John Larkin wrote>On Wed, 9 Jan 2019 18:27:30 -0600, "Tim Williams" ><tiwill@seventransistorlabs.com> 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!! >> >>Tim > >I've connected 0805 resistors across the AC line. Try it. > >They usually just go PIP.DC is different and sustains the arc.
using an smt resistor as a fuse
Started by ●January 9, 2019
Reply by ●January 10, 20192019-01-10
Reply by ●January 10, 20192019-01-10
On Thursday, 10 January 2019 06:48:05 UTC, 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/YegLM > > Best regards, PiotrI hear you can get them in white as well as red. Re using a jellybean resistor for fusing, there are some general issues. Voltage rating is low for sm parts Carbon resistors sometimes hot tunnel when overloaded. This means R drops precipitously. General purpose small resistors can also initiate a fire. Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value. Fusible Rs address some of these issues. NT
Reply by ●January 10, 20192019-01-10
tabbypurr@gmail.com wrote:> I hear you can get them in white as well as red.And anything in between.> Voltage rating is low for sm partsThere are really robust HV SMD resistors, good to 10kV (although I prefer the VR37 THT series), but they are way more expensive than a proper fuse, so there is no point.> Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value.There's even a negative resistance region over a significant fraction of the I-V curve, so the glowistor will be happy to slurp a bit more. And more.> Fusible Rs address some of these issues.Fusible transistors do that too, LOL. "Im an arcist"... Best regards, Piotr
Reply by ●January 10, 20192019-01-10
On Thursday, January 10, 2019 at 7:56:28 PM UTC+11, tabb...@gmail.com wrote:> On Thursday, 10 January 2019 06:48:05 UTC, 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/YegLM > > > > Best regards, Piotr > > I hear you can get them in white as well as red. > > > Re using a jellybean resistor for fusing, there are some general issues. > Voltage rating is low for sm parts > Carbon resistors sometimes hot tunnel when overloaded. This means R drops precipitously.Carbon resistance elements have negative temperature coefficient of resistance, so they are very likely to hot tunnel. One of my bosses had party trick where he could persuade a 10k carbon film resistor to pass roughly an amp indefinitely, and it still measured 10k afterwards - all you could see afterwards was a dark line in the paint above where the hot channel had been. He was on the "intrinsically safe" technical review committee, and that trick had been part of the case against allowing any carbon-based resistors in intrinsically safe equipment.> General purpose small resistors can also initiate a fire. > Carbons are also prone to arcing. BTDT - suffice it to say the arc has no ability to limit the short current to a sane value.Nor a hot channel in a carbon composition or carbon film resistor.> Fusible resistors address some of these issues.But it does pay to read the data sheet carefully. -- Bill Sloman, Sydney
Reply by ●January 10, 20192019-01-10
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).
Reply by ●January 10, 20192019-01-10
If the fuse or fuse able resistor is connected to the mains it needs UL approval It will then be listed as a Critical Component Cheers Klaus
Reply by ●January 10, 20192019-01-10
On 09/01/19 21:43, Winfield Hill wrote:> I've placed 22-ohm 0805 resistors in series with the HV supply lines in the > latest version of my easy-to-build AMP-62 high-voltage amplifier board. This > version is often made with aggressive output current-limit values, e.g., 150mA. > But it's intended that such currents last only a short time, e.g., to rapidly > charge a capacitive load (during the rapid output slewing, the 22-ohms only > drops 3.3 volts). By comparison, the original version of this board, as > described in our x-Chapter book, has a wimpy 5mA current limit. But we assume > that users of this amplifier version who need its extreme slewing capability > understand the risks, when they select the 150mA current-limiting resistor > values. > > If the output were to be shorted, and draw 150mA from the supply, firstly it's > hoped the supply voltage will soon fall dramatically, due to its own say 20mA > current limit.Could you /ensure/ that happens?> Meanwhile the 22-ohm 0805 resistor would dissipate 0.5 watts, or > 4x its rated power, and hopefully after a while it would fail open. But if it > doesn't fail fast enough, one of the high-voltage MOSFETs, dissipating 40 watts, > might short. At that point the current would increase further, and 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.What's the *benefit* of not using a "proper" fuse or equivalent?
Reply by ●January 10, 20192019-01-10
"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message news:u18d3ep9k2k51emo173eeeii060tcr8c85@4ax.com...>> >>"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!! >> >>Tim > > I've connected 0805 resistors across the AC line. Try it. > > They usually just go PIP.Sure, it pops... How many hundreds of times was the transistor dead before it cleared? Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
Reply by ●January 10, 20192019-01-10
On 1/10/19 1:47 AM, Piotr Wyderski wrote:> Tim Williams wrote: > >> The transistor protects the fuse, not the other way around!! > > 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/YegLM > > ����Best regards, PiotrGood thing I wasn't drinking coffee! 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 10, 20192019-01-10
On Wednesday, January 9, 2019 at 9:53:56 PM UTC-5, Winfield Hill wrote:> George Herold wrote... > > > > OK no more x-chapter teases, unless I get to proof early copies*, > > or it's being published soon. > > I'm happy to send a draft of the HV amplifier section to anyone > who wants to look at it. It's a good example of the advanced > material in the x-Chapters.OK, I'd be happy to read it. (gherold-at-teachspin.com) The technical book i'm reading now is Feynman's gravity text... kinda hard slogging, so something a little lighter would be welcome.>(We're aiming to be out this year.)Oh dear, well now you've done it, don't you know that predicting how long something will take immediately causes God to multiply that time by pi. :^) George H.> > > -- > Thanks, > - Win
