Schematic problem - Page 1
Forum section: electronics / beginners

author: GFXMihnea
author_rank: Contributor
timestamp: November 14, 2013, 11:00:55 am
content: Hi, i want to make this circuit :http://www.next.gr/uploads/224-1c277448f9.pngBut my question is, i have all the components, but i don't have the irf640, i just have two irf630, and two irf634b, i can use that in my circuit because i don't know how to read a datasheet and see if they are equivalentThanksAnd another question: would that circuit work if i use it for car ignition coil , and mount the irf on a heatsink ?

author: madshaman
author_rank: Frequent Contributor
timestamp: November 14, 2013, 11:34:11 am
content: This might help you:http://en.m.wikipedia.org/wiki/Safe_operating_area

author: madshaman
author_rank: Frequent Contributor
timestamp: November 14, 2013, 11:37:47 am
content: This might help you even more (and more appropriate for that circuit):http://www.nxp.com/documents/application_note/AN11158.pdf

author: GFXMihnea
author_rank: Contributor
timestamp: November 14, 2013, 11:40:00 am
content: I don't know english so well, so i get tricky when i read that...but an overview of the datasheed of irf640 and irf630 are kind of the same.I want to know if so, the 555 trigger 5volts to open the gate on irf640, but an gate on irf630 can be triggred with 5v on the gate ? because i don't know where to search the gate-open value on the datasheet

author: madshaman
author_rank: Frequent Contributor
timestamp: November 15, 2013, 05:41:52 am
content: No worries, I want to give the best answer I can, so may I ask you a couple of questions first?What is your native language?Can you give me a brief description of your mathematical background?Do you know linear algebra?Do you know calculus, specifically basic integration?If you have a mathematical foundation, are you more comfortable with graphical or analytical methods? (Measuring values from a graph vs formulas)There's a *lot* going on in that circuit, but you are specifically interested in how to determine when the the mosfet will act as an open or closed circuit between the drain and the source; I want to do my best to show you how you can determine that for yourself for any circuit using the information in a data sheet.

author: madshaman
author_rank: Frequent Contributor
timestamp: November 15, 2013, 07:47:24 am
content: P.S. Two things with regards to your specific application:1) Can you post links to the data sheets for the devices you actually have and also indicate which package they are in? These devices are available from more than one manufacturer and in more than one package.The package will influence how much power the device can dissipate without going BOOM.Device characteristics can vary from unit to unit and also from manufacturer to manufacturer. I will also help to be looking at the same data sheets you are.2) One parameter you should look at right away is in the "absolute maximum ratings" sections, and that's the "maximum power dissipation", usually denoted as: Pd. Start by comparing the Pd of the 630 to the 640.I know you want a single number for the "on" voltage, an ideal mosfet will have such a number, and certain mosfets will come close; but, there is no such thing as an ideal mosfet.This number is called the "threshold voltage" and is usually denoted as Vth or Vgs(th). It might be given as a range, and in this case you are interested in the maximum of the range.A mosfet can be simplistically characterised by a function: Id = f( Vds, Vgs, T ), that is, drain current is a function of drain-source voltage, gate-source voltage and temperature.The main characteristic curves you'll find in the data sheet will usually show Id vs Vds for a number of values of Vgs (one curve per Vgs) at a given temperature on a single graph. It may show a number of graphs at different temperatures.To turn the the mosfet "on", you want to be in the "flat" part of those curves, that is, outside the mosfet's "linear region" of operation and inside the "saturation region". Also notice that the higher Vgs is, the higher Vds must be for the mosfet to operate in this region.The Pd (maximum power dissipation) parameter will imply a region within the Id/Vds graph called the " safe operating area". To complicate things, this area will be time-dependent for non-continuous, non-static operation (like in your circuit).Determining where in this area the device will be operating with regards to your circuit is a good starting point for determining the suitability of a replacement part (once you've assured yourself the mosfet will be acting like a digital switch; that is, in the saturation region).Others please correct me if I've given misleading information.

author: madshaman
author_rank: Frequent Contributor
timestamp: November 15, 2013, 08:08:30 am
content: Addition: I'm not sure if I'm getting too deep here, but there's one more important factor in determining if a given mosfet is suitable for your circuit. It has to do with the driving ability of what's connected to the mosfet's gate.In short, the gate is a capacitor and a certain number or charge carriers need to be moved into the gate for the true Vgs to be seen by the mosfet. If your driving circuit can only source a tiny amount of current (current is charge carriers per unit time), this process takes longer. What that means is that the mosfet will spend more time in its linear region where it behaves as a resistor and will dissipate more power.A similar problem exists when you want to turn the mosfet "off".The shape of the driving signal itself will also have influence over this; this is where calculus or graphical methods would come in.Here's a link which talks a bit about this, I only skimmed it and I realise it may be difficult due to English not being your native language:http://www.vishay.com/docs/73217/73217.pdfPlease let me know if you find this information useful or have any questions.

author: GFXMihnea
author_rank: Contributor
timestamp: November 15, 2013, 10:54:32 am
content: What is your native language?A:Romanian is my native language, i'm a 16 year old boy at the start of electronicsi'm in 11 grade highscool.Can you give me a brief description of your mathematical background?A:My highschool profile is "electronics and electromechanic engineering" , in one week i have 20 hours of electronics tutorials and practical tests(but i can't understand anything from what my teacher teach us)Do you know linear algebra?A:Basics, but i can hire an meditator professor to teach me more if i need to.Do you know calculus, specifically basic integration?A:Yes, the basics, and i can research the internet to find out more.1) Can you post links to the data sheets for the devices you actually have and also indicate which package they are in? These devices are available from more than one manufacturer and in more than one package.A:All my components are ordered from internet, rezistors, the 555, capacitors, potentiometer, perfboard, but i can't find the irf640, just the irf630 wich i have and my scrap components (two) - I understand how the nchanel mosfet works, when gate recives +5 volt, the current flows trought source and drain.Like in my circuit, the 555 sends a signal of 5 volts trough gate and closes and open the circuit verry fast...that's what i understand.In my country, it's hard to find electric components and resources, so i talk'ed a little bit with dave, and he send me a nice multimeter (extech ex320)Also, it's my first perfboard cirucit, so i ordered a perfboard.The mosfet will be mounted on a heatsink and a cooler fan.I ordered a irf630 mosfet (http://pdf.datasheetcatalog.com/datasheet/SGSThomsonMicroelectronics/mXtwqrx.pdf) This i the datasheet.The irf640 mosfet datasheet:http://www.vishay.com/docs/91036/91036.pdfBut i can't find the power disipation category...

author: madshaman
author_rank: Frequent Contributor
timestamp: November 15, 2013, 02:08:59 pm
content: Quote from: GFXMihnea on November 15, 2013, 10:54:32 amThanks for the answers GFXMihnea, I'll try to help you out if you're willing to do some of the work on your own.To determine if your IRF630 can be used in place of the IRF640 in your circuit, let's try to answer the following question first:In the circuit you posted, when the output from the 555 (pin 3) via the resistive divider is high ("on"), what state would the IRF630 be in versus the state of the IRF640?To start, you need to figure out what voltage appears on the gate of the mosfet when pin 3 of the 555 is high.Do you know how to do that?A couple of additional questions which can help me guide you are:Do you know what a "resistive divider" is (there is one sitting in front of the gate of the mosfet)?Do you know how to determine the voltage appearing on pin 3 of the 555 in the circuit you posted (hint: it's related to the voltage on pin 8 )?There is a diode (1N4005) in front of the drain of the mosfet, do you know what effect this diode will have on the voltage appearing on the drain?How much have you learned about the charging/discharging of a capacitor; would you be able to calculate how long it would take a capacitor to charge/discharge?This question isn't directly related to helping you out, but I'm curious:The circuit isn't hard to understand at all if all you want to know is what it does and how it does it from a high level point of view, but it's actually incredibly complicated to analyse for a beginner, particularly because of the presence of the ignition coil (which we'll try to avoid dealing with as long as possible): did your instructor ask you to understand this circuit for your class or is it a circuit that you want to build for yourself?Quote from: GFXMihneaI'll try to keep the math to a minimum and avoid any calculus.Quote from: GFXMihneaFor the IRF630 datasheet you linked it's called "Ptot" and is 75W at 25ºC. For the IRF640 datasheet you linked it's called "Pd" and is 125W at 25ºC. This tells you that the IRF640 is designed to handle a lot more power than the IRF630; that's cause to suspect that it may not work in that circuit without exploding, but it's better to figure that out (we'll get to it eventually). :-)edit: changed "pin?" to "pin 8 )?"

author: GFXMihnea
author_rank: Contributor
timestamp: November 15, 2013, 02:58:32 pm
content: Do you know what a "resistive divider" is (there is one sitting in front of the gate of the mosfet)?Yes, it divides the input voltage, and the output voltage, right ?Do you know how to determine the voltage appearing on pin 3 of the 555 in the circuit you posted (hint: it's related to the voltage on pin 8 )?Well, pin 8 it's +vcc wich it's ~12v, the 3, or the "out pin" it's "-minus-" in that circuit, so we can measure with a multimeter right ?There is a diode (1N4005) in front of the drain of the mosfet, do you know what effect this diode will have on the voltage appearing on the drain?Yes, it's all about inductive tensions, it's protects the irf640 and the 555 from being destroied by inductive tensions flowing back.And like al the diodes , isn't perfect, so will have an rezistance.How much have you learned about the charging/discharging of a capacitor; would you be able to calculate how long it would take a capacitor to charge/discharge?At the school, we are just at tranzistors, we have much more to reach capacitors. BUT, i watched some videos on the youtube, and do some research about capacitors, and YES, i can calculateIt's for me, and for instructor also. My first circuit was a electric fence using...car ignition coil, and a 12v relay. I made a "relay buzzer" wich closes and opens the circuit verry fast, and i hooke'd up to car ignition coil, and it workes.That project was presented at school and i had a two 10+ note.Now i'm just want to make a v2 of that project, a little more clean, and more...electric (because with that relay was a little bit mecanic) and present at school.Now i'm have some questions.I have a lot of components scraped from: four crt tv,two radio-tape-cd players (rezistors, inductors, capacitors, tranzistors, diodes, flybacks etc.)Also i made a nice lab power supply using a atx power supply(650watts).For heatsink for irf630 i can use an cpu heatsink with a fan, it would help not to destroy the mosfet ?I'm beginner of tranzistors and mosfets, but i can do a big lists of all my tranzistors and mosfets wich i have if that will help doing my circuit.It's my english easy to understand ? Because all my english it's learned from youtube-vids, movies, and reading, not from school...EDIT: I watched a datasheet for an FAIRCHILD IRF630 and i've seen this:Gate Threshold Voltage (min=2; max=4)I didn't know what "threshold" means, so i took up the dictionary and search.The gate threshold voltage represent the voltage applied to the gate to be opened right ?I think in my circuit, on the "OUT" pin, isn't trigger more than 4 volts, right ?

author: madshaman
author_rank: Frequent Contributor
timestamp: November 17, 2013, 12:26:02 pm
content: Hi GFXMihnea, thanks for answering the questions I've asked.I'm traveling over the next few days so I may not be posting regularly.I want to address your questions well and we'll start with a couple simplified versions of sections of the circuits you posted so that you'll be able to answer some of your questions for yourself.I'll go over the behaviour of a diode and how it's different from a resistor (when "forward biased") and how it drops voltage.To answer your question about the threshold voltage, it actually has a special meaning with regards to how a mosfet physically operates, but essentially you are right. However, we're still going to calculate voltages on both the gate and across the drain-source and look at the characteristic curves once we have these values.We'll go over every part of your circuit and make sure we understand exactly how each part is working (and maybe even make some improvements).We'll also go over how the 555 generates the pulses (it involves charging and discharging that capacitor which will determine both the duty cycle and the operating frequency).Finally we'll look at the inductance effects in play (do you have any plans to acquire an oscilloscope? If not, I might find the time to mock up a similar circuit to show you some subtle details on one of mine).It can be really fun to play with an ignition coil, and they are *relatively* safe because of their driving ability (the amount of current they can source is limited by the internal resistance of the secondary coil). However *please* be aware that even driving one from your "buzzer" circuit is potentially even (even fatal) and driving one from a 555 timer can be even more so. There is also danger on the primary side of the coilPlease don't underestimate the danger that arise from being startled, disoriented or having your muscles spasm involuntarily.Bear with me, it's fun to make this kind of circuit, but fully understanding this circuit in detail well help you design and build even more cool stuff in the future.Cheers!

author: GFXMihnea
author_rank: Contributor
timestamp: November 17, 2013, 03:49:25 pm
content: do you have any plans to acquire an oscilloscope?No at the momment, in my country this things are really expensive and, at first i will try to buy some other tools(soldering station, etc.)Now to the the ignition coil, i made a little relay, and hook-up a "X capacitor" of 0.56uF in paralel of the plus and the minus of secondary now, i have 1cm spark, and...well i get a little sock from that and it's like an stun gun.Now, i'm looking everyday at the circuit trying to understand every part, so first of all it's the mosfet.I find in my scrap one irf630 mosfet and i try to make a simple circuit, this is my schematic and my problem:Now i have a question, should i order the components, will this circuit work ? First of all, i must have the components and after i will understand all category.

author: Chris_PL
author_rank: Contributor
timestamp: November 17, 2013, 05:29:59 pm
content: Quote from: GFXMihnea on November 17, 2013, 03:49:25 pmYour problem is called 'gate capacitance'.

author: GFXMihnea
author_rank: Contributor
timestamp: November 17, 2013, 06:29:04 pm
content: I searched for "gate capacitance" on google, but it's a tuff english for me, since my english isn't native language for meCould someone explain, what it's really a gate capacitance, and why it's this happening ?Because of the human capacitance ?

author: IanB
author_rank: Super Contributor
timestamp: November 17, 2013, 07:02:41 pm
content: Quote from: GFXMihnea on November 17, 2013, 06:29:04 pmIf you look at the gate symbol on the schematic, you will notice it is not connected to anything inside the MOSFET. This means that if you leave the gate terminal unconnected on the outside it will be "floating" and will have an unknown voltage on it. This means the MOSFET will be neither on nor off, but will be in some poorly defined state.Short anwer: the gate of a MOSFET must always be tied to a definite voltage with a low impedance path. You can connect it to a positive voltage or to a negative voltage or to ground, but never leave it dangling.

author: GFXMihnea
author_rank: Contributor
timestamp: November 17, 2013, 08:35:11 pm
content: I understand now, thank you for all your help.Now, let's get back to the subject, shall we?This is the circuit:I searched on the web for that schematic and find this, wich is the same, just it's mounted on a pcb.Now: as 1uF tantalium capacitor, i can use a electrolitic one ?

author: IanB
author_rank: Super Contributor
timestamp: November 17, 2013, 08:46:34 pm
content: Quote from: GFXMihnea on November 17, 2013, 08:35:11 pmPossibly, but if electrolytic I would make it bigger, say 100 uF.

author: GFXMihnea
author_rank: Contributor
timestamp: November 18, 2013, 04:21:17 am
content: Or, i can connect a 12v/50a relay at the drain of the mosfet , i thinkg inside the relay will be an electric arc and that would destroy the relay, but i think that would help irf630 from staing "alive" since i've got no irf640.

author: madshaman
author_rank: Frequent Contributor
timestamp: November 18, 2013, 06:05:59 am
content: Hi GFXMihnea, I think it will be easier for you to answer some of your questions once you fully understand this circuit.Here's a first step: I've drawn a small schematic which is a simplified version of part of your circuit.Can you tell me:1) What part of the circuit is it?2.1) What is the voltage appearing on node 'a' relative to ground?2.2) How did you calculate this voltage?If you will answer these questions, we'll move forward. If you have some difficulty, we'll try to resolve that.

author: GFXMihnea
author_rank: Contributor
timestamp: November 18, 2013, 04:03:19 pm
content: Well, if i can i want to give a name for all the resistors.1ohm=R10.50ohm: R2I do some research on the internet to know how to calculate and find that:http://en.wikipedia.org/wiki/Voltage_dividerI have thisVin=12VVout=node "a"I'm rearange the formula to:Vout = (R2 / (R1 + R2)) * VinVout= [ 0.5 / (1 + 0.5)] * 12Vout= (0.5 / 1.5) * 12Vout= 4The node a has a 4voltsBUT!The diode has an voltage drop right ?I searched for the diode datasheet and find thatMaximum Instantaneous Forward Voltage Drop= 0.934-0.93= 3.07Answer: node a has 3.07 Volts.

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2013, 04:46:37 pm
content: The diodes don't provide any back-EMF protection. You don't want back-EMF protection anyway as it's responsible for the inductive kick-back which generates the high voltage. Make sure the MOSFET can handle the voltage (you may need to get a higher voltage rated part, 400V will probably do) and remove the diodes.

author: GFXMihnea
author_rank: Contributor
timestamp: November 18, 2013, 05:23:04 pm
content: IN MY IRF630 Mosfet datasheetDrain-source Voltage= 200vAnd irf634Drain-Source Voltage= 250So...fail, right ?

author: madshaman
author_rank: Frequent Contributor
timestamp: November 19, 2013, 07:11:35 am
content: Quote from: GFXMihnea on November 18, 2013, 04:03:19 pmGood stuff! Thank-you for making the effort; it's going to pay off. I'm currently travelling, but will continue with introducing Kirchhoff's circuit laws and how to analyse that mini-circuit with those.You can read a bit in advance here:http://en.wikipedia.org/wiki/Kirchhoff's_circuit_lawsmaybe there is also Romanian translation.

author: madshaman
author_rank: Frequent Contributor
timestamp: November 19, 2013, 07:13:31 am
content: P.S. Don't worry about the scary math, I'll go slowly and show you it's simpler than the wiki page might indicate.

author: GFXMihnea
author_rank: Contributor
timestamp: November 19, 2013, 12:33:35 pm
content: I've learned in at highschool about Kirchhoff law(current and voltage)I've find it in romanian and i'm gonna do some lecturing:http://ro.wikipedia.org/wiki/Legile_lui_Kirchhoff_(circuit_electric)Today at school i've teardown an old osciloscope(i think it's rusian , inside him it's a long catodic tube):http://images4.okr.ro/auctions/2009/10/25/240451694-4264078-700_700.jpglike thisand my professor gived me some parts likeA verry big transformator(9cm x 9cm x 9cm) with a lot of output wires, and the two white sleved wires i think are the primary(i've measured and have continuity)A couple of 100k potentiometers (great for my project)Two ad131 transistors (i didn't find a datasheed)An 100uF, 385Vcc capacitor (i think isn't polarized)http://images8.okr.ro/auctions.v3/215_300/2012/02/25/e/f/677629966305701242077668-6149250-215_300.jpgSorry for crappy images, taken with my web-cameraQuestion: how to find the primary ?That two white wires resistance is: 1 ohmAnd the two red wires are 40 ohm and have an bigger diameter form the others.I think the two red wires are the primary, because i know that the primary has more turns(in a stepdown trans.)EDIT: It seems it's a stepup and stepdown transformer, the primary coil are 2 red wires comming from trans.On one secondary i've find 320v :O320v stepped up from 230 xD

author: madshaman
author_rank: Frequent Contributor
timestamp: December 17, 2013, 03:15:41 pm
content: Hi GFXMihnea, I'm back from traveling and the aftermath. Let me know if you're still around and would like to continue!Sorry for the long pause.