This Astable Multivibrator doesn't work - Page 1
Forum section: electronics / general

author: xzswq21
author_rank: Frequent Contributor
timestamp: June 03, 2021, 05:12:16 am
content: HelloI wanted to build a MOSFET tester and I found a circuit in the bellow:https://www.electronics-lab.com/project/mosfet-tester/I've built it but it doesn't work, so I decided to simulate it in the LTspice, in simulation it works! (I have attached the circuit)Q2 is a LED driver, M1 is under test and you can replace it with any MOSFET, If MOSFET is working it will operate in the Astable Multivibrator circuit causing the LED to flash.Could you please check it to help me? why the circuit doesn't work?Thanks

author: xzswq21
author_rank: Frequent Contributor
timestamp: June 03, 2021, 07:25:19 am
content: I found the problem! if you see there is a 10M resistor between the Gate and Drain pins of the MOSFET so it says this area is very sensitive to any leakage, you can test it in the simulation file, just replace the 10M resistor with a 1M, the output goes wrong, so when you built the tester place at least 3mm free space around the MOSFET pins and it's better to place some slot around the MOSFET pins. now the Circuit works good.

author: richard.cs
author_rank: Super Contributor
timestamp: June 03, 2021, 01:42:14 pm
content: I don't see any reason for this part of the circuit to be so high impedance unless the intention is also to test for MOSFET gate leakage. Making R10 1MOhm and then increasing C1 to 1 uF should keep the overall circuit behaviour the same at 1/10 the impedance and be a lot less sensitive to construction. You could probably also simplify by omitting Q2 and connecting the LED to the collector of Q1.

author: Labrat101
author_rank: Unknown
timestamp: June 03, 2021, 04:14:40 pm
content: I don't see this a practical test . Just because it will oscillate doesn't mean that it is good.First if its only flashing a led at a few Hz .Evan a bad fake from China will pass this test .Also its recommended to put at least 1k in series with the Gate.I think if you have an oscilloscope at hand even a simple one and do an x y of the DUT . Would be more practical.90% of most parts coming out of China are clones .

author: xzswq21
author_rank: Frequent Contributor
timestamp: June 03, 2021, 06:51:51 pm
content: Quote from: Labrat101 on June 03, 2021, 04:14:40 pmI agree with you, Actually I can not be sure about the performance of the MOSFET, in my application the MOSFET uses in a 220VAC flyback power supply and there is 200VDC across flyback transformer and the MOSFETQuote from: Labrat101 on June 03, 2021, 04:14:40 pmDo you mean in series with the 100nF capacitor? Why it's important?Quote from: Labrat101 on June 03, 2021, 04:14:40 pmCh1 and Ch2 of the oscilloscope between which nods? Between Gate and Drain pins of the MOSFET in this circuit?

author: Labrat101
author_rank: Unknown
timestamp: June 03, 2021, 08:41:55 pm
content: Quote from: xzswq21 on June 03, 2021, 06:51:51 pmFirst if your talking about a High voltage Flyback there is properly a internal diode across the source & drain . I would recommend reading the spec sheet on your MosFet and there is often a diagram with examples .If the FET you are using is rated @ 200v plus and was used to run a flyback. Run at high frequencies depending on design etc etc and so on .Most manufactures recommend a 1k as close as possible in series with the Gate also mention in the data sheet if required .If you google transistor tester / mosfet this has been mention else were in this forum . If you want to make yourself this type of project there are 100 of tester ideas that work . also there are $20 testes on Ebay etc.But its more fun to build it . Transistor curve trace is a nice project and there is also a mosfet tester as well .I built a transistor curve trace which is a must have if you want to match transistors . I found the circuit via this forum along long time back so I don't remember were .This circuit you have only tests that a LED can flash .. .without being rude do some Googling and a little research you will find something that fits your Bill .Update :: there are 1000s of types of MosFet ranging though manyusages :: some mosfets do have isolated Gates on the High voltage types and are hard to test with out a good tester ..Have Fun

author: Labrat101
author_rank: Unknown
timestamp: June 03, 2021, 09:29:18 pm
content: This tester No comment .Quick & Dirty .

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2021, 01:41:12 pm
content: Quote from: Labrat101 on June 03, 2021, 09:29:18 pmYou might want to add a 1M to 10M gate-source resistor, otherwise the MOSFET's gate can pick up noise, causing the LED to flicker and, to avoid damaging the gate from ESD, always touch the source side first, before the gate.

author: Labrat101
author_rank: Unknown
timestamp: June 04, 2021, 05:31:37 pm
content: Quote from: Zero999 on June 04, 2021, 01:41:12 pm@Zero999That's WHY I wrote NO Comment .. Its a Demo ONLY.Personally I use a full Tester & or Curve Tracer Test

author: xzswq21
author_rank: Frequent Contributor
timestamp: June 04, 2021, 06:04:18 pm
content: Quote from: Labrat101 on June 04, 2021, 05:31:37 pmWhat do you mean full tester? Could you please introduce me a curve tracer or a full tester?Plus I have an idea, can I modify the circuit to generate a 1KHz pulse instead of a few Hz? I will see the output with an oscilloscope, is it more effective?Thanks

author: Labrat101
author_rank: Unknown
timestamp: June 04, 2021, 07:13:11 pm
content: Quote from: xzswq21 on June 04, 2021, 06:04:18 pmhttps://www.kandh.com.tw/uploadfiles/592/Products/Testing-Instrument/KI-3020A/KI-3020A-Catalog/_en_ki-3020a_10801.pdfI built my one Many moons ago years ago . This one is about the same . in fact looks very similar .There are quite a few circuit designs floating around read up on this they are not so complicated .the transistor tester side uses an pulse staircase design with XY out.The Mosfet is a lot different as you can Not putmore than a few ua on the gate only voltage. & has a extremely high hfe . and when active switched will have a verylow internal resistance usually less than an Ohm between the source and Drain .If you are a beginner I would look at the arduino component tester there was a whole project up on GitHubwhich would get you on the right track and it tests all components and about 90% + accurate .and will recognise most MosFet's , Transistors pnp npn ,caps ,diodes , resistors etc.As I don't know what you want exactly . If you are starting out . which I guess you are by the first Question.Or Look on Ebay etc there are component testers for less than $20 .. (Dave did a YouTube on 1 long time back)If you have CRT scope then curve tracer will give you a visual of what's what .Personally i am OLD school analogue only . Non of my test gear is newer than 15 years . My Tech scope is having its 35 birthday soon .I think that the arduino component tester would be good . its cheap and will give you fun building it and learning .https://create.arduino.cc/projecthub/plouc68000/ardutester-v1-13-the-arduino-uno-transistor-tester-dbafb4

author: Labrat101
author_rank: Unknown
timestamp: June 04, 2021, 10:12:35 pm
content: UPdate ::Read this it will explain how to build a curve tracerit simple and it works well .FET CURVE TRACER (recommended)http://eprints.utar.edu.my/88/1/EE-2011-0703010-1.pdfRead the entire Document its a bit long as it explains how each section worksand how to use with examples . the power supply is towards the end of docThe one I built I added another row of DAC .. ie D1 to D6 . with a switch HI/Low so I could testPower transistorsusing the other outputs from IC1 4024. .. also adding another switch for 4 or 8 lines (traces)Just make sure that the switch S1 & S2 are wired correctly as it will burn a Fet gate if testedas a pnp or npn . Recommend putting a separate socket for the Fet . as not to make error whentesting .Enjoy

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2021, 07:43:13 am
content: Quote from: Labrat101 on June 04, 2021, 05:31:37 pmWhy post it, if you didn't want anyone to comment?Another improvement to that circuit would be to replace the finger, with a switch. A small gate resistor, say 1k would also be a good idea, to avoid short circuiting the supply, if the MOSFET's gate is damaged. Also note that the gate-source voltage rating of some small, low gate threshold voltage, MOSFETs is under 12V.

author: Labrat101
author_rank: Unknown
timestamp: June 07, 2021, 08:21:35 am
content: Quote from: Zero999 on June 07, 2021, 07:43:13 am@ Zero999If only you had a Brain and could Read the whole subject you would have realised why I put it thereBut I guess you only want to add Stupid remarks that are not only off subject but are not relevant or helpful in any way .The diagrams that were added are 100% operation and also allow for modification .and Based on the Original Design .Have a good life, and go Patent some thing ..

author: Halcyon
author_rank: Unknown
timestamp: June 07, 2021, 08:47:41 am
content: I'm too tired for crazy. Play nice or I start deleting/locking stuff.

author: xzswq21
author_rank: Frequent Contributor
timestamp: June 07, 2021, 08:49:25 am
content: Quote from: Halcyon on June 07, 2021, 08:47:41 amPlease edit their comments

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2021, 09:17:15 am
content: Quote from: Labrat101 on June 07, 2021, 08:21:35 amI've merely suggested ways to improve the circuits you've posted, nothing personal. For the benifit of the orignal poster, here are some schematics illustrating them. Have a nice day.