Electronic fuse
in [Design]
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From: Jim Thompson on 4 Jul 2010 13:01 On Sun, 04 Jul 2010 08:50:15 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 3 Jul 2010 22:07:56 -0700 (PDT), dagmargoodboat(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. > >This circuit is perfectly serious in some situations. As a plus, it >sets off geezers who were taught never to do it, back in the days of >alloy diffused transistors. > >We do use polyfuses in situations like yours, but the surface-mount >ones have low voltage ratings and their trip current depends on the >pads and traces, the major heat sink mechanisms. The leaded parts are >generally better. > > >> >>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 > >The equivalent has been done with a mosfet as the pass device, in an >IC even as I recall. > >But why beta limit once, when you can do it twice? > >ftp://jjlarkin.lmi.net/Beta_Twice.JPG > >Geezeer-seizure for sure. > >John Why bother? You're beyond hope. Where's the product you used it in? With details... no pimpy hand-waving :-) ...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: dagmargoodboat on 4 Jul 2010 13:10 On Jul 4, 10:50 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.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. > > This circuit is perfectly serious in some situations. As a plus, it > sets off geezers who were taught never to do it, back in the days of > alloy diffused transistors. > > We do use polyfuses in situations like yours, but the surface-mount > ones have low voltage ratings and their trip current depends on the > pads and traces, the major heat sink mechanisms. The leaded parts are > generally better. > > > > > > >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 > > The equivalent has been done with a mosfet as the pass device, in an > IC even as I recall. > > But why beta limit once, when you can do it twice? > > ftp://jjlarkin.lmi.net/Beta_Twice.JPG > > Geezeer-seizure for sure. > > John That's hilarious! Yes, geezer seizure for sure! 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. -- Cheers, James Arthur
From: Dave Platt on 4 Jul 2010 13:38 >>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_Fuse.gif > >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. -- Dave Platt <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: Tim Williams on 4 Jul 2010 15:32 "Dave Platt" <dplatt(a)radagast.org> wrote in message news: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_Thing.png Again, could be simpler, blah blah. Rise time isn't a big deal for driving motors on and off, so the 4001 is fine, or you could replace each flip-flop with a pair of transistors (or even an SCR!). The pin count is about the same, and the 4001 saves on resistors. If you don't mind the Vbe tempco (for this, you don't), the LM311 can be replaced easily with a transistor, which would save a few pins. If you're partial to relays instead of FETs (or need to drive a higher voltage motor), you can do the same thing with relay logic, saving a few parts thanks to the DPDT contacts (as with your switch). Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 4 Jul 2010 15:44
On Sun, 4 Jul 2010 10:10:57 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: >On Jul 4, 10:50�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.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. >> >> This circuit is perfectly serious in some situations. As a plus, it >> sets off geezers who were taught never to do it, back in the days of >> alloy diffused transistors. >> >> We do use polyfuses in situations like yours, but the surface-mount >> ones have low voltage ratings and their trip current depends on the >> pads and traces, the major heat sink mechanisms. The leaded parts are >> generally better. >> >> >> >> >> >> >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 >> >> The equivalent has been done with a mosfet as the pass device, in an >> IC even as I recall. >> >> But why beta limit once, when you can do it twice? >> >> ftp://jjlarkin.lmi.net/Beta_Twice.JPG >> >> Geezeer-seizure for sure. >> >> John > >That's hilarious! Yes, geezer seizure for sure! > >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 |