Electronic fuse
in [Design]
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From: Jim Thompson on 4 Jul 2010 20:46 On Sun, 04 Jul 2010 17:22:34 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 4 Jul 2010 16:05:10 -0700 (PDT), dagmargoodboat(a)yahoo.com >wrote: > >>On Jul 4, 5:11�pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- >>Web-Site.com> wrote: >>> On Sat, 3 Jul 2010 22:07:56 -0700 (PDT), dagmargoodb...(a)yahoo.com >>> wrote: >>> >>> >>> >>> >I need a current limiter, so I looked at poly fuses. �They're pretty >>> >sad. �And regular fuses are awfully crude--make one mistake and you >>> >have to desolder a fuse? �That's no fun. >>> >>> >So I was thinking about the Larksonian current limiter... >>> >>> >Fig. 1 � � (View in Courier font) >>> >====== >>> > � � � � � � Q1 >>> > �>-------. � � .-------> >>> > � � � � � V � / >>> > � � � � �------- >>> > � � � � � � | >>> > � � � � � � | >>> > � � � � � � | >>> > � � � � � �.-. >>> > � � � � � �| |R1 >>> > � � � � � �| | >>> > � � � � � �'-' >>> > � � � � � � | >>> > � � � � � � | >>> > � � � � � �=== >>> >>> >...which was offered here as kind of a half-serious lark by a certain >>> >John Larkin. >>> >>> >As John pointed out, Q1 can be a 2:1 Hfe-binned part, giving a 2:1 >>> >limit tolerance, which isn't too bad for a non-critical application. >>> >>> > �i.limit ~= Hfe * i.b >>> >>> >So, I'm supplying a little +5V to the real world via a small >>> >connector, to communicate with and power a small external device. �I'm >>> >not sure what the draw is, maybe 50-100mA, max., but I do want to >>> >supply the full +5V and not a lot less--the voltage drop should be >>> >low. >>> >>> >John's circuit's tolerances are fine--I'd set it to 200mA or so, and >>> >get 150-300mA. �That's great. �But if the output's shorted, that's >>> >possibly 5v x 300mA--too much dissipation for Q1. >>> >>> >So I added foldback-- >>> >>> >Fig. 2 >>> >====== >>> > � � � � � � �Q2 >>> > �>--+-----. � � �.-------+----> >>> > � � | � � �V � �/ � � � �| >>> > � � | � � -------- � � � | >>> > � � | � � � �| � � � � � | >>> > � � '>|Q3 � �| � � �R2 � | >>> > � � � |------|----/\/\/--' >>> > � � �/| � � �| >>> > � � | � � � �| >>> > � � '--------+ >>> > � � � � � � �| >>> > � � � � � � .-. >>> > � � � � � � | |R3 >>> > � � � � � � | | >>> > � � � � � � '-' >>> > � � � � � � �| >>> > � � � � � � �| >>> > � � � � � � === >>> >>> >There, that's better. �If Q2 ever desats, Q3 comes on, robs Q2's base >>> >drive, and the output collapses. �Once shorted, an idling current >>> >flows through Q3 e-b and R2 to the load, so the thing restarts once >>> >the short is removed. >>> >>> >(The classic foldback uses a divider from Q2(c) to GND, tap goes to >>> >Q3(b). �I didn't do that here--we don't need that much of a hair- >>> >trigger.) >>> >>> >Hmmmm. >>> >>> >It's pretty fast. �Maybe a cap to slow down Q3, so we can tolerate a >>> >brief spike without triggering... >>> >>> >Fig. 3 >>> >====== >>> > � � � � � � � � �Q4 >>> > �>--+-----+---. � � �.--------+----> >>> > � � | � � | � �V � �/ � � � � | >>> > � � | C1 --- �-------- � � � �| >>> > � � | � �--- � � �| � � � � � | >>> > � � '>| � | � � � | � � R2 � �| >>> > � Q5 �|---+-------|---/\/\/---' >>> > � � �/| � � � � � | >>> > � � | � � � � � � | >>> > � � '-------------+ >>> > � � � � � � � � � | >>> > � � � � � � � � �.-. >>> > � � � � � � � � �| |R3 >>> > � � � � � � � � �| | >>> > � � � � � � � � �'-' >>> > � � � � � � � � � | >>> > � � � � � � � � �=== >>> >>> >Hmmmmmm. >>> >>> >--James Arthur >>> >>> Or do it right... >>> >>> http://analog-innovations.com/SED/CurrentSense.pdf >> >> >>That logic-output current-sensor's pretty close technique-wise to my >>electronic circuit-breaker of Fig. 5 above, except for the method of >>generating the less-than-Vbe offset voltage for the threshold. That's >>cute. I recognize it as an old analog IC trick, but I've never done >>it. I like the hysteresis too. > >It just doesn't do the electronic fuse function. > >John Gaw-w-w-wd Larkin, you are a nit-picking old hen. Anyone with half their brain tied behind their back can add the pass device... to something I posted here in 2005, then again in 2006. Wait-a-minute, I guess _you_ can't ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Obama: A reincarnation of Nixon, narcissistically posing in politically-correct black-face, but with fewer scruples.
From: Grant on 4 Jul 2010 22:09 On Sun, 04 Jul 2010 17:14:11 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 4 Jul 2010 15:28:33 -0700 (PDT), dagmargoodboat(a)yahoo.com >wrote: > >>On Jul 4, 2:44 pm, John Larkin >><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> On Sun, 4 Jul 2010 10:10:57 -0700 (PDT), dagmargoodb...(a)yahoo.com >>> wrote: >> >><snip> >> >>> >Meanwhile I was going exactly the opposite way, trying to minimize the >>> >transistor dependencies. I was just looking at some BJT datasheets. >>> >The main problem is d(Hfe)/dt. >>> >>> >Here's a compromise: >>> >>> >Fig. 6 >>> >====== >>> > Q60 R60 >>> > >--+------+---. .--/\/\/---+---> >>> > | | V / | >>> > | C60 --- -------- | >>> > | --- | | >>> > '>| | | R61 | >>> > Q61 |----+-------|----/\/\/----' >>> > /| | >>> > | | >>> > '--------------+ >>> > | >>> > .-. >>> > | | R62 >>> > | | >>> > '-' >>> > | >>> > === >>> >>> >It's simple, self-resetting, drifty and crude, but R60 tames the >>> >transistor dependencies considerably. >>> >>> >To keep droop reasonable I'd set the limit waayyy high, so the 0.6v >>> >trigger point doesn't happen until the load is already majorly >>> >pathological. >>> >>> If you're willing to put a sense resistor in the current path, well, >>> you're cheating. You may as well do a vanilla 2-transistor foldback >>> current limiter. Adding a schottky diode can reduce the voltage loss >>> to a few tenths. >>> >>> John >> >>By "vanilla" do you mean an emitter-follower with a sense transistor >>that robs its base-drive? >> >>James > >Right. Add a schottky to take away some of the Vbe requirement, and a >couple more resistors to add foldback. > >Or go with an opamp and a small-value sense resistor, and a p-channel >fet. > >Or maybe one of these... > >ftp://jjlarkin.lmi.net/Goofy_Ilims.JPG > > >The polyfuse is looking better and better. Except they take many seconds to reset? But my experience is just the one for rotary tool. Certainly better than letting the smoke out when one stalls the drill or some heavier work tool. > >Foldbacks can have strange behavior with some non-resistive loads. The polyfuse is foldback too (less than half the trip current to reset), might have similar problems you see with foldback? >That's why a shutdown with periodic retry is good. > >Hey, have a uP port pin turn on a series PNP or pfet, and use a spare >ADC channel to digitize the load voltage. Then do the math. I'm doing >some fairly complex shutdowns in software on the 4-20 mA gadget we're >doing... software lowpass filtering, trip thresholds, automatic retry, >stuff like that. Another thing I like to do is measure power fet >voltage and current and heatsink temp, run a realtime simulation of >thermal dynamics, and shut down based on junction temperature. Well, all that is tempting to add, once you have the smarts onboard :) Grant.
From: dagmargoodboat on 5 Jul 2010 00:45 On Jul 4, 7:46 pm, Jim Thompson wrote: > John Larkin wrote: > >dagmargoodb...(a)yahoo.com wrote: > > >>On Jul 4, 5:11 pm, Jim Thompson wrote: > > >>> Or do it right... > > >>>http://analog-innovations.com/SED/CurrentSense.pdf > > >>That logic-output current-sensor's pretty close technique-wise to my > >>electronic circuit-breaker of Fig. 5 above, except for the method of > >>generating the less-than-Vbe offset voltage for the threshold. That's > >>cute. I recognize it as an old analog IC trick, but I've never done > >>it. I like the hysteresis too. > > >It just doesn't do the electronic fuse function. > > >John > > Gaw-w-w-wd Larkin, you are a nit-picking old hen. Anyone with half > their brain tied behind their back can add the pass device... to > something I posted here in 2005, then again in 2006. Wait-a-minute, I > guess _you_ can't ;-) Here's the fuse implementation (below). It's identical to my Fig. 5, except that I got the threshold with a two-resistor divider on Q71(e), whereas you saved a resistor with the aforementioned trick on Q72 to get a drop for Q71(b) that's less than Vbe. Same result. You had a hysteresis resistor but that doesn't make any sense in this usage, so I killed it. I also had to update yours to straddle the switch, like mine, so that it can stay folded-back when the output's hard-shorted, and added the cap so it can tolerate spikes. I think the new guy has the potential to latch on start-up; the fix is the same as before. Fig. 7 ====== Q70 R71 >--+---------+-----. .---/\/\/---+----> | | V / | | | -------- | | C71 --- | | | --- | '>| | | | Q72 |--. | | | /| | '>| | | | | Q71 |-------+---------|--------------+ | /| | | | | | .-. | | | R72 | | | | | | | | | | '-' | | | | | '-------------------+ +----' | | .-. .-. | |R73 | | R74 | | | | '-' '-' | | === === So, scratch one resistor off the parts list, performance is the same, and 253 green jobs were created or saved! -- Cheers, James Arthur
From: dagmargoodboat on 5 Jul 2010 01:52 On Jul 4, 12:38 pm, dpl...(a)radagast.org (Dave Platt) wrote: > >>I need a current limiter, so I looked at poly fuses. They're pretty > >> sad. And regular fuses are awfully crude--make one mistake and you > >> have to desolder a fuse? That's no fun. > >http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Electric_F... > > >Of course, this is a latching type, so you want to slow it down with a base-emitter cap for surges. > > Here's my own version: > > http://www.radagast.org/~dplatt/hamradio/Limiter-1.pdf > > It's shown in-circuit in its intended application - hooked up to a > mobile "screwdriver" antenna's motor, to act as an automatic shutoff > when the antenna reaches the top or bottom of its travel and > starts to clutch-stall. This circuit's behavior is a bit like a > polyfuse - it's tolerant to short overcurrent surges, and it's > self-resetting - but it's somewhat easier to adjust the current > set-point, and it's far less temperature-sensitive than a polyfuse. > > Unlike the Circuits_2008 version, mine can be treated as a two-terminal > device - it doesn't need a separate ground connection. It does have a > significant voltage drop (between 2 Vbe and 3 Vbe) which may make it > unsuitable for certain applications. That's cute. It trips & latches to a lower current, which it holds. How does it reset itself? The holding current to the latch looks robust--just glancing it looks to me like it'd stay latched until the power's interrupted. -- Cheers, James Arthur
From: dagmargoodboat on 5 Jul 2010 01:55
On Jul 4, 2:32 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > "Dave Platt" <dpl...(a)radagast.org> wrote in messagenews:1ht6g7-am7.ln1(a)radagast.org... > > It's shown in-circuit in its intended application - hooked up to a > > mobile "screwdriver" antenna's motor, to act as an automatic shutoff > > when the antenna reaches the top or bottom of its travel and > > starts to clutch-stall. This circuit's behavior is a bit like a > > polyfuse - it's tolerant to short overcurrent surges, and it's > > self-resetting - but it's somewhat easier to adjust the current > > set-point, and it's far less temperature-sensitive than a polyfuse. > > Ahh -- I drew one of those for an automatic window drive. I never tested it, but apparently the subject built it and it worked first time. I like it when that happens.http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Motor_Thin... Spike-zilla! (How do you avoid big shoot-through currents? High Vth FETs?) -- Cheers, James Arthur |