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TC1 lcr-meter transistor-tester fix

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masster:
1) What is the maximum voltage/current supported by the input ports of ATMega?
2) Is there a better solution to protect the input ports against charged capacitors?
3) Is there a better solution to protect the input ports against wrongfully connecting tested parts? Fool proof and full proof.

indman:

--- Quote from: masster on August 12, 2022, 03:42:03 pm ---1) What is the maximum voltage/current supported by the input ports of ATMega?

--- End quote ---
This information can be found in the datasheet of the AVR. MaxVoltage -5.5V, MaxCurrent for 1 port - 40mA,for all ports -200mA


--- Quote from: masster on August 12, 2022, 03:42:03 pm ---2) Is there a better solution to protect the input ports against charged capacitors?

--- End quote ---
You can replicate the protection circuit using a relay with 3 contact groups as shown in the Karl-Heinz manual.
But as correctly noted by the authors of the project - no protection provides a 100% protection guarantee. The same goes for the answer to your question №3

masster:

--- Quote ---Most likely the ATmega was damaged too.
--- End quote ---
If the display says "Testing..." and after 5 seconds it shows "Cell, 1.40 V between pin 1 and 2" , is that definitive proof the ATMega is damaged?

indman:
Yes, this is one of the characteristic signs that one or more measuring ports of the processor are damaged. To make sure of this definitively, you can check the resistance of each port relative to GND and VCC. Provided that the SRV05-6V8A protective assembly is removed and nothing else is connected to the ports in parallel.You can also believe whether the nominal values of the measuring resistors 680ohm and 470k are in order. If they are in good condition, and the port resistance is very different, then this indicates damage to the processor.

masster:
I decided to remove the 6V8A and SRV05-4 to measure the resistance between each pin of ATMega and GND. See schematic below. Where there is no resistance value, the resistance is higher than 2 Mohm.
I find abnormal the resistance between VCC and GND, which is still low IMO. I have used an external power supply for VCC and I have measured a current consumption of 170 mA. That is way too high, over what 78L05 can deliver. Hopefully there is another faulty filtering capacitor on the VCC bus. I didn't remove them all. I still have hope that ATMega is not toasted.

Please have a look at my resistance measurements and tell me what else you find odd. I don't like the values on pins 34 to 37, corresponding to PA0 to PA3.

Right now, with external VCC supply and battery connected, I get the Cell of around 1.2V between 1 and 2. Sometimes I get a Cell between pin 1 and 3.

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