing bulbs. Probably quicker, easier and safer to implement too. 30 minutes tops.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2020, 10:57:32 am
content: Mind that the above article is a trip relay; quite useful by itself, but it won't limit current, and relays aren't fast enough to save fast-blow fuses, let alone transistors. (Diodes and thyristors are probably okay, assuming they are fused appropriately.) It would work well in combination with a light bulb (or other load resistance) bank.To do better, you need a current limiter. I'd like to make one myself, actually; a resettable, current limiting fuse, that is. I've already made a smaller version, 30V 20A, which works very nicely, fits in a handy enclosure, and is battery powered.The challenge is, to withstand mains power, the circuit must dissipate many kW at a time, and it may occasionally be subject to overvoltage swells and surges. (Wouldn't want your project suddenly kicking on full throttle just because someone turned on/off their air conditioning nearby, or because it's stormy outside!) Semiconductors can't handle this for long at all. There aren't many other things you can dump that energy into (big resistor, stack of MOVs?), and none of them are very compact or reliable. It would probably cost a few hundred dollars, though.Tim

author: Kilrah
author_rank: Unknown
timestamp: January 28, 2020, 12:28:25 pm
content: Just a series cap?

author: DarkZero
author_rank: Regular Contributor
timestamp: January 28, 2020, 07:02:24 pm
content: hello!so the only solution for me is big variable wire wound resistor. ah i was hoping there is something cheaper

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2020, 07:29:26 pm
content: Thanks for reading the replies!

author: DarkZero
author_rank: Regular Contributor
timestamp: January 29, 2020, 08:15:13 am
content: Hello!I read the replies and ill probably do what u suggested gyro. I was just thinking that there is fast ac current limiting circuit out there because even thou light bulb will limit current i cant set the limit and if something is wrong with the circuit other component will still die, well more of them.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2020, 12:45:11 pm
content: Sure, it's a compromise between supporting the initial charging surge (assuming smps) and allowing proper startup without passing enough current to kill too much if it doesn't.As T3sl4co1l mentioned, dissipation mount's up pretty quickly when you are current limiting at mains voltage. That's (pretty much) the only reason that people use lightbulbs, they're good at dumping power.Just one hint - If you go for a big circular wirewound rheostat, check the wiper current rating very carefully - it tends to be a lot less than the element current and dissipation might lead you to believe, particularly the Far Eastern ones.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2020, 01:49:07 pm
content: Indeed, and not only that, but the element rating is only however much of it you're using. A 300W resistor tapped at 10% won't handle anywhere near 300W!Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 29, 2020, 02:12:10 pm
content: With a light bulb bank, you can also play with different response times.The thermal mass of a 100 watt filament is larger than that of a 25 watt filament. Putting four 25 watt bulbs in parallel will yield a faster current limiting times.Several years ago, while using my very first digital scope I captured some waveforms and "saved" them by taking a photo...on a film camera.I can't find the photos anymore, but if I recall correctly the 25 watt filament would fully limit the inrush within the first cycle, 16.6 msec.

author: all_repair
author_rank: Frequent Contributor
timestamp: January 29, 2020, 02:26:18 pm
content: Bulb is good, it is a live heart beat indicator too. Only issue now is it is getting harder to get one.