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| This Astable Multivibrator doesn't work |
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| xzswq21:
Hello I 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 |
| xzswq21:
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. :scared: |
| richard.cs:
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. |
| Labrat101:
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 . |
| xzswq21:
--- Quote from: Labrat101 on June 03, 2021, 04:14:40 pm ---I don't see this a practical test . Just because it will oscillate doesn't mean that it is good. --- End quote --- I 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 MOSFET --- Quote from: Labrat101 on June 03, 2021, 04:14:40 pm ---Also its recommended to put at least 1k in series with the Gate. --- End quote --- Do you mean in series with the 100nF capacitor? Why it's important? --- Quote from: Labrat101 on June 03, 2021, 04:14:40 pm ---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. --- End quote --- Ch1 and Ch2 of the oscilloscope between which nods? Between Gate and Drain pins of the MOSFET in this circuit? |
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