Introducing another current-limiting switch - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2016, 03:06:59 am
content: Some time ago, I designed and built this:http://seventransistorlabs.com/Fuse/Images/Fuse2.jpgI'm preparing a new model, this time with accurate current limiting, a longer duration, thermally controlled trip, and auto-retry. It's basically a built-up version of a "protected MOSFET" switch.Render:Upsides:- All discrete design, easy(?) to see the operation of- Input (on/off) and output (fault) connector- Nominal 24V 5A load capacity; basic design is scalable to any voltage and currentDownsides:- Not isolated- Much larger than commercially available parts of same nominal rating- Relatively high power consumption (12V logic supply, ~8mA)But, I'm thinking this:Once scaled up to, say, 400V and 30A, not only will it hugely surpass any integrated part's V/I switching capacity: it will then be usable for prototype, EV and industrial switching applications, substituting for complicated and bulky capacitor-precharge and contactor circuits!Comments?Tim

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 16, 2016, 02:50:37 am
content: As it's targeted for prototype use, replace P2 with a Phoenix connector like P1. Perhaps add an extra pin or use a 0.150 instead of a 0.200 if they need to be differentiated. That allows screw-terminal connectors to be used instead of the crimp connector.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2016, 10:27:39 am
content: Hmm, not a bad idea.Y'know, they make screw terminals pretty small, don't they..?Or even spring cage terminals:http://media.digikey.com/pdf/Data%20Sheets/Phoenix%20Contact%20PDFs/COMBICON%20Spring-Cage%20PCB%20Term.%20Blocks.pdfLooks like 3.5mm might be the smallest pitch that's practical and available. Which isn't too bad. It'll fit between the screws. Think I still have to grow the board, but I don't have any particular requirements there yet, aside from "make it small".And pluggables are great, since you can insert a screw/spring retention housing or a crimped plug.I really don't mind the Molex SL connectors, even for proto. There might even be preloaded housings (socket to loose wires?) out there, or you can still fit a generic (no key, latch) header socket into there. (Which is intended: I've put \EN by GND, so you can jumper it always-on if needed.)There's always plain bare pin header, but I don't really like using nonpolarized stuff when VCC/GND is available (which is the case, here).Tim

author: ZeTeX
author_rank: Frequent Contributor
timestamp: September 16, 2016, 10:40:03 pm
content: That's nice PCB, but what does it actually do?EDIT: "With a response time as low as 5?s, this electronic fuse can provide current limiting and fault protection for electronic circuits, especially in development, research or laboratory environments, where the circuitry being tested may not yet have protection circuitry incorporated."Now I know what it is, it is basically a smart fuse isn't it?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2016, 04:42:10 am
content: Yes, fast acting and (in this case) accurate inrush current limiting.Handy part is, it's fast enough it can prevent destroying transistors. Rather than the other way around, where the $10 transistor protects the $0.10 fuse.Tim

author: batteksystem
author_rank: Regular Contributor
timestamp: September 17, 2016, 10:01:25 am
content: How to adjust the current limit? If there is a digital interface like I2C would make the programming job a lot easier.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2016, 03:03:17 pm
content: Fixed at the moment.I'm curious, what applications would you have for an adjustable limiting switch?Tim

author: ajb
author_rank: Super Contributor
timestamp: September 17, 2016, 06:05:36 pm
content: It seems like if you've gone to the trouble to design a discrete electronic fuse it'd be simple enough to make the setpoint selectable at least. Solder jumpers to select from a few fixed options, maybe, or a trimmer (with a test point for the reference voltage so you can set it accurately without trial and error!). That gives a good advantage over a lot of the integrated overcurrent-protected switches that are out there, which often have rather poorly-defined fixed trip points, especially at the higher current/voltage levels.I wouldn't think it's worth putting an i2c device on board or anything, but maybe bring out a set point pin so the user can supply a programming voltage with an external DAC or pot. It would also be nice to have a current sense output for monitoring.Oh, and a couple of status LEDs would be useful, too.

author: batteksystem
author_rank: Regular Contributor
timestamp: September 17, 2016, 11:33:48 pm
content: Quote from: T3sl4co1l on September 17, 2016, 03:03:17 pmFor Automotive use, we allow the user to set adjustable limiting switch to suit different cable gauge. The idea is not to let too much current passing through a wire in the harness, otherwise it may melt and mess up the whole harness, which is expensive to repair. I was using smart MOSFET and only need to read ADC input from MCU I control.

author: nctnico
author_rank: Super Contributor
timestamp: September 17, 2016, 11:42:24 pm
content: Quote from: T3sl4co1l on September 17, 2016, 04:42:10 amBut where do you dump the inrush energy? Surely at higher voltages the MOSFETs won't do you any good due to the SOA limit.

author: batteksystem
author_rank: Regular Contributor
timestamp: September 17, 2016, 11:49:15 pm
content: Quote from: nctnico on September 17, 2016, 11:42:24 pmProbably some of these diodes are TVS?

author: langwadt
author_rank: Super Contributor
timestamp: September 18, 2016, 12:28:59 am
content: Quote from: T3sl4co1l on September 17, 2016, 04:42:10 amI've done something similar on AC with back to back FETs, to protect a number of triacs

author: rx8pilot
author_rank: Super Contributor
timestamp: September 18, 2016, 12:39:12 am
content: Quote from: nctnico on September 17, 2016, 11:42:24 pmI was wondering the same thing. I have some similar designs based on IC's from Linear and the SOA requirements are serious. I ended up using a giant MOSFET for a 10A nominal load. Pulling the FET into the linear region can only happen for a very short period. Heat sinks don't help either since they are too slow. The FET is destroyed before the outside even gets warm.I like the discreet design. The integrated solutions insulate me from the real challenges. Would be fun to learn the details of this circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2016, 05:41:12 am
content: It sounds like people have different ideas about how much power and energy a thing like this can absorb. Let me explain.Protecting wiring: this requires a tremendous amount of energy, to even begin to be a hazard! I would recommend a device with an equally high trip energy, like a polyfuse. (Or if repetitive action is needed -- 10k's of cycles rather than 100s -- a ceramic disk type PTC. They are out there -- unfortunately no one categorizes them separately from polyfuses, so they are hard to search for. Vishay and TDK, among others, have lines of these.)Motor inrush: medium energy. Motors are just wire, again, but the wire is smaller and piled up internally, so it can heat up faster. You might also want an active current limit -- not just a fault-clearing action -- to limit locked-rotor current (LRA) or startup/inrush.This might be for several reasons in turn: the mechanicals might occasionally be stalled, and the motor needs to tolerate that continuously (it delivers limited torque, without burning up); improving life/reliability by limiting surge current and peak torque; reducing supply droop or powering a relatively beefy motor from a supply of limited capacity; etc.For motor limiting, the dissipation might be high, but not like fault current. A semiconductor (or a few) with heatsink can endure this indefinitely. (Or even better, a proper switching motor controller could be used, saving all that power.)Electronic inrush: this is usually charging capacitors. The surge current (unlimited) can be enormous! That tiny little spark, when you connect a power supply to an electrolytic capacitor, can be over a hundred amperes! (Rough calculation: 0.1ESR, typical of say 1000uF, charged from a 12V car battery, will easily deliver a peak of 12V / 0.1= 120A. (This will decay over a few hundred microseconds, and won't exhibit ringing unless the series inductance is over 10uH, which would be quite a length of hookup wire for a car battery.)So next time you make sparks off a rectifier, go easy on the poor things! It's so easy to think that a circuit rated for 1A simply won't do much more, but this is how things die, very easily; and that most diodes can endure that kind of fault current without thinking, is a testament to how robust they are. Transistors aren't so robust, though!And also, that sparks are very low resistance, and an innocent electrolytic capacitor can deliver huge gulps of current when forced to!Anyway, capacitor inrush is the prime application of these circuits. My earlier fuse device works very nicely to protect devices (under 10 microseconds!), but very poorly to charge capacitors: I can sit there tapping on the "on" button, and each pulse delivers only a tiny bit of charge. If the load has resistance, the charge leaks away and I'll never get anywhere! I have to use two in parallel, one with a current limiting resistor, to start such a load.With active current limit, the pulse duration can be much longer, and the delivered charge, much higher. But the power dissipation is much higher, too, so it needs to be carefully limited.This circuit monitors voltage drop and temperature, so:1. If the load is inductive (like a motor), there will be some milliseconds spent in saturation, charging up that inductance. This doesn't count against the fault timing -- because, why should it?2. If the load is marginal, like an incandescent light bulb, the circuit will go into current limiting at first, then saturate as the lamp heats up. As long as it heats up fast enough, the limiter will go back into saturation without switching off. No problem!3. If the load is faulty, the circuit will draw current limit at full voltage drop, dissipating a lot of power, and switching off quickly.Tim