mosfet diy tester or low cost - Page 1
Forum section: electronics / beginners

author: amtpdb
author_rank: Regular Contributor
timestamp: January 19, 2014, 10:33:17 pm
content: Hi:I need a way to test the mosfets on a subwoofer I am looking at for a friend. I cannot see anything burnt or bulging so I need to test the mosfets and transistors. I am trying to learn electronic on my own. I built a tester off youtube that used a flashlight, but I don't seem to get the same results as they show. 2 positive and 1 negative wires off the battery. I need to test an irf 530 and an irf 530n(are they the same or test the same?) 1rfz14. I need to test a l7900 and a l7800 series voltage regulator also. Do all of these need to be removed from the circuit or can they be tested in place. Any help will be greatly appreciated. I have always wanted to learn about electronics, but never had anyone around that could help and answer questions. I have watched alot of youtube, but some answers that are very basic are never mentioned. Such as which pin gets the positive lead and which gets the negitve lead when testing.Thanks in advance for any help..Don

author: txescientist
author_rank: Regular Contributor
timestamp: January 19, 2014, 11:12:47 pm
content: Find datasheets for regulators and transistors. Google part number and you will find many useful info on specific part. First test voltages. Find pinouts for voltage regulators and check voltages - input and output. Transistors and mosfets you can't test in circuit. You must first desolder them and test. Check caps, first discharge capacitor under test, and then use multimeter to check it. Measure capacitance and measure resistance. Some tests you must do when it's connected to mains voltage.Don't touch any MAINS connections (fuse and mains transformer) !!!Post pictures or just tell symptoms... Check speaker if it woks, sometimes output cap shorts, and DC current flows trough speaker and damages its coil...mosfet tester circuits:https://www.google.rs/search?q=mosfet+tester&client=firefox-a&hs=n02&rls=org.mozilla:en-US:official&tbm=isch&tbo=u&source=univ&sa=X&ei=ZFncUoqhKeqAywOVloKYBA&ved=0CCQQsAQ&biw=1036&bih=548

author: amtpdb
author_rank: Regular Contributor
timestamp: January 20, 2014, 06:00:44 am
content: Thanks for the information. The subwoofer came over apart. One of my friends told him it was most likely a cap. I pulled the main power cap and it tested fine. He only told me the light telling him it was on but nothing came out of the speaker. I will not look through it. I re soldered a fue iffi joints and one input jack joint. The input was not one he was using!Thanks again and if you have any suggestions where to start looking, chime in. I am going to put it back together and plug it in and see what I find.Thanks.Don

author: txescientist
author_rank: Regular Contributor
timestamp: January 20, 2014, 08:35:23 am
content: If power supply voltages are OK, then you should use signal tracer. This is what you can do: Connect input to an sound source and use another pair of speakers to trace signal. You can build something like this:http://www.learningelectronics.net/circuits/pulse-generator-and-signal-tracer.htmlor use one cap in series with input of speakers you use to trace signal - general idea is to protect the input of the working speakers. Google signal tracing and you will find all that you need. Where signal is lost that component is faulty. Better than this is using oscilloscope. Also you should download schematics for it if it is available. Problem with my 5.1 system was input channel digital selector chip. I didn't have replacement so just shortened input and output on the chip and now it works just with one input source - PC, which is OK for me. Sorry if my explanation is little confusingRegards

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2014, 09:36:13 am
content: You can test mosfets quite easy by wiring them to a load and a powersource and then touching the gate with one finger while touching VCC or GND with another.If you can turn it on touching VCC and off touching GND it's probably fine.Keep the powersource under 15V to avoid pushing the gate too high.12V is good as most people normally have some 12V lamps available for the load and a 12v DC source.

author: amtpdb
author_rank: Regular Contributor
timestamp: January 21, 2014, 02:17:11 am
content: Hi:I want to thank both of you for the information. As I said I am trying to learn this on my own by reading and watching. I will build the meter this weekend. I will try youtube to see if I can watch someone use one. Do you need to have the unit on or does the battery used input the signal? I don't quite understand the process, but am more then interested and willing to learn how to use this meter.Thanks again to both of you and it looks like I might learn something here, this is what it is all about!Don

author: amtpdb
author_rank: Regular Contributor
timestamp: January 21, 2014, 06:01:46 am
content: Ok don't laugh, I went to digi-key and tried to get the transistors and the capacitors ordered to build the signal tracer. The transistors had so many different listings for them with different letters after them I don't know what to order. The capacitors don't list as noted for the signal tracer circuit. Could someone tell me which ones would be best and educate me as to how you made the choice looking at all the different choices with all the different letters after them and or numbers!I really appreciate your taking the time. 2n2 630v cap- ceramic or polyester4n7 63v ceramic or polyester, bc557 45v 100ma PNP, bc547 45v 100ma npn.Don

author: Sigmoid
author_rank: Frequent Contributor
timestamp: January 21, 2014, 06:11:25 pm
content: I wonder, is there a way to determine the channel type and pinout of a JFET or MOSFET with nothing but a single diode tester, like you can for a BJT?Since I mentioned it, I guess I'll just hash out the BJT test method for those who don't know it. It's invaluable.NPN:base(+) to collector(-): smaller diode dropbase(+) to emitter(-): larger diode drop (the actual difference is often only a few millivolts)all other pairings: "infinite" resistancePNP:collector(+) to base(-): smaller diode dropemitter(+) to base(-): larger diode dropall other pairings: "infinite" resistance

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 21, 2014, 10:23:24 pm
content: Yes.Using just a multimeter, you can identify and test a mosfet to see if it works, at least partially, these tests work best on power mosfets because of the large gate capacitance.First, using the diode test function on your multimeter, check with the leads both ways to find the pin not connected to any other - it may seem not connected if you check one way but may appear as a diode the other.This pin is the gate.If you find more than one pin not connected to anything else, it is either a dual gate MOSFET or a N/C pin.Now, short all the pins on the FET to the gate, to empty the gates charge, this should turn the FET off.Using the continuity tester, check to see what other pins are connected and make a note. While doing this, take care not to probe the gate.If there are 2 different groups of pins, one is the drain, the other is the source. If there are 3 different groups of pins, the additional pin is the body.On the 3 pin device, you need to find out what voltage turns it on, positive or negative.Switch your multimeter back to the diode test mode.Testing to see if it is an N-Channel MOSFET:Put the positive lead on the gate and touch the two unknown pins with the negative lead.This should give the gate a positive charge, relative to both the drain and source, or a positive Vgs.Check between the two unknown pins with your multimeter in diode test mode. If it is a short in both directions, or a very small diode drop (much smaller than before), the FET is an N-Channel FET, it required a positive Vgs to turn on. You can verify this by holding the negative lead on the gate and touching the other pins with the positive lead, to turn the FET off, then check that it is no longer a short circuit, but instead a diode.Testing to see if it is an P-Channel MOSFET:Put the negative lead on the gate and touch the two unknown pins with the positive lead.This should give the gate a negative charge, relative to both the drain and source, or a negative Vgs.Check between the two unknown pins with your multimeter in diode test mode. If it is a short in both directions, or a very small diode drop (much smaller than before), the FET is an P-Channel FET, it required a negative Vgs to turn on. You can verify this by holding the positive lead on the gate and touching the other pins with the negative lead, to turn the FET off, then check that it is no longer a short circuit, but instead a diode.Now that you have worked out if it is a P-Channel or N-Channel MOSFET, measure the diode drop and take note which pin is the C/Kathode(negative lead) and which is the Anode(positive lead). On a P-Channel MOSFET, the c/kathode will be the source, and the anode will be the drain. On an N-Channel MOSFET, the c/kathode will be the drain and the anode will be the source.You have 3 pins and the channel type of your MOSFET resolved, tada!If you have a 4 pin MOSFET, you need to probe all the unknown pins, aside from the gate with the diode test functionality, If the MOSFET has a double diode with a common anode (positive lead stays on the same pin and if you probe the other two with the negative they both give you diodes) it is an N-channel MOSFET, If the MOSFET has a double diode with a common c/kathode it is a P-Channel MOSFET.The pin that the common dual diodes both connect to is the body/bulk. The other two pins can be either the source, or the drain, depending on which you choose to connect the body diode to.