Light bulb instead of fuse idea - Page 1
Forum section: electronics / beginners

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 28, 2020, 12:18:49 pm
content: Greetings,I am currently wrapping up by build of a power supply and at the beginning I was bowing fuses when there was even no load on the output of the rectifier. It was a pain in the butt to go through my bin of salvaged fuses every time and replace it, just to have it blow up again.That project is now nearing completion, but I kept thinking about those wasted fuses.Literally this morning I woke up with a idea (I must have been creaming about this at night).What if we could use a high wattage Incandescent light bulb, in series, instead of a fuse? I know they are hard to find but I still have some.In this case the light bulb would light up bright when there is a short circuit. This would tell me that there is a short and I could troubleshoot and I would not have wasted a fuse.There could still be a fuse in series, but at least it would reduce the number of fuses being wasted during a build process.What do you guys think of this idea?Or am I just reinventing the wheel here?Thanks...

author: Raj
author_rank: Frequent Contributor
timestamp: April 28, 2020, 12:24:49 pm
content: Thou shall measure your current.Then get a proper rated fuse...If you wanna know when the thing is shorting out, power it up using current limiting power supply and keep an eye out at how much the circuit is consuming.Do post more details of said project, for us to determine if it's dangerous or not.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 28, 2020, 12:26:11 pm
content: Very good idea indeed(despite as you said it is reinverting a weel)I use an incandescent lamp (40 - 150 W) for powering 230 V AC devices. It saved a lot of fuses, devices, time, and money.The lamp works as a current-limiting device and as an indicator simultaneously.

author: Raj
author_rank: Frequent Contributor
timestamp: April 28, 2020, 12:34:09 pm
content: A bulb acts like a PTC and won't react to sudden surges of current.Thou shall know how much current consumption to expect.

author: Dave
author_rank: Super Contributor
timestamp: April 28, 2020, 12:43:28 pm
content: Yup, you're reinventing the wheel. The trick that you mention is regularly used by repairmen.The filament has a low resistance when it's cold, so it doesn't cause a significant voltage droop and the resistance increases dramatically when it gets hot, thus limiting the fault current.It's perfectly fine even if you don't know the exact current the device is supposed to be drawing, because when you're fixing things you usually don't do maximum load stress tests.

author: digsys
author_rank: Unknown
timestamp: April 28, 2020, 12:57:12 pm
content: It is a very common method used in battery UPS / charger circuits and similar. As pointed out though, be aware of the attack / release profiles. ie it is not a precision protection method, but a useful 2nd backup.

author: langwadt
author_rank: Super Contributor
timestamp: April 28, 2020, 01:05:29 pm
content: also used in some speakers to protect the tweeter

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 28, 2020, 01:05:53 pm
content: Thank you guys.I had a feeling I could not possibly have thought of something that others have not thought of before.Nevertheless, I am going to install an old halogen light (that I have no use for) under my overhead cabinets and wire it up so that I can connect any new test circuit in series. If that's a trick repairmen have been using for a while that means it's been time tested and although it is not the answer to every problem it does offer some benefits.The circuit I was blowing fuses on was actually a factory made transformer hooked up to a rectifier, that I salvaged from a discarded piece of equipment. I wanted to use that ready made transformer/rectifier to provide DC to the bench power supply I'm building. So, at the very beginning I wanted to test what was coming out of the rectifier. With nothing hooked up the the rectifier output I plugged it in and tested the voltage. After I tested the voltage I turned the power switch off and I actually saw the fuse light up as it blew. When I did that same test again, same thing happened again, every time I turned off the power.So, I started using a variable transformer to slowly bring the power up and down.Again, this was a fairly new, salvaged, factory made circuit. There was a power cord, switch, going through a fuse, EMI filter, center tapped toroidal transformer with output connected to a bridge rectifier and two 4700uF caps on each side, outputting +/-25V. There was also a trace going to a bank of 3 power regulators that I was not using. So, I later broke off that trace, anyway, and I just used the raw +/-25V output.I do not know if this would continue blowing the fuse as I kept using it through the variable transformer, and I later switched to a different salvaged rectifier.I do have a build thread for that project and the unit is almost finished. But I had this separate light bulb idea so I thought I'd share the idea separately.I also do intend to post other projects. I will definitely need help.Thank you all...

author: rdl
author_rank: Super Contributor
timestamp: April 28, 2020, 01:26:58 pm
content: Two 4700uf caps will draw a lot of current when power is applied and they charge up, easily enough to blow an improperly sized or wrong type of fuse (not a slow blow). That's enough filter capacitance for a five amp or more power supply. Why the fuse would blow when power is removed is a mystery to me.

author: digsys
author_rank: Unknown
timestamp: April 29, 2020, 01:14:34 am
content: If the Inrush current to the main filter caps is the problem, there is also a "fix" for that - use the lamp (or similar) idea from main bridge > caps, AND a higher speed / more precise fuse / current limiter / etc AFTER the caps to the regulator / other circuits. Depending on what you are powering, you can even have more than 1protectedbranch OFF the caps.

author: eliocor
author_rank: Unknown
timestamp: April 29, 2020, 11:11:40 am
content: You surely have used the wrong type of fuse: your choice should have been of a T or TT one (slow or very slow).just some hits on google:https://www.swe-check.com.au/pages/learn_fuse_speed.phphttps://www.schurter.com/content/download/676506/13549060/version/3/file/Guide_to_Fuse_Selection.pdfhttps://www.doityourself.com/stry/knowing-when-to-use-a-time-delay-fuse