Protection Circuit? - Page 1
Forum section: electronics / beginners

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 04:26:37 am
content: Hello all!I'm hoping someone could help me.I have a circuit that draws (according to the ammeter) 334mA max at 5volts. I want to set-up a resettable poly fuse, a diode, and a zener diode as a protection circuit. I am not sure on the amp rating or sizes I need.I want to protect the circuit in case the polarity of the input was reversed by accident, or if the voltage applied was over the 5 volts the circuit needs.One problem though, there is a sensor in the circuit that operates as low as 4.5volts, below that, it does not operate.I was thinking maybe a 5.6 or 5.1 zener diode, but not sure on the current rating or wattage.I know I need to use a shottky diode, because the forward voltage drop is much lower than a regular diode.The poly fuse will be a case size of 1812, and I was thinking maybe a 0.5amp rating?I was looking at this for the shottky diode.http://www.mouser.com/ProductDetail/ON-Semiconductor/NSVR0320MW2T1G/?qs=sGAEpiMZZMtQ8nqTKtFS%2fIMZb%2fvX%2fZJHlCuzGQtxqeU%3dI am not sure on the Zener though.I'd like to keep the footprint size of the components as SOD-323 or even smaller if I can. I may even change the Polyfuse to a 0805 depending on cost.Here is a picture of the circuit I was thinking. see attachment.Any help will be appreciated!Thanks in advance.

author: David_AVD
author_rank: Super Contributor
timestamp: April 02, 2013, 05:27:48 am
content: Without the series diode, you will still have some reverse polarity protection as the zener will be forward biased and the current will be limited by the polyswitch. I guess you could put the Schottky diode in parallel with the zener to take the brunt as it will have a lower Vf than the zener. Can your circuit tolerate a small reverse voltage?

author: smashedProton
author_rank: Frequent Contributor
timestamp: April 02, 2013, 05:37:18 am
content: A p channel mosfet sgould do the job

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 05:47:55 am
content: I am not sure david. It uses 3 ULN2003 IC's, 7 A3144 hall sensors, and and an NPN transistor (resistor biased), as well as various resistors and LED's. It's a fairly basic circuit for switching LED's on and off with an output for each hall sensor.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 06:03:46 am
content: smashed,I'm totally lost. I'm learning, but this electronic stuff is overwhelming.Why the Pch mosfet? Is it because no voltage drop like the diode? Will that solve both the overvoltage and the reverse polarity problem?I'm looking at mousers site, and not sure what I need to select on the filtering for the j channel mosfet.Any suggestions? I want to keep the cost down as well.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:24:36 am
content: Reverse voltage protection with a P-FETThis eliminates the voltage drop, leaving you with just the on resistance of the FET. Doesn't do anything for overvoltage. Nice little "circuit" (if you can even call it that), though.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 06:48:58 am
content: Thanks c4757pstill confused though.In the video, he said when the battery is first connected, the diode drops 1 volt across it, leaving 11 volts for the circuit. So, if I connect it up, I will have only 4 volts going to my circuit? That won't work.Also, I'm still confused on how to select the proper mosfet to make my circuit work. (if the above is NOT true)

author: David_AVD
author_rank: Super Contributor
timestamp: April 02, 2013, 08:51:23 am
content: Very shortly after the circuit is powered up, the MOSFET conducts and (almost) eliminates that 1V drop.

author: smashedProton
author_rank: Frequent Contributor
timestamp: April 02, 2013, 02:36:56 pm
content: It will be worth looking at Dave's multimeter protection video

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 04:24:04 pm
content: Okay, will this work? (see attachment)The data sheet for the MOSFET can be found herehttp://www.vishay.com/docs/63199/si2365eds.pdfIt's cheep, very cheap. 0.001 cents each through Mouser. But they aren't available to ship till middle of next month, but that's okay, I'm in no rush.I'm not sure if the zener diode is enough watts. If I go higher, it will require a SMA (do-214AC) case. Much bigger pad footprint.If this works, it will give me about 4.981v to the circuit? (ran a simulation on LT Spice using a mosfet with an Rds of .040 Ohm) That would be great!The simulation also showed a 11.687 volts when 12 volts was supplied. That shouldn't be correct?? It would ground out through the Zener Diode, blowing the fuse I thought? (I didn't have a fuse in place, I'm just assuming it would have blown the PolyFuse)

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 10:10:15 pm
content: Anyone have any advice on this circuit and chosen components? I'm designing the board currently on DipTrace, just need to make a BOM on the parts I hope to be using.BTW Dave, thanks for your videos. I've learned ALOT. I bought my Hacko FX-888, Flush Cutters, 0.2mm Solder with Flux! I downloaded (and currently using) DipTrace and started using SMD components, all because of you showing how easy they were to work with. For years I was struggling with soldering little things here and there. I never even knew what flux was or a decent soldering iron until I saw your videos. I was always using that THICK solder from places like HomeDepot, you know, the stuff the plumbers use for their pipes. Keep the videos coming Dave! They help people like me learn!Oh, btw Dave, if you're reading this. When I get my circuit design finished, I'll send you a kit and explain what all it does.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 10:20:23 pm
content: VGS(max)on that PFET is only ±8V. Also, I'd suggest putting the PTC right at the input. The more it protects the better, and the cost of moving it is zero. The chances of the FET shorting to the gate are slim, but definitely possible (especially with those ESD protection Zeners).Also, I'm a bit confused that you're specifying 4-16V in but have a 5.6V Zener straight across the rails. Which of those is wrong? A 5.6V Zener will trip the PTC just under 5.6V, so the circuit will not be running at 16V...

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 10:27:04 pm
content: But if I do that, and the voltage is too much for zener (>5.6v), it will ground out, blowing the fuse. Won't that then damage the MOSFET as well?and +-8 volts......That was the smallest I could find. Don't I need one that's at 5.6v or lower to activate the MOSFET? I tried reading up, I am just not understanding the mosfet stuff. Even looked at some videos, as suggested.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 10:28:26 pm
content: Quote from: Falcon69 on April 02, 2013, 04:24:04 pmRead the price again. It's not 0.001 cents, it's 0.001 dollars (0.1 cents), and that's if you buy 1000 of them.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 10:28:56 pm
content: I was saying 4-16v just in case someone had plugged in a power supply rated at 16v (usually they will either plug in a 5volt or a 12volt by accident) The circuit runs off of 5volts. All my LED's and other chips are set for a 5 volt supply source.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 10:30:23 pm
content: QuoteYa, that's what I meant, still pretty cheap

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 10:32:49 pm
content: Quote from: Falcon69 on April 02, 2013, 10:28:56 pm4-16V implies that it will function over that range. You need to clarify: What range does it operate over, and what range can it protect itself over?Quote from: Falcon69 on April 02, 2013, 10:27:04 pm±8V is the maximum voltage you can have on the gate before you damage the FET. "4-16V" is not within this range. Anything in excess of this maximum is likely to trip the ESD protection diodes and blow the shit out of the transistor. You're going to have to limit the voltage on the gate likethiswith a Zener and resistor, or find one with a higher rating.QuoteVGS(max)is the maximum allowed voltage, not the threshold voltage (the voltage where it turns on). That's a separate parameter.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 10:34:18 pm
content: In the datasheet, look at the "Transfer Characteristics" graph. That will tell you the voltage required to actually turn the transistor on (it's around 1V).

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 10:42:51 pm
content: okay, so place the PLC at the input of PWR, then the Zener diode across the pwr to the ground, then place the MOSFET. Find MOSFET with at least a 16vgs. That should make the circuit perform correctly?The Hall sensors operate at 4.5v to 24v, the ULN2003's operate at up to 50v.However, all my LED's (40 of them) I have on the circuit are set up with resistors for a 5 volt input source. So I need 5 volts to goto the circuit, less alittle for voltage drop across the MOSFET, of course. So that's why I want to set up a Zener Diode for over voltage protection, but, I don't want the Zener Diode, the MOSFET, or my circuit to fry if someone hooked up 12volts by mistake. I rather the PolyFuse blows, and then resets after some time.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 10:51:01 pm
content: Quote from: Falcon69 on April 02, 2013, 10:42:51 pmSounds good to me, as long as this is what you mean. Your description is a bit obtuse.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 11:18:11 pm
content: yes, exactly what I was thinking.I found two that's within my budget, and they have fairly low Rds. Which I think that means less voltage drop, more volts going to the circuit?http://www.mouser.com/ds/2/427/sia441dj-107049.pdfhttp://www.mouser.com/ds/2/427/si5419du-114911.pdfWill the 5.6v 550mW Zener be enough? That's the highest I could get in a small case. the next size up (SMA), looks like they make a 5w. I'd rather stay with the small if I it's sufficient enough. The Poly fuse is rated at .5amps, but trips at 1amp. My circuit pulls a total of 334mA when everything activated (according to the ammeter).

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 11:20:14 pm
content: Those FETs look fine - but be careful, they might be a pain in the ass to solder. How are you planning on assembling this?Quote from: Falcon69 on April 02, 2013, 11:18:11 pmWhat is the actual part number?

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 11:28:08 pm
content: http://www.nxp.com/documents/data_sheet/BZX84J_SER.pdfThis is the one I was looking at. If I could go lower on the specs, I can probably get cheaper, and maybe even smaller package

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 11:31:31 pm
content: Nice. That one has an IZSM(maximum non-repetitive peak Zener current) of 12A.I don't suspect you'll get 12A through that PTC before it trips.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 11:33:17 pm
content: Quote from: c4757p on April 02, 2013, 11:20:14 pmI made a reflow oven using a toaster oven and an Auburn Instruments PID Controller. It works great! Best money I've spent so far.Ahh, so I can possibly look for a lower Izsmand possibly find a cheaper and smaller chip? I will look, thank you! So Should I look for something that's like 5 amp or so?

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 11:36:14 pm
content: Quote from: Falcon69 on April 02, 2013, 11:33:17 pmNice.QuoteI wouldn't go much lower for a crowbar like that. Peak short-circuit currents can be high if you're discharging a filter cap. If you want to go lower, you should add a bit of series resistance to the input to keep the short circuit current from peaking above the maximum.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 11:39:32 pm
content: Great! thank you. I will go with the one I posted. And I just need to choose between those two FETs i posted? Prolly the smaller of the Rdswould be better, I'm guessing.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 11:42:38 pm
content: Yeah, pretty much the only parameter that matters here is Rds(on), as long as it won't blow up. Of course, there might not be any reason to go for lower resistance if the higher resistance one is cheaper - you should actually calculate how much current you need at what voltage and figure out what voltage drop is acceptable. Just blindly engineering something to be "good" ends up wasting a lot of unnecessary money.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 11:49:18 pm
content: ya, I understand. I'm just mainly worried about the hall sensors requiring minimum of 4.5volts. I'm not sure on total resistance, as it will change. The hall sensros will be anywhere from a couple feet away to 100 feet away from the control board. So the resistance of the wiring will come into play, though, I doubt it will be much. There's a $40 difference in price when ordering 500 of them. $40 isn't that much more for added voltage, if needed.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2013, 11:50:19 pm
content: Thank you for your help c4757p and others. I will redesign the board to fit these chips.

author: c4757p
author_rank: Super Contributor
timestamp: April 03, 2013, 12:00:50 am
content: You said the Hall effect sensors work up to 24V. If you supply them with 6V or so at the beginning of the cable, there's no way it will be down to 4.5V at the end. If you want a closely regulated voltage on them for some reason, supply 5.5V into the cable and stick a local LDO regulator on each one - but that's probably overkill. Just make sure to stick a capacitor on them at the end of the cable.

author: Falcon69
author_rank: Super Contributor
timestamp: April 03, 2013, 12:11:06 am
content: that won't work everytime. The object is to grab power from a USB wire from the computer CPU. BUt, most people aren't very elctronic savvy, so they may end up clipping the 12 volt one and connecting that instead. That's what I'm trying to protect the circuit from. So, if don't right, there is no 5.5 or even 6v input, only 5volt (or 12volt if you do it wrong)C, can you take a look at that data sheet again, the Si5419DU. I'm confused on the recommended pads. I'm guessing that the very big square is just copper for heat, and the small squares are all the actual pads? Or is the WHOLE thing for the pads, meaning, I need my stencil cut for the solder paste for the WHOLE thing, big and little squares.?

author: Falcon69
author_rank: Super Contributor
timestamp: April 03, 2013, 12:12:18 am
content: and yes, there is a capacitor on each circuit for the hall switches. I put them there because I was afraid of noise.

author: Falcon69
author_rank: Super Contributor
timestamp: April 03, 2013, 09:27:26 pm
content: Okay, I redrew it up. Is this correct now? I've included part numbers. I just want to double check before I send this thing off to have a prototype board made up.Again, circuit draws around a total of 334mA. It operates on 5volts (all resistors are set-up for 5volts)I want to protect the circuit from over voltage (if someone accidentally plugged in a 12v source for example) and I want to protect it from reverse polarity and also ESD. I don't want the thing blowing up chips when people (or myself) handle the finished board.Should I put in a diode between the ground and the zener diode? And, if so, a 1w, 3w, 5w? I don't want anything to fry or blow, except for the fuse.Should I have a capacitor in there somewhere? And if so, what size?I need as close to 5 volts I can to the circuit if someone was to plug in a 5 volt power source (for example, if they stole the power from a USB), so that's why the MOSFET.Also, can I get away with a 5.1v Zener? That's awfully close to the USB power source of 5 volts, and I know the voltage will fluctuate a little.Here are the data sheets.http://www.mouser.com/ds/2/427/si5419du-114911.pdfhttp://www.mouser.com/ds/2/302/BZX84J_SER-115414.pdfhttp://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_1812L.pdfThis fuse will be a 0.5A ratingI'm also concerned with the size of the MOSFET. Can it really be that small and do everything it says? I mean, it's really really tiny! (which is great!)Thanks again guys!

author: Falcon69
author_rank: Super Contributor
timestamp: May 03, 2013, 01:02:20 am
content: I got my circuit boards in, soldered them up, and did some testing.I have changed the resistor values for the LED's (from original schematic) so they aren't too bright on the panel LED"s and so that they are about the same light intensity on the switches. I hope that running them lower than the forward voltage of the LED will be okay. I'm guessing that will actually make the LED's last longer.The other thing, I went with a zener and schottky diode set-up for the over-voltage and reverse polarity protection. The schottky i selected for the reverse polarity protection is working great. The 5.6v Zener is not. It was only rated for 550mw. It blew the diode right away when I hooked up a 12 volt power source, which then blew the fuse, as it is suppose to. I ordered some more zeners from Mouser, and tried them. The 1W Zener blew right away, just like the 550mw. The 3W Zener is holding, however, the PTC fuse (resettable fuse) which is rated at 500mA and is suppose to trip at 1A, just gets hot, too hot to touch. It does not blow. But, with that 3W zener and 12v power source, when I click on the switch for the lamp test, the green LED's all dim. But, even with the 12volt source, it seems the LED's are all within the operating forward voltage. I'm assuming this is because 5.6v of that power source is grounding out through the Zener, only leaving roughly 6.4v for the circuit.So, something is wrong here. That fuse should be blowing, instead of getting hot. The entire circuit is pulling 260mA. Should I change the fuse to a 250mA hold current and a 500mA trip? Would that then make the 12volt power source blow the fuse through the 5.6v 3W Zener Diode? I'm not sure if the PTC fuse will work, since it is only rated for 250mA current hold, and the circuit draws just slightly above that?Actually, the 200mA PTC (400mA trip) fuse is cheaper, if that would work. The thing is, the circuit only draws about 172mA, until the test lamp switch is kicked on, then all the LED's go on, shooting the current draw to just alittle over 260mA. This switch doesn't stay on forever. It's only on momentarily, for a visual indication of which LED, if any, has gone bad.So,Questions.......1. Will running LED's at less than forward voltage of the LED bad for them?2. Can I run 260mA through a 200mA PTC fuse with a trip rating of 400mA and still be okay if the current is only momentary? (like, less then a couple minutes).3. If I use that fuse, will a 550mW 5.6v Zener diode then work properly, instead of blowing like it does with the 500mA PTC Fuse? (550mW Zener Diode is really small footprint)4. Using a 5.6v 3W (or 550mw if above question works) Zener Diode with cathode to fuse (200mA PTC Fuse) and anode to ground blow the fuse if 12v is supplied as the power source? Power to Fuse, than Fuse to circuit/cathode of Zener as in picture of first post.Any suggestions?Thanks for all your help guys.

author: Corporate666
author_rank: Unknown
timestamp: May 04, 2013, 08:51:25 am
content: Quote from: Falcon69 on May 03, 2013, 01:02:20 am1) No, no harm at all2) It depends. It's probably not good design practice. Between I-Hold and I-Trip is a gray area where the fuse will heat up gradually and either it's resistance will go up which will drop the current and prevent it blowing, or it will just blow very very slowly. It depends how close you are to I-Trip.3) To answer the question, you have to keep in mind how much energy it is going to take to heat up that fuse and cause it to blow.. and you have to remember that your diode is going to have to handle all that energy and then some (for derating purposes). I don't think the lower value fuse will trip fast enough to let you use a 550mW zener for overvoltage protection until the fuse trips.4) I would guess you would be OK with a 3W zener (don't have the motivation to do any calculations at the moment) - based on your results with the heftier fuse.Having said all that - I hate PTC fuses. I remember when I first learned about them and they seemed like a wonderfully easy solution to overcurrent problems - but I have found they have very specific applications and do not work in many cases. Your situation is one example of how tricky they can be to use. Another was a metal core PCB where the heat dissipation was so good that the fuse didn't blow even close to it's rated trip current. The problem with resettable fuses is that they are sooo slow to trip that by the time they do, most likely whatever you were trying to protect is fried already. The other problem is they are by nature temperature devices so their performance swings wildly over temperature. They work OK in a "sledgehammer" approach where you want to protect against huge overcurrents, but when you try to use them in the manner you are - you run into these kinds of problems, especially over temperature swings.I personally do not think that a zener and PTC fuse is a very good solution for overvoltage protection - certainly not in a commercial device. I would use a resistor divider on the voltage input fed into a comparator along with a voltage reference to control the base of a P-fet which will also give you reverse hookup protection, will generate almost no heat, and give excellent protection and very little voltage drop. If you are making a hobby device (I guess you are since you said you just have resistors on the LED's and run them right from 5V) then a PTC fuse/zener would be OK if you get the numbers worked out. But even then, I personally would go with the reference/comparator or I would use a chip that is designed for this work. They are small and inexpensive and readily available. Maxim makes a bunch specifically for USB overvoltage protection. Check out the MAX4840A.

author: Falcon69
author_rank: Super Contributor
timestamp: May 04, 2013, 05:16:25 pm
content: That Max chip would definitely work. I like it, however, at a price tag of $2.22 each (or .99 per 2500) is pretty steep for this design. Plus, add the $0.39 for the Mosfet plus added cost for the resistor and capacitor needed. That's going to be about $3.00 added per circuit. An additional $2.32 over the Zener/Schottky set-up. I ordered some 200mA PTC fuses. If those don't work, then I will need to go with the Maxim chip. Thank You Corporate666

author: codeboy2k
author_rank: Super Contributor
timestamp: May 05, 2013, 08:44:30 am
content: You don't need the MAXIM chip. Yes, a single chip specialized over-voltage solution can get expensive, and if you have the room for five or six additional SMT parts in SOT-23 and 0604... then I often use something like the following. I just put it together on LTSpice for you to see it in operation. If I am building something that needs over-voltage and reverse voltage protection I often use something like this and modify the values.The zener diode conducts when the voltage exceeds the zener voltage + 1 diode drop of Q1. When the zener diode conducts, it turns on Q3, which pulls away base current from Q4, turning it off, and thus turning off MOSFET Q2. R2 can set the current through the diode and thus the trip point. R2 should be of size at least 1210 (1/2 Watt), it will get hot at input voltages above 20V or so. R5 can be decreased to set the speed. The actual speed to trip depends on the gate capacitance, because it has to discharge through R5. The 10k value shown takes about 100us to turn off. 1K is 10us and 100 ohms is 1us. Choosing a lower R5 value will increase the quiescent current when not tripped, and since this is the normal operating mode, you want to pick a balance between tripping speed and wasted current. The advantage of this solution is it's really cheap, less than $0.50 in parts when ordered in quantity. It also doesn't use a power zener, like you needed a 3W zener. There is very little current and power wasted in the over-voltage circuit.You can improve the circuit with a PNP transistor to actively pull up the gate when it trips, and this will give a fast tripping speed and little to none wasted power.A PTC can still be used ahead of the first MOSFET Q1 for extra protection against short circuits, but it no longer functions as part of your over voltage protection. It's simply overcurrent protection, as it should be.Finally, on a last thought, you can also consider a voltage regulator to replace all these 5 or 6 parts needed for overvoltage protection. A jelly bean 7805 type regulator can be $0.50 to $0.80 and has the advantage that it doesn't shut your circuit down when there is an overvoltage, but instead regulates it and keeps on going. You then need to worry about heat and power dissipation. But sometimes a simple regulator can also work for overvoltage protection.(but if you use a voltage regulator, it will have input voltage limits, and if you still think your circuit might experience long-lived overvoltages above those limits, you should still put in overvoltage protection for voltages above the regulator's input limits -- and note that MOVs don't work here because MOVs are for transient overvoltages)EDIT: updated the circuit to respond in less than 500ns at overvoltage. This is also a good starting point for an over-current electronic fuse.Added the 500n zip files

author: Falcon69
author_rank: Super Contributor
timestamp: May 06, 2013, 06:18:09 am
content: Thanks Codeboy!The problem is, I don't think I have the footprint for that, but, I might be able to if I could use 0603 resistors and then everything else smaller then an SOT363 case. I also noticed, that the Si4403DY is discontinued, but Mouser has an Si4403BDY-T1-E3 listed, but at a quantity I need is .68 cent each. It has a fairly large footprint as well, a soic-8, and is also only a single, I'd need a dual or run two of those, according to your circuit (the first one). I don't have much space. I could enlarge the boards, but I'd rather not.Which of those two circuits is better? My circuit operates at 5volts. The most voltage it would probably see is 12volts. A computer power supply will probably be the supplied voltage, but just in case the 12volt supply is hooked up by accident, I don't want all my LED's blowing, or becoming waaaaaaaaaay to bright. The other circuits (hall effect sensor and ULN2003) can operate at 12 volts just fine. I have all the resistor values set up on my resistors and circuits for 5 volt power supply, NOT 12volts. The entire circuit draws a total of 260mA when all LED's are turned on.I'd like to try and keep the cost down as much as possible. If that means enlarging the boards slightly, I can probably do that, but would rather not.My circuit is designed for the DIY CNC machine builder. It's suppose to simplify the problem of adding multiple limit switches. It uses three ULN2003 Darlington Transistors IC's, in addition to the hall sensors and the LED's for visual indication of limit status.Corporate666, if I go with the maxim4840, the FLAG output (pin 3), can I connect that to the ground pin of an LED, then the power of the LED to a resistor then connect to power? If I read the directions right, The FLAG ouput supplies a low (or ground) output for 50ms. That would be perfect if I hook up a red LED and it lights for a brief moment, signaling that a voltage greater than 5.8 volts was supplied. I'm not understanding what it means by a pullup resistor connected to the FLAG output. It looks like I can just leave the FLAG output unconnected as well, but if I can hook up the LED for a visual effect of overvoltage, that would be kinda cool.Again, thanks for all your help guys

author: codeboy2k
author_rank: Super Contributor
timestamp: May 06, 2013, 08:17:00 am
content: You're welcome.Yes, it's good to weigh on the various choices regarding cost vs. amount of protection needed.1. You could just say it's designed for 5V and if someone hooks up 12 volts and it blows, well that's their own damn fault.2. you could shutdown as I have drawn the circuit. .. lots of circuitry required. zeners, mosfets, BJTs etc.. more space, more cost, more work for you3. you could regulate the 12V down to 5V and keep going, saying your device operates from 5-12V now.If you are only concerned for the LEDs then perhaps a simple option is to just regulate the supply to the LEDs, using a 5.1V zener regulator for those parts that can't run at between 5-12V. If everything else will operate as you intend them to, with any voltage between 5 - 12V , except the LEDs, then that seems like a simple solution.Just don't bother about over-voltage protection, but regulate the LED voltage, so your resistor values work at the zener voltage.Then say your device is designed for 5V but if they hook it up to 12 it will still work, or say 5-12V is the operating range. Doing it like this will probably save you the space and cost of the extra components needed for true over-voltage protection. You don't even need the second MOSFET then, just the first one.

author: Corporate666
author_rank: Unknown
timestamp: May 06, 2013, 09:21:40 pm
content: Quote from: Falcon69 on May 06, 2013, 06:18:09 amThe circuit that Codeboy gave you is what I was talking about when I said you could use a divider/transistor to turn off the FET (he used a zener which is fine too). That is the cheap way at the cost of size. The Max chip does the same thing in a tiny package, but of course you pay for it in terms of cost.As for the FLAG output, I just looked at the datasheet, and the FLAG line on the chip is asserted low by the device whenever under or over voltage occurs. The 50ms time it mentions is how long the FLAG line will stay low after the under/over voltage condition disappears. The FLAG line is open drain, so it will sink current - you could connect an LED to it, but the problem would be that line is only going to be low when you have too little or too much voltage, and since you would need to connect your LED to power before the Max chip, you would not know what value resistor to use. You could use a resistor assuming 12V and it would just be dimmer on undervoltage... or you could put a regulator on the input (but you could just use a regulator to accomodate over voltages anyway), or you could put an LED driver in that handles a range of input voltages, which is a bit of an elaborate design just to have an LED for over voltage.If you really want the LED, you can buy an SPX5205 regulator which, IIRC has a Vin range up to 16 volts and can be had for $0.15. It will not support a lot of current, and it comes in a very small package SOT23-5, but the regulator plus one cap is all you need for a regulated 3V output, then a small resistor and your LED-for-over-voltage idea would work.The same LED idea would be pretty easy to add to Codeboy's circuit too, though. Or you could implement an "error" LED separately (and easily) from the FLAG line of the chip as well.