2N7000 strange behaviour - Page 1
Forum section: electronics / beginners

author: igeorge
author_rank: Contributor
timestamp: April 12, 2017, 10:34:06 pm
content: I want to use a 2N7000 as a switch to protect my circuit for over voltage.I have a scenario when i feed 5 volts and everything should be fine.But is i feed 24vdc the fet should start conducting and blow the fuse.On data sheet of 2N7000 trigger si 0.8 to 3 vdc.I attached 2 pictures which should explain better.Even if i make GS 1.92 volts, the fet stay open and the fuse does not blow.I need help as it is first time when i work with Mosfets.

author: Audioguru
author_rank: Super Contributor
timestamp: April 12, 2017, 10:58:51 pm
content: Which will blow first, the fuse or the Mosfet? I think the Mosfet with its absolute maximum allowed continuous current of only 280mA will blow much faster than the fuse.The datasheet for the 2N7000 Mosfet shows that some sensitive ones conduct 1mA when the gate-source voltage is 0.8V and others that are not sensitive conduct 1mA when the gate-source voltage is as much as 3V. The gate voltage requirement changes when the temperature changes and when the Mosfet is replaced so it cannot be used as a comparator like you have. All 2N7000 Mosfets partially turn on when their gate-source voltage is 4.5V and all of them completely turn on when their gate-source voltage is 10V and they are not hot.You need a 5V reference and a comparator circuit to provide 10V to the gate ofa power Mosfetwhen the supply voltage is higher than 5V. Since the inputs of a comparator do not work as high as its supply voltage then use a lower reference voltage, maybe 1V and use a voltage divider to feed the scaled down supply voltage to the other input of the comparator.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2017, 11:06:55 pm
content: You should use a crowbar circuit, which uses a zener diode and an SCR to short circuit the supply and blow the fuse.https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/over-voltage-protection.php

author: timb
author_rank: Super Contributor
timestamp: April 12, 2017, 11:09:32 pm
content: What you're after is called a crowbar circuit and it's traditionally done with SCRs, as they latch. However, if you want to use a MOSFET, try it with a Zener instead of a voltage divider:In this circuit I used a 12V Zener, but you could go with a 20V one if you want. Basically it will trigger at the Zener voltage + the N-FET's threshold voltage. So if your FET has a threshold voltage of 5V and you want the crowbar to fire at around 25V you'd use a 20V Zener. (Ideally you'd use maybe an 18V Zener to give you a bit of margin.)In this image you can see it in action:The fuse blows about 100ms after the crowbar triggers.You may also want to either add a resistor to your input to limit the current to about twice what the fuse is rated for (to protect the MOSFET) *or* (ideally) use a large MOSFET rated for the pulse current of your source.(That is to say, if your source can put out 10A during a short circuit, make sure your FET has a pulse rating of at least 10A. Keep in mind the pulse current rating is different from the sustained current rating; it may only be rated for 1A sustained current, but can handle ten to one hundred times that for short durations [commonly 100ms max]. Remember, even a fast blow fuse can take 100ms or more to open, so it doesn't hurt to generously derate the MOSFET!)

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2017, 11:18:04 pm
content: Quote from: timb on April 12, 2017, 11:09:32 pmI wouldn't recommend a MOSFET at all for a crowbar or any form of overvoltage protection. Power MOSFETs typically aren't specified for analogue operation, only as a switch which either on or off and the gate threshold voltage varies considerably. If the zener doesn't have a high enough power rating on its own, then add a BJT to it.

author: timb
author_rank: Super Contributor
timestamp: April 12, 2017, 11:21:21 pm
content: Quote from: Hero999 on April 12, 2017, 11:18:04 pmAs a crowbar it *is* working as a switch. The normal working voltage (5V) of the circuit is *well* under the Zener's turn-on voltage, so it shouldn't really be an issue. In a case like this, the circuit shouldn't be hovering around the threshold voltage, so I don't see an issue. It should be on or off, period.Edit: Of course there won't be a sharp turn on, like with an SCR. The MOSFET will turn on over a couple of milliseconds as the voltage rises and the MOSFET's threshold voltage is overcome, but that's no different than how a MOSFET-as-a-Switch normally operates (if the dV/dT isn't super fast there's a "soft knee" to the turn on curve).

author: igeorge
author_rank: Contributor
timestamp: April 13, 2017, 12:21:20 am
content: Thank you everybody for help and suggestions.For me Analog is a strange language.I know the basic, but i am focused mostly on Microprocessors.I know i did a bo bo with the mosfet selection, but as i said is first time i touch them.Timb, i want to ask you two questions.First, is what software you use for simulation.I use Proteus, but is not the greatest.I use it because i can load a schematic an processor hex file and simulate it there.For rest, is kindergarten.I like your and i like to try it.Now, about the circuit. The power supply is short circuit protected at 3A. Max voltage is 24VDC. Can you suggest me a through hole fet that can resist and i can try it before i put it on the board.My problem i that during the assembly process in the plant, the electrician wire some times the board at 24vdc , instead of 5 vdc.Also can you suggest me the other parts number to use, like the zenner, or value of other components.I know i ask the moon, but i need help this time.Thanks again to all for helm.

author: timb
author_rank: Super Contributor
timestamp: April 13, 2017, 11:58:31 am
content: Quote from: igeorge on April 13, 2017, 12:21:20 amFor normal analog simulation I use (and recommend) LTspice. There's a bit of a learning curve, but there's plenty of videos and tutorials online to get you started. The UI isn't much to look at, but it's got one of the best SPICE engines in the world (and certainly the best free one).I also use iCircuit quite a bit (which is what's in my images). It's only a couple of dollars and runs on iOS, Mac and Windows. It's got a fairly basic (though complete) simulation engine, but I find it nice for quickly sanity checking analog circuits. It also simulates digital parts as well (basic logic gates and a handful of 74-series and 4000-series logic). The nice thing is the simulation runs in real time and it lets you visualize current flow and magnitude through your circuit.I assume your FET needs to be through hole, or is SMD OK? I can give you part numbers for a couple good candidates once I know.You should be able to pick out the Zener yourself. There's nothing special about it; any 1W 5% Zener of the appropriate voltage rating will suffice.

author: igeorge
author_rank: Contributor
timestamp: April 13, 2017, 12:32:47 pm
content: Thank you Timb,I use also LT Spice , but like you said it needs time to get use to it.I use it for small blocks of the whole circuit , just to make sure they works.I found yesterday Icircuit, and i have to install it on my Chrome browser.I do not have a pad, and my phone screen is too small.The Mosfet i will use a through hole for start.When you pickup the Mosfet , please suggest me the voltage for zenner.Does the 4.7k stay the same ?Sorry for questions but this is not MikroBasic or MikroC, then i am lost.For future, i will put some time aside to start exploring at least the basic analogThank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2017, 08:44:01 pm
content: You can use a MOSFET to blow a fuse, sure.It needs to be fuckoff massive, like, a couple milliohms Rds(on). You also need a schmitt trigger circuit: this is not optional, as the zener-dropping circuit will simply shunt regulate and vaporize the transistor.If a shunt regulator was sufficient, you should choose a TVS diode instead: they are much faster, cheaper per peak watt capability, and much sharper (the voltage is limited to a modest value, guaranteed in the datasheet).An SCR is much cheaper per peak ampere of capacity, has a much lower voltage drop under surge conditions, and is latching all its own (though you should still have a schmitt trigger circuit, so it fires abruptly).Remember that surges deliver currents of tens or hundreds of amperes, and fuses need tens to thousands of amperes to blow. And remember that, even under such fault conditions, fuses take tens of milliseconds to blow, while the poor 2N7000 expires in ten microseconds.Tim

author: exoticelectron
author_rank: Regular Contributor
timestamp: April 13, 2017, 08:57:37 pm
content: A 0.300ma fast blow fuse from bussman will blow in 100ms at 800ma.http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyBK-AGC-1-2I dont think there is anything wrong with your circuit from a topology standpoint. Just get a different mosfet that can definitely handle the crowbar current and will turn on enough with the VGS you can supply.Also, you need to make sure the mosfet does not go into a linear region at some intermediate point between your normal (5V) and fault (24V) situation or it will destroy even a high power mosfet. A comparator would make sense.Looks like a fun project!

author: igeorge
author_rank: Contributor
timestamp: April 13, 2017, 11:00:00 pm
content: To T3sl4co1lMaybe you suggest like here ? I used 3 diodes in parallel for REVERSE VOLTAGE PROTECTION 1N5819 and a 6 volts TVS SMAJ6.To exoticelectronI mentioned before that in rush i did a mistake and i did not read properly the data sheet, and i end up with 2N7000, but what is the suggested Through hole mosfet i can use ?I get lost reading the parameters because it is first time i use mosfet, so what i need for my project. Do you have a part number ?Also, how and where i should find the trigger voltage ? If i trigger it on, do i need to read any other parameters to keep it on until the fuse blow(latch it) ? My power supply, either 5 or 24VDC is short circuit protected, and the max current is 6A in the big one.

author: exoticelectron
author_rank: Regular Contributor
timestamp: April 13, 2017, 11:11:57 pm
content: Quote from: igeorge on April 13, 2017, 11:00:00 pmYour divider + the mosfet Vgs to Igs characteristics are acting as your comparator right now..and a very poor one that will be destroyed by a slow rising 24VDC. Replace the divider with a comparator circuit that drive the mosfet gate.For the mosfet, find something with a low enough RdsOn that it will not dissipate anywhere near enough power to hurt itself, even if the fuse doesn't blow, continuously. Also, it needs to have a low enough Vgs @ Igs relationship so that its fully on at the voltage your comparator can drive it to. Uhhhhh other than that pick a package and price. Digikey search engine is your friend here. Oh probably make sure its Vds is well above 24VDC tee heeDoesnt have to be through hole...see what you can come up with with the above requirements before you worry about package

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2017, 11:24:58 pm
content: A current-limited power supply will still deliver very large surge currents, because of capacitors.The above circuit is okay as long as there is enough series resistance that the fuse and diodes are not overloaded.MF-R050 is rated for 0.41 ohms minimum, and 40A maximum. A surge of -24V therefore delivers I = (24V - Vf) / (0.41 ohm) = 53 amps. I would suggest adding a 0.33 ohm surgeproof resistor in series with the fuse, to limit current safely.You can also use a series blocking device for polarity protection, usually a diode or PMOS.There are also polyfuse+TVS diode combo parts, so that overvoltage causes the polyfuse to heat up rapidly.Tim

author: igeorge
author_rank: Contributor
timestamp: April 13, 2017, 11:40:47 pm
content: Thank you guys.For polarity protection i use the 3 diodes in parallel like in the above drawing.If polarity is reversed the diode will conduct, and all 3 in parallel will take the hit.I will order the suggested mosfet and the resistor.But as you suggest to do a search at digikey it means i have to know what to look for.What i did after my first mistake with 2n7000, was to check for DS voltage to be above 30V, Trigger voltage to be high enough so at 5 volts it will not be triggered.Never looked at I/gs because i do not know what it is.I also selected a fet with a DS current above 6 Amps.As i do not have a clue about them, and no time to study them now, of course i select the wrong part.Here you guru you come, to take the poor blind man by hand and helping him to cross the road

author: timb
author_rank: Super Contributor
timestamp: April 14, 2017, 12:08:36 am
content: Thinking more about this afternoon, you could use a TL431 as a cheap comparator with a much sharper turn on than a Zener diode:A simple 2N3906 would work fine for the PNP transistor. Essentially the TL431 acts as the programmable comparator. You can set the trip voltage by adjusting the voltage divider feeding the ref pin.This circuit is commonly used for SCR crowbars, so it should work fine with a properly rated MOSFET. It's the circuit I would use.

author: igeorge
author_rank: Contributor
timestamp: April 14, 2017, 12:54:09 am
content: Thank you Timb.I will get the parts next week (FedEx went for egg hunting:) and try it on a bread board.What is the part number for FET ?

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2017, 01:57:20 pm
content: Attached are two ways of implementing over-voltage protection, on in the same circuit.A crowbar with an RC delay. This will will short circuit the supply and blow the fuse but I've added a capacitor to slow it down. The trigger voltage is just over 6V: the TL431 is set to 5.5V and the SCR triggers when its gate hits above 0.6V.An amplified TL431 which clamps the supply when the voltage exceeds 7V and acts fairly quickly. At extremely high currents, the supply voltage will rise considerably, as Q1 can't increase its collector current indefinitely. You might be able to swap it with a lateral MOSFET, the kind designed for analogue operation and used in some audio amplifiers.The result is. A 25V pulse is clipped to 15.5V fairly swiftly, without triggering the SCR and blowing the fuse but applying 7V continuously will blow the fuse. This is desirable, since most components can withstand very brief, extreme over-voltages for short periods of time, yet can be destroyed by continuous, less extreme over-voltages.Note that you can see there is a small delay before the amplified TL431 circuit clamps, resulting in a spike of nearly 20V, so you might want to consider adding a much faster transient suppression diode, such as the 1N6276A, in parallel with the whole the circuit.

author: igeorge
author_rank: Contributor
timestamp: April 14, 2017, 04:10:47 pm
content: Thank you Hero999,A complete different solution. And according with your LTspice graphs seems to work.By "adding a much faster transient suppression diode, such as the 1N6276A, in parallel with the whole the circuit." you mean replacing D1?In your opinion, what do you suggest as being the most reliable and cost effective ? Transistor or SCR ?My processors, mostly from Microchip support up to 6.5V, but i prefer to have the whole system to blow the fuse at 6.1VDC.Any changes to do here to achieve this limit ?With all this support and many detailed solutions,i do not know what to do? I will buy for next week parts and experiment each proposal and keep you posted.All of you were so kind to help me.Thank you all again.

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2017, 08:00:23 pm
content: Quote from: igeorge on April 14, 2017, 04:10:47 pmYes.QuoteSCR because it acts as a switch with a sharp on action and is very rugged compared to transistors.QuoteIs 6.5V the absolute maximum rating or operating voltage? Most parts rated for 5V will tolerate 7V for a short length of time.The values of R1 & R2 set the tripping point for the SCR circuit. With the values shown it will trip at about 6.1V. C1 just slows it down and can be removed, if an instant trip is required. The trouble is, if the voltage is set too near the nominal supply voltage and it trips quickly, then there's a risk of nuisance tripping. This is why it's a good idea to have a transient suppressor for, short, fast voltage spikes and a crowbar for the longer pulses. The crowbar should always be set to a slightly lower voltage than the transient suppressor (whether it be a diode, TL431 & BJTs or both) otherwise the crowbar is unlikely to trip.Vtrip = (1 + R1/R2)×2.495 + 0.62.495 is the typical reference voltage for the TL431The 0.6V is the nominal trigger voltage for an SCR, which varies depending on the temperature and characteristics of the SCR.Since R1 and R2 form a potential divider, a potential divider calculator program can be used to help you select resistors from the standard values available. Here's a link to the online one I use:http://www.random-science-tools.com/electronics/divider.htm

author: igeorge
author_rank: Contributor
timestamp: April 14, 2017, 09:01:41 pm
content: OK Hero999,I am still waiting and answer from Timb aboutthe part number of the used mosfet, but yours I will try it next week and let you know the real life.About the processor ,6.5 it is the MAXvalue of VDD"Voltage on VDD with respect to VSS .... -0.3V to +6.5V"from datasheet.VDD is the positive side and VSS is the negative side, usually called ground , or digital ground and is 0.Can you post please your LT file ?Thank you

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2017, 09:53:06 pm
content: Quote from: igeorge on April 14, 2017, 09:01:41 pmI didn't answer your question on the MOSFET because I wouldn't recommend using a MOSFET. If you do go for this option, you want something in a large case, with the appropriate voltage rating. The current rating an on resistance aren't very important.Attached is the LTSpice file. It looks different than the picture I posted previously because there's no way to embed symbols so had to use a generic 8 pin DIL for the TL431. If it doesn't work on your PC, it'll be because a model is missing. If so, let me know and I'll fix it.

author: igeorge
author_rank: Contributor
timestamp: April 14, 2017, 10:01:20 pm
content: Thak you.I will try it and let you know if it works on my LT or not.You see, my mind is simple. For me, in digital is 0 or 1, no"50 Shades of gray"I assumed that if the fuse is 300 mA, i do not need a 10A mosfet. I was wrong again.

author: igeorge
author_rank: Contributor
timestamp: April 14, 2017, 11:32:51 pm
content: I try it and is missing theTecor SCR S4025LDo you have it ?Also is missing definition for model"BC327S"The capacitor is ceramic or tantalum ?To be clear in my mind, i will use just the part of the schematic with SCR, and ignore the part with transistors.Is this correct ?Sorry for so many questions, but i want to see it working and move back to my programs.

author: timb
author_rank: Super Contributor
timestamp: April 15, 2017, 12:35:41 am
content: igeorge, you can use my last circuit with the single TL431 and simply replace the MOSFET with an SCR. That would work equally well, if not a bit better.That said, if you're intent on using a MOSFET, it'll still work fine. I think Hero999's circuit is way, way too complex for what it does. In this specific case, considering you have a current limited power supply and a fast blow fuse, an appropriate MOSFET should be able to crowbar the circuit without damaging the MOSFET. You can always add a 500mOhm resistor to the FET to limit current, which will prevent damage if your power supply has a large amount of capacitance on the output.I thought I posted that mosfet part number in my last post, but I guess I didn't. Let me look it up again.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 12:42:09 am
content: for this"500mOhm resistor"do you mean milliohms or megaohms.I know it might look like stupid question but i need to know.

author: timb
author_rank: Super Contributor
timestamp: April 15, 2017, 12:46:58 am
content: Quote from: igeorge on April 15, 2017, 12:42:09 amm=milliM=Mega500 milliohms, yes.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 12:49:41 am
content: Thank you

author: timb
author_rank: Super Contributor
timestamp: April 15, 2017, 01:01:05 am
content: Quote from: igeorge on April 15, 2017, 12:49:41 amFor your MOSFET, try the STP16NF06. It's got a very nice dV/dT rating and can handle up to 50A for at least 10mS. That should be more than enough time to discharge any output capacitance on the power supply. (A power supply should, ideally, have less than 100uF on the output.)Add a wirewound 500mOhm resistor to the drain of the FET, rated for 2W or 3W (wirewound resistors have high peak surge current capability).Based on some tables from fuse data sheets, a 1A fast blow fuse shouldn't have any trouble opening before the MOSFET is damaged. You could also add a small heatsink to the FET if you need to extend the pulse duration.I can also get you a part number for an SCR that would be a drop in replacement for the MOSFET in my circuit, if you want.Edit: As I understand it, you only need this crowbar to protect the rest of the circuit from overvoltage stemming from a technician hooking up the wrong power supply during initial install, correct? Even if the MOSFET is somehow damaged, it'll still save the rest of your circuit.

author: Ian.M
author_rank: Super Contributor
timestamp: April 15, 2017, 01:10:50 am
content: Quote from: Hero999 on April 14, 2017, 09:53:06 pmIf you make a new folder for the project save the schematic to it and copy the symbol (.asy) to the same folder as the schematic (.asc), and also put the model (.sub, .lib. or .mod, or hierarchical .asy) there, and use the Top Directory dropdown in the Component browser to add the parts from the current folder when creating the schematic, instead of your original copies you have added to the installed component library, then zip the project folder (after cleaning unwanted generated files), the result is *ENTIRELY* portable.A TL431 model and symbol can be found here:http://www.audio-perfection.com/wp-content/uploads/TL431.zipI seem to have the BC327S model installed. I probably upgraded my standard.bjt transistor library from the Yahoo group or the LTspice wiki at some point.The symbol for the Tecor SCR S4025L (Misc\SCR_Teccor) is missing (and presumably the model as well) which means your crowbar sim is still broken for anyone else . . . .I try to keep symbols for 3rd party parts NOT supplied by L.T. in a separate folder (Lib\Sym\3rd Party) to reduce the risk of me inadvertently posting a non-portable simulation. It doesn't help with transistors or other stuff that lives in a standard.* file as there is no easy way of keeping them separate and still easily accessible in the selector.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 01:13:39 am
content: Thank you for all info.I do not mind the part number for SCR.i can experiment with both.One thing i do not understand.Why everybody oversize Mosfets, Scr, or resistors.If the fuse is just 300 milliamps fast blowing fuse, whey i need components rated at 5W or 30A ?For testing i want to do it on a breadboard, but in real life, it will be on a PCB using SMD.Oversized components will need a big heat sink or space around, which i do not have on the PCB to spare.I was under the impression that a simple SOT23 SCR of mosfet will do the job.Maybe i am wrong.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 01:19:31 am
content: Hi Ian,Can you please post a working zip for me ?Thank you.

author: Ian.M
author_rank: Super Contributor
timestamp: April 15, 2017, 02:22:49 am
content: I don't have the Teccor SCR model or symbol so cant easily post a working zip.The BC327S model I have is:Code:.MODEL BC327S PNP(IS=0.230E-12 ISE=39.284E-15 ISC=0.118E-15 XTI=4.800+ BF=504.327 BR=23.0 IKF=0.780 IKR=0.195 XTB=1.700+ VAF=26.0 VAR=4.500 VJE=1.0 VJC=1.0+ RE=0.203 RC=0.262 RB=0.800 RBM=0.400 IRB=0.100E-3+ CJE=57.177E-12 CJC=28.600E-12 XCJC=0.650 FC=0.750+ NF=1.0 NR=1.002 NE=1.944 NC=3.025 MJE=0.470 MJC=0.520+ TF=0.664E-9 TR=2.600E-9 PTF=1.0 ITF=0.250 VTF=2.0 XTF=6.350+ EG=1.110 KF=1E-9 AF=1 MFG=SIEMENS)Copy it and paste it onto the schematic as a dot command.There seems to be a model for the SCR at:http://www.littelfuse.com/technical-resources/technical-centers/semiconductors-technical-center/~/media/Electronics_Technical/SPICE_Models/Switching_Thyristor/Thyristor_SCR_Sxx20x_Sxx25x_A_SPICE_Model_lib.htmThe key bit you need is:Code:.SUBCKT S4025L 1 2 3* TERMINALS: A G KQpnp 6 4 1 Pfor OFFQnpn 4 6 5 Nfor OFFRfor 6 4 500MEGRrev 1 4 500MEGRshort 6 5 15Rlat 2 6 2.64Ron 3 5 9.4mDfor 6 4 ZbrkDrev 1 4 ZbrkDgate 6 5 Zgate.MODEL Zbrk D (IS=10F IBV=1U BV=400).MODEL Zgate D (IS=10F IBV=100U BV=10 VJ=0.3).MODEL Pfor PNP(IS=10P BF=3.5 CJE=200p CJC=200p TF=0.3U).MODEL Nfor NPN(IS=10P ISE=1E-9 BF=100.0 RC=13.6M CJE=1000p CJC=200p TF=0.3U).ENDSAgain add it to the schematic as a dot command.There's a SCR symbol in Misc. Insert it in the gap by the bottom left corner of U1 and change its value fromSCRtoS4025Lwhich should fix that problem albeit with a different symbol.I'll give Hero999 a chance to post a fixed portable version in case I've found different models.P.S. its better to use a TRIAC as they can be triggered by current OUT of the gate, then you can directly drive it from the TL431 cathode, as any resistance or other load in the anode circuit of a TL431 adds negative feedback and makes its setpoint rather mushy. See Figure 24. TRIAC Crowbar in theOnSemi TL431 datasheetYou can use a SCR but it needs an inverting driver as used in Hero999's 2N3055 baced circuit - just drop in a S4025L in place of the 2N3055! See Figure 25 in the TL431 datasheet.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 02:33:36 am
content: Thank you IanI got it and works.On the probe i got the same signal like Hero999

author: Ian.M
author_rank: Super Contributor
timestamp: April 15, 2017, 02:38:47 am
content: At the right hand end of the LTspice toolbar you will find an icon '.op'. Hover over it and it says 'SPICE Directive'. Click it. Copy/paste the command text I posted into the edit box, click  then click on a blank part of the schematic where you want to put the new dot command.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 02:50:07 am
content: Thank you Ian.Founded did it and posted.I can sleep well now.I will play with it tomorrow morning.But i rephrase again my previous question.If the fuse is fast blowing fuse, only 0.3Amps, why i need the over rated components to work with 10-20 amps, or 3-5Wats ?I believe the fuse will blow before the component.Am i wrong ?

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2017, 08:44:48 am
content: Quote from: timb on April 15, 2017, 12:35:41 amThen you have a similar circuit to mine, plus an extra PNP transistor, which helps because it takes the variance in SCR trigger voltage out of the equation.QuoteThat's because it showstwo methodsof accomplishing the same thing. It may not be necessary here but it's often a good idea to have both non-latching, as well as a hard latching crowbar form of overvoltage protection.QuoteA resistor may help but I don't see how the MOSFET in your circuit remains fully switched on. When the power supply voltage falls, the gate voltage will also fall, turning it off, until it reaches equilibrium.Quote from: timb on April 15, 2017, 12:46:58 amYes, m = milli and M = Mega but watch out for LTSpice which is not case sensitive and treats M as mill, so you have to use Meg for Mega.Quote from: igeorge on April 14, 2017, 11:32:51 pmYes, you can just use the part of the schematic with the SCR and ignore the transistors. If you want fast acting remove C1.Sorry about the missing models, I was unaware they weren't included in the default install.Quote from: Ian.M on April 15, 2017, 01:10:50 amI did think about posting a zip but quite often people are wary about downloading zips from unknown sources because they can contain anything. Having it all embedded in one asc file makes life much easier, until someone like me makes a mistake and they have to hunt for models.Thanks for posting the missing models.Quote from: igeorge on April 15, 2017, 02:50:07 amBecause huge currents can flow before the fuse blows which is not instantaneous. Everything in the current path of the fuse needs to be able to pass the huge short circuit current, at nearly the power supply voltage, until the fuse blows.

author: timb
author_rank: Super Contributor
timestamp: April 15, 2017, 12:58:07 pm
content: Quote from: Hero999 on April 15, 2017, 08:44:48 amA 300mA fuse *should* blow well before the voltage bottoms out, but I suppose that depends on the impedance of the supply. I have used a similar circuit (with a MOSFET) before to crowbar a power supply. If the fuse can act quickly enough it works fine.That said, an SCR *is* the traditional way to do this, and the most robust.Also, I didn't look closely at your circuit, but I see now you were showing two methods.Frankly, if it were me, I'd just use a TL431 and an SCR, the PNP transistor isn't even needed (it just adds precision to the threshold).This is only being used to prevent someone plugging the wrong voltage in during initial setup of whatever this device is, so it doesn't need to be precise or durable.To be honest a crowbar might not even help him here as, by the time it activates and the fuse blows the damage could be done to the rest of the board. Even tens of milliseconds is a long time when it comes to over voltage and silicon.A better solution may be one of Linear Tech's Ideal Diode/Over Voltage Protection parts. Basically, these place a MOSFET inline with your power supply input and monitor the voltage, if it goes above a set threshold it will control the gate of the FET and either shut down the output *or* try to regulate the output (like a linear regulator), depending on the part.The would be the most robust and safest choice in this scenario, and would be basically guaranteed to work.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 03:03:52 pm
content: To Timb:Thanks Tim, but i am still waiting for your Mosfet to buy Monday.To both of you:Wow ... I made a new world war here from a simple dummy question, but reading the comments i can see a truce coming.Just to give you more info. The board is made and from time to time the electrician which wire the machine and boards make one of two mistake.1.wire the input of the board at 24VDC which is the main voltage in the whole machine. The 24 is wire some time correctly + to + and- to -, or in reverse polarity.2.wire the proper 5VDC but reverse polarity.The customer refuse to have a new board done with 24VDC in (that will solve part of the problem). He sticks with5 VDC.This end up with complain that the supplied boards are defective and NO WAY to prove the wrong job of electrician. I come with the idea to have a NON REMOVABLE fuse soldered to the board. Electrician make a mistake, the fuse is blown.Now, to make my life simpler, i wanted to still protect the board and not trow it in the garbage.In this way i will stop arguing with the customer that his man did a lousy job.The customer agreed with this, but he does not want to spend the sun and the moon for an upgrade of the board. Like any other customer he is cheap.He buys 30-50 boards a year so his budget for this is already fixed, and him must absorb the cost (he is just a machine builder).I hope now you are more clear for my constraints.Attached are the look of the whole board and the zoom on the protection side i tried to make it with diodes and TVS.I did not try it yet.

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2017, 03:25:09 pm
content: That's pretty standard solution, as far as over voltage protection goes. The TVS diode will be good for clamping short high voltage spikes but is likely to blow before the fuse does.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 03:33:41 pm
content: I have to comply.Ian suggested to use a Triac instead of SCR like here"P.S. its better to use a TRIAC as they can be triggered by current OUT of the gate, then you can directly drive it from the TL431 cathode, as any resistance or other load in the anode circuit of a TL431 adds negative feedback and makes its set point rather mushy. See Figure 24. TRIAC Crowbar in the OnSemi TL431 datasheet You can use a SCR but it needs an inverting driver as used in Hero999's 2N3055 baced circuit - just drop in a S4025L in place of the 2N3055! See Figure 25 in the TL431 datasheet."Data sheethttps://www.onsemi.com/pub/Collateral/TL431-D.PDFAny suggestion for part number to test for Triac?

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 04:24:35 pm
content: To Ian and Hero:Where i can find a fuse and put it in the LT circuit just before D1.On the latest LT i download yesterday is no fuse symbol or component.I would like to see the graph with a fuse on the circuit.Can you help me on this ?Thank you

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2017, 04:58:26 pm
content: Quote from: timb on April 15, 2017, 12:58:07 pmIt sounds like a similar thing could be implemented using the TL431. Obviously it won't be as fast and won't protect against reverse voltage, unless another MOSFET is added.Quote from: igeorge on April 15, 2017, 03:33:41 pmA TRIAC could be used but the circuit would need to be modified. The part number is not important. Anything which can pass the current for long enough to blow the fuse will do.Quote from: igeorge on April 15, 2017, 04:24:35 pmI'm not aware of any fuse models for LTSpice. I don't see any benefit in simulating it. because it's something which varies quite widely, depending on the temperature and there's quite a wide variance from fuse to fuse. If you know the short circuit current, then you can predict whether the fuse will blow and how long it will take by looking at the fuse's datasheet.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 05:07:15 pm
content: Thank you HeroI will try the triac version after Easter holiday when Digikey and FedEx start working again.

author: timb
author_rank: Super Contributor
timestamp: April 15, 2017, 05:10:29 pm
content: Quote from: igeorge on April 15, 2017, 03:03:52 pmI already recommended it several posts back:Quote from: timb on April 15, 2017, 01:01:05 am

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 05:18:53 pm
content: Thanks Hero, i missed.It comes in 2 flavorsvgs(th) 2.5 v or 4 vAny preference?

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2017, 05:42:43 pm
content: Quote from: igeorge on April 15, 2017, 05:18:53 pmAs this is running off 5V, the lower threshold is better.

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 05:45:19 pm
content: Thank you very much.

author: timb
author_rank: Super Contributor
timestamp: April 15, 2017, 08:45:40 pm
content: Quote from: igeorge on April 15, 2017, 04:24:35 pmAs Hero999 said, you don't need one. You can just determine when the fuse should trip based on the various data points in your plot.That said, I did build a "fuse" sub-circuit in my other simulation software (iCircuit) for fun:Unfortunately, LTspice doesn't have digital parts like D-Latches (though I suppose you could build one out of discrete transistors, but it might significantly slow the simulation down).In my circuit, I used a dependent voltage source whose voltage is equivalent to the current (1V=1A) flowing through the switch labeled "Fuse" (top right). This feeds into an RC network, to give a slight time delay of a real fuse (and smooth out pulsed loads).This time delayed and smoothed voltage is then fed to the non-inverting input of a comparator and if the voltage is higher than the reference voltage on the inverting terminal (which represents the trip point), its output goes high, clocking the D Flip-Flop, which in turn causes the output to change states and open the switch ("Fuse")! (The switch is then latched in the open state, as no more current can flow and thus there's nothing to trip the comparator and toggle the tit-flop again.)The switch and RC network in the bottom right are responsible for pulsing the reset line of the D Flip-Flop once during startup. 5V goes across the switch to the R input of the flip-flop, the RC network at the control node of the switch is set to open it a few microseconds after power up. Thus the R input of the flip-flop sees a 5V pulse a few uS wide.You could actually make an electronic fuse with a real circuit like this. You'd need to add a current sense amplifier in place of the dependent voltage source and use either a relay (for a slow acting fuse) or beefy MOSFET (fast acting) for the main switch, but it would work!The more you know! ~~~*

author: igeorge
author_rank: Contributor
timestamp: April 15, 2017, 09:01:02 pm
content: Thank you Tim,It is ingenious way of solving the fuse problem.For sure i will get Icircuit for windows and start playing with it.It looks more user friendly than Proteus.

author: Ian.M
author_rank: Super Contributor
timestamp: April 15, 2017, 10:30:53 pm
content: Actually LTspice has digital parts: INV, BUF, AND, OR, and XOR, with the last four having complementary outputs, SRFLOP and DFLOP. See A devices in the help file.

author: Circlotron
author_rank: Super Contributor
timestamp: April 16, 2017, 12:21:36 am
content: Quote from: igeorge on April 14, 2017, 10:01:20 pmFor this particular kind of circuit:SCR = 1MOSFET = 0--------------SCRs are *hugely* strong and simple and cheap and reliable and will tolerate a "near enough" design much better than a MOSFET will. They are very dumb and simple, like a blacksmith's anvil, but that is exactly what you need for this kind of circuit.MOSFETs need all these extra parts around them and can be delicate and fussy if everything is not just perfect. The circuit may only work once because the MOSFET blows just before the fuse.

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 11:58:30 am
content: Thank you ALL for helping me on this tedious task.I still have a dumb question.As i explained through this post, analog for me it is a strange territory.Using basic resistors, capacitors or OpAmp, i did , the rest, is somewhere in the dark.Here is the question:We use the TL431 to trigger the SCR, and a resistor divider to trigger TL.How about using just Resistor divider to trigger the SCR, without using TL ?

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2017, 01:24:43 pm
content: Quote from: igeorge on April 16, 2017, 11:58:30 amYou could but the gate trigger voltage is highly variable.http://www.littelfuse.com/~/media/electronics/datasheets/switching_thyristors/littelfuse_thyristor_sxx20x_sxx25x_datasheet.pdf.pdf

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 01:28:52 pm
content: Thank youI found out the hard way.Removed in LTspice.I will stick with the proposed solution.Waiting for next week to get part on my breadboard.Have a nice week end

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2017, 02:41:31 pm
content: The minimum parts count circuit is Figure 24. TRIAC Crowbar in the OnSemi TL431 datasheet. Three resistors, one Triac and the TL431. The only reason to go with the Fig 25. circuit is if you need a beefier TRIAC or SCR than the TL431 can directly drive or if using a SCR rather than a TRIAC is going to save you more money than the extra parts cost.I'm wondering what it would take to successfully crowbar a polyfuse, and whether that could be done fast enough to protect a sensitive load.@igeorge,What's the expected peak current consumption on the 5V rail (excluding switchon transients)Its probably worth putting an anti-parallel pair of high brightness red LEDs with a 3K3 series resistor, across the fuse. These fault LEDs would be labelled 'Reverse Polarity' and 'Overvoltage/Overcurrent'. If its possible to use a polyfuse the board would then be far more resistant to idiot installers.

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 03:12:24 pm
content: To make things clear, the fuse it is not polyfuse. It is a one time fast blowing fuse 0.3A.The maximum current on the board is 200 milliamps.All the IC are rated to Max 6VDC.We do not want to use polyfuse for the simple reason, that who made mistake and blow the fuse, has to go to his supervisor and get a replacement (electricians do not like to do that). In this way we can make them to look twice before putting power.Based in fig24, and your choice of Triac, can you please suggest me the value of the 3 resistors and a part number for triac.To test it first on the breadboard i need a through hole version.Later i will find a similar or the same on smd.I read the formula to calculate the Vout, but i do not know what should be.The whole circuit should go on short for 6 VDC.Once the triac start conducting, the fuse will blow and everything will be safe.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2017, 03:53:05 pm
content: Quote from: igeorge on April 16, 2017, 03:12:24 pmI've already posted that circuit, with appropriate resistor values for 6.5V.https://www.eevblog.com/forum/beginners/2n7000-strange-behaviour/msg1186642/#msg1186642If you want to change it to 6V, then the values for R1 & R2 can be calculated using the site linked belowhttp://www.random-science-tools.com/electronics/divider.htmThe TRIAC part number is not important. Most TRIACs in a TO-220 package will be able to withstand the current pulse for long enough to blow a 300mA fuse at 24V. Here's an example datasheet, I fount by searching for TO-220 TRIAC:http://www.farnell.com/datasheets/1708247.pdf

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 04:02:40 pm
content: Thank you Hero999, but you have to understand me.I had so many variants discuses here, and i for got about it being posted.I think i will stop now and wait until i have the parts to test.I put a micro to flash a led on the breadboard on 5VDC and when i have the protection i will supply 24 and pray.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2017, 04:23:22 pm
content: Quote from: igeorge on April 16, 2017, 04:02:40 pmWhy do that? Simply set up a DSO to trigger at +1V rising, in one-shot mode. One channel before the fuse, one after, with a simple resistive load in place of any delicate parts like MCUs flashing LEDs. For initial testing, as you want the powerup to be clean and sharp to get the worst-case performance of the crowbar, apply +24V via either a Mercury wetted relay with an adequate current rating (at least 10 times the fusing current) or a really beefy P-MOSFET (greater surge current rating than the SCR or TRIAC) driven by a high current gate driver controlled by a debounced switch. If you've got more than two scope channels, put one on the TL431 cathode. Study the resulting traces to see how effective it is at crowbarring the supply. You can probably do initial testing with the fuse shorted out and the PSU set to current limit at a few Amps, but eventually you'll have to blow a fuse per shot to gain some confidence that it will work as expected. Ideally, do about ten shots, downloading the scope traces so you can compare them to see how sensitive it is to fuse variations. It may also be worth checking it with 150mA and 250mA fuses, again to see how sensitive it is to (extreme) component variation.The micro and LED only gets added for the final dog & pony show when you need to convince your boss its worth doing. Or if you have developed 100% confidence, build the crowbar + fuse on protoboard, and feed its output to one of the original boards so you can demonstrate everything except the fuse survives.

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 04:42:02 pm
content: That is exactly what i want to do.Crowbar on breadboard and because the micro is cheap i wanted to put it there too.If it dies, RIP, but if it is still flashing after i replace the fuse, then is OK.Of course i will use a scope also to see it.For power i will just use one of the Omron 24VDC power supply.I will wire it directly and have a switch on the 120VAC. It is limited at 3A. After that it shut down until the short is removed.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2017, 04:46:10 pm
content: Switching on the AC supply to the 24V PSU will give a relatively slow rising 24V rail with plenty of time for the crowbar to function. If you want to do a worst-case test, you need to switch the PSU output.There's no point in trying to kill a MCU before you are preparing the dog & pony show. Depending on the MCU it may tolerate >>7.5V on Vdd momentarily, but anything over its abs. max. Vdd limit *could* kill it or compromise future reliability, so whether or not it survives doesn't tell you anything - unless you used *exactly* the same type of MCU on the original board.

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 05:10:11 pm
content: Ian, please understand. This is the real plant life.They wire the machine , then they apply power.If the board is wired correctly at 5VDC, it is no way in the world that the power swing to 24.For other applications, i understand. But here, the 5vdc is also a shelf CSA or UL approved supply.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2017, 05:55:33 pm
content: Fair enough, but you *DO* need to test at a faster supply rise rate than the actual machine's PSU can produce. If the peak voltage before the crowbar fires is acceptable at double the rise rate you can be confident it will always be acceptable (neglecting tolerance variations) in actual installations.I suggest scoping the output of an unloaded Omron 24VDC PSU (if that's what's in the machine) during powerup to get the rise rate, finding its output capacitance, then using a bench PSU set to 24V, + external capacitance equal to the Omron PSU, with the bench PSU's current limit set to charge the capacitor twice as fast as the Omron PSU can, for twice the rise rate. Due to the extra capacitor, you can use a simple switch between the PSU and the capacitor as bounce will no longer be such a critical issue. Scope the result and confirm the rise rate is near to double the Omron's rise rate.Have you considered adding a 'witness' fuse? A separate low current fuse, tapped off upstream of the main fuse, feeding only a 12V Zener - if that fuse is blown >12V or reverse polarrity must have been applied to the 5V rail. That way you can distinguish between board faults that blow the main fuse and bad installation

author: igeorge
author_rank: Contributor
timestamp: April 16, 2017, 06:07:28 pm
content: Thank you Ian.I will do it.

author: igeorge
author_rank: Contributor
timestamp: April 19, 2017, 06:02:51 pm
content: The great UPDATE/DAY for everybody.I got the parts and tried the simple version with the triac at 24VDC.It did works with no problem.The 300mA fuse blow and the circuit was protected.I have on it and Ti-RS232 chip, and AD8541 and a Pic16F882..None of them was damaged.I replaced the fuse and put 5VDC and everything function normally.After that, i tried my old version with some diodes for reverse protection (started with 3 in parallel and went down to ONE) 1N5819, and a TVS for 24 VDC which break at 6VDC, ST Micro part.I tried the same at 24VDC normal and reverse polarity. Blow the fuse but the circuit was OK.Bellow it is the drawing for the diodes. I believe it is simpler and cheaper this way .I will keep the version with triac for new projects and incorporate it in the price at the beginning.Thank you all of you for the effort and support. It was amazing and i learned a lot.Beside the part number, are the Digikey part numbers also.The one which finish with ND.

author: Ian.M
author_rank: Super Contributor
timestamp: April 19, 2017, 06:28:20 pm
content: The TRIAC + TL431 is a close tolerance crowbar. Its easy to hold the trip point to 2% tolerance if you use a 1% TL431A and 1% resistors, and fractionally over that for a 2% TL431 and 0.1% resistors, so with the trip point set at 5.8V, you can comfortably stay under the AD8541's abs max supply voltage rating of 6.0V and the PIC16F882's abs. max. supply voltage rating of 6.5V without a significant risk of nuisance tripping due to normal fluctuations of the 5V railHowever your chosen SMAJ6.0CA TVS diode has a breakdown voltage range of 6.67V to 7.37V, so will *NEVER* provide adequate protection for the PIC and the OPAMP as they are guaranteed to see more than 6.6V before the fuse blows. Depending on the peak fusing current, Vdd could easily go over 8V, as the only guaranteed clamping spec is 10.3V @38.8A pulse.It will reduce the probability of failure, but doesn't eliminate it, and because the parts on the 5V rail have been over-stressed, they are likely to have a higher probability of failing in-service.

author: igeorge
author_rank: Contributor
timestamp: April 19, 2017, 06:49:43 pm
content: Thank you Ian.Maybe i was over excited that it did works.I do not know how it did not blow the AD8541 chip or the PIC.I did it only two times.OK, back to the drawing board.I will present to my boss the triac solution and let him deal with the customer.5.8 volts it is excellent.Thank you for letting me know.

author: Ian.M
author_rank: Super Contributor
timestamp: April 19, 2017, 07:08:25 pm
content: The next thing you should do is get some dataloggers that can log the maximum transient voltage reached and hook them up to the 5V rail in a few of the machines, so you can see if 5.8V gives you enough margin to avoid unwanted crowbar trips. If the 5V rail is running 5% high and is noisy, you may not have enough safety margin. As you are using a non-resettable SMD fuse, an unwanted trip means the board has to be returned fir rework before it can be put back into service.This may be an argument for using a polyfuse that can be reset by powering down the machine, with a separate witness fuse, Zener and indicator LED for detecting installer idiocy.

author: KL27x
author_rank: Super Contributor
timestamp: April 19, 2017, 07:58:06 pm
content: Real plant life sounds strange.If the blown fuse stops the worker from producing and makes him come to supervisor, yes he is more likely to remember that "this sucks." But it doesn't change the fact that he is human and makes mistakes. After the fact, he still doesn't know how/why he made this mistake in the first place. This just increases his anxiety level.If there is mistake which gives instant feedback and it also instantly correctable without major drawback, he gets to make this mistake many times, thus not only learning it is a mistake but also hopefully learning how and why he made it and perhaps know better how to avoid it in the most efficient way.Dog chases a firecracker just once... then he avoids it. You need the dog to keep chasing. So maybe make the stick not explode in his face. If I want to learn how to finish Mario Bros in the fastest possible time without dying, I would learn a lot faster if I could restart that last boss battle instantly, rather than finding someone to restart the game for me and getting a demerit.

author: Zero999
author_rank: Super Contributor
timestamp: April 19, 2017, 10:32:03 pm
content: Quote from: Ian.M on April 19, 2017, 06:28:20 pmYes, the TRIAC + TL431 will providefasteraccurate clamping but I question whether it will be as fast as a transient suppression diode. The TL431 isn't the fastest of comparators and experience tells me the SPICE models for it aren't great. It's certainly something that needs to be bench tested.QuoteYou're right there. I think it survived so well because the overvoltage was very brief and I believe manufactures often specify their parts very cautiously. It wouldn't surprise me if a simple MCU can stand around 8V for a few hundred ms with no lasting damage.Another possible problem with this set up is if the power supply doesn't supply enough current to blow the fuse in time. Not only is there a greater risk of destroy the MCU but also the TVS diode, which doesn't form a sharp short circuit. It will sit there a bit over its clamping voltage, passing as much current as the supply will allow, until it overheats. The worst case is if the power supply is current limited to around 300mA, causing the power rail to sit at around 7V to 8V, the TVS diode cooking, failing open circuit, then the MCU being hit by the full 24V.Perhaps combining the crowbar with a transient suppressor diode is a better approach?

author: igeorge
author_rank: Contributor
timestamp: April 19, 2017, 10:56:12 pm
content: Thank you again to all of you.Let me explain again the scenario.A machine builder build a machine where he got 2 sources of voltage; 5 VDC and 24 VDC.He buys boards from a supplier which test them before and guarantee that they are good.The electrician wiring the machine can do one of the mistake:1.Wire 5 VDC in reverse polarity; solved by the reverse Diode on the input AFTER the fuse. Result = blow the fuse, no damage.2.Wire 24VDC in reverse polarity. The solution and result is the same as above.3.Wire correctly 24 VDC. Solution is the proposed TVS (still needs more testing). Result is = Blow the fuse.What ever the electrician do wrong will be discovered on the first test run.Assuming a board is destroyed, it will be replaced and the voltage applied correctly ; 5 VDC.The machine is usually under test for 1-2 weeks before it will leave the shop and go to customer. During that time, what ever can be wrong it might happen, but it will be corrected immediately.The reason do not put a polyfuse, or socket fuse is to prove to machine builder that the boards were destroyed during assembly or tests.In other words the seller of the boards cover his a..During production in the plant, it is no way that somebody will go and reverse polarity or change the voltage supply.Like i said, for future projects, the triac solution will be incorporated on the price. For the existing contract , the TVS is the most acceptable solution.

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2017, 11:35:36 am
content: Quote from: igeorge on April 19, 2017, 10:56:12 pmOh I understand why you need a one shot system which can't be reset by someone who doesn't know what they're doing. You need definitive proof that the electrician is making mistakes. If you went for a PTC fuse, then repeated overvoltage could still damage the components, causing reliablity problems later and you'd have no way of proving it.My concern is the TVS might not always blow the fuse. It worked when you tested it but only because you used a power supply with a high enough current capacity to ensure the fuse opened before any damage was done. If it's connected to a smaller power supply with a lower current limit or via a long piece of thin cable (high resistance) then the fuse might not blow before the TVS and anything else on the board fails.Do you know the current capacity of the 24V power supply the electrician will connect the board up to, the length of the wires and cross-sectional area of the conductors?Are you sure it will be able to supply enough current to reliably blow the fuse before anything else is destroyed?If all of the above is known and you're sure it will blow the fuse, then fine, otherwise you should consider the crowbar.

author: igeorge
author_rank: Contributor
timestamp: April 20, 2017, 11:50:27 am
content: The power supply is big enough to supply a full 24 VDC to the whole machine.It is in the range of 20-30 A. It is overrated.The wires are usually 16 gauge, big wires with a very low drop.The length of the wire is no more than 30 feet.The supply, provide power radial.From a main terminal strip to each oh the users is one wire.Like i said. The board builder, us, wants to prove to machine builder that the board are damaged during the assembly of the machine.Before, they use to come back to us complaining that a number of boards does not works,WERE delivered defective.In this way we can prevent the complains, and get paid for repairing the board or replacements.

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2017, 05:25:49 pm
content: Quote from: igeorge on April 20, 2017, 11:50:27 amThat's good: you can be sure the fuse will certainly trip very quickly, just make sure the fuse has adequate breaking capacity for the maximum short circuit that beefy PSU can deliver. A ceramic fuse will give better protection than a glass fuse, which would be borderline in this case.

author: igeorge
author_rank: Contributor
timestamp: April 20, 2017, 06:50:25 pm
content: Got it.Thanks for suggestion.