On Wed, 09 Jan 2019 18:27:30 -0600, Tim Williams wrote:> What are you fusing? To what end? > > "High voltage" is particularly onerous because said fuse will continue > arcing the whole time the supply is draining down. > > The transistor protects the fuse, not the other way around!! >It tends to be the other way round with SCRs though. :-) -- Johnny B Good
using an smt resistor as a fuse
Started by ●January 9, 2019
Reply by ●January 11, 20192019-01-11
Reply by ●January 11, 20192019-01-11
On 1/10/19 1:01 PM, John Larkin wrote:> On Thu, 10 Jan 2019 04:42:00 -0800 (PST), whit3rd <whit3rd@gmail.com> > wrote: > >> On Wednesday, January 9, 2019 at 1:44:00 PM UTC-8, Winfield Hill wrote: >>> I've placed 22-ohm 0805 resistors in series with the HV supply lines ... >> >>> If the output were to be shorted, and draw 150mA from the supply,... the 22-ohms >>> would certainly fail, stopping the high power dissipation and removing the load >>> from the supply. >>> >>> OK, I see I'm going to get roasted for this one. >> >> OK, I'll say just use a fuse <https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/466%20Series%20Drawing.pdf> >> because they're available, in a variety of specifications. >> >> Three things the fuse has to do: carry the current, break the connection on overcurrent, >> and recover from the fault afterward. A metal-film resistor will get hot enough in places to kill its >> metal film, or ceramic core, or endcaps, when it fuses, and THAT can damage >> the PCB underneath it. Using a non-fuse-rated component risks the heat >> damage (potentially, fire) that a fuse was intended to prevent. >> >> It's that third part, repairing the damage and returning the gizmo to service, that >> is the question: is it OK to throwaway the board on an overload (potentially, just >> a stray wire) event, or would you want to repair/rebuild? And, is heat damage >> limited to the resistor/fuse, or does it scorch the printed wiring? >> >> I've never greatly admired surface-mount fuses, but you can also get fuseholders. >> I HAVE repaired boards with failed SMD fuses a few times, and found surface-mount to >> be only slightly annoying (tweezer-type irons help). > > We've found surface-mount fuses (and surface-mount polyfuses) to be > erratic. > >Polyzens are very good for overvoltage. We current-limit elsewhere to protect against short circuits. (RCs in the supplies of the output amps, often.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●January 15, 20192019-01-15
On Wednesday, January 9, 2019 at 10:48:05 PM UTC-8, Piotr Wyderski wrote:> If the transistor is not properly selected for fusing, one may also try > to augment it with the Soviet LED technique: > > https://imgur.com/gallery/YegLMTransparent bolt! (The light from the junction is escaping.)
