The LM339 zero-crossing comparison circuit has input but no output

neu_zouqilei

[ attach ] 704644 [ / attach ]
Figure ...

Further ahead is the circuit that needs to be sampled. The ground on this side is connected to the ground on the primary side of the transformer, and the ground of the zero-crossing comparator is also this ground. All of the above parts are the following one Can any big guy guide me where the problem lies?

Attached is the schematic diagram of this module

neu_zouqilei

I'd like to ask if anyone can provide some ideas.

maychang

neu_zouqilei posted on 2023-6-12 18:55 I'd like to give you a thumbs up. If anyone can provide some ideas, that would be great

According to the first picture in the first post, R27 and R28 are both 100 kilo-ohms, R31 is 10 kilo-ohms, and R25 and R26 are 5.1 kilo-ohms. Therefore, even if I_To_Rec is grounded, the output of U6_2 will be at a low level because the potential of the non-inverting input is lower than that of the inverting input.

Then, I_To_Rec must be lower than ground so that the output of the comparator can output a high level. Is I_To_Rec not negative enough to ground?

damiaa

LM339 - Input resistor size configuration 100k 10.2k + input 100k 10k This is definitely not OK

Try 100k 5k + 100k 10k at the input end. You can make an adjustable resistor to try.

neu_zouqilei

damiaa posted on 2023-6-12 19:35 LM339 - Input resistor size configuration 100k 10.2k + input 100k 10k This will definitely...

You mean the problem of unbalanced resistance, right? But I am confused about one thing, that is, U1A of LM339 also uses this circuit, and the effective value and peak value of the input signal are almost the same as the value of the input signal of U1B. U1A has a good output according to this, but U1B does not. Can you guide me?

damiaa

neu_zouqilei posted on 2023-6-12 20:00 You mean the problem of unbalanced resistance, right? But there is one thing that confuses me, that is, U1A of LM339 also uses this circuit, and the input signal...

Please test it with an oscilloscope.

maychang

neu_zouqilei posted on 2023-6-12 20:00 You mean the problem of unbalanced resistance, right? But there is one thing that confuses me, that is, U1A of LM339 also uses this circuit, and the input signal...

Since the circuits and peripheral components of U6_1 and U6_2 are exactly the same, you may want to swap V_To_Rec and I_To_Rec. This can be achieved by flying two wires on the circuit board to see if one still has output and the other has no output after the swap. This way, you can determine whether it is a problem with the input signal or a problem with the numerical error of the peripheral components.

不亦心

If the problem is not found in the schematic diagram, cut the output pin and test again to eliminate the problem of the subsequent circuit failure

neu_zouqilei

maychang posted on 2023-6-12 19:06 According to the first picture in the first post, R27 and R28 are both 100 kilo-ohms, R31 is 10 kilo-ohms, R25 and R26 are 5.1 kilo-ohms, so even if I_To_Rec is grounded, U6_2 input...

Thank you! The main problem is that R28 may be soldered through, and then when using the DC coupling of the oscilloscope to measure, it is found that the voltage of pin 7 is 5V, while the input peak of pin 6 is only 2.5V, so there is no signal.

neu_zouqilei

Bu Yixin posted on 2023-6-13 09:26 The schematic diagram does not show any problems. Cut the output pin and test it again to rule out the problem of the subsequent circuit failure

Thank you! It has been ruled out that the problem is that R28 is soldered through.

neu_zouqilei

maychang posted on 2023-6-13 08:04 Since the circuits and peripheral components of U6_1 and U6_2 are exactly the same, you might as well swap V_To_Rec and I_To_Rec, which can fly two wires on the circuit board...

Thank you for your ideas. The problem has been solved. It was mainly caused by the reference voltage of the zero-crossing comparator.

neu_zouqilei

damiaa posted on 2023-6-12 22:16 You can change the oscilloscope and test it.

Thank you! Using an oscilloscope to measure the DC coupling, I found that the reference voltage of pin 7 was 5V, and found the problem with R28.