Good afternoon.
First of all, sorry for the unfortunate diagram I've done, but I still think it's understood.
My problem is the following. I try to measure if current flows through a shunt resistor (0.2 ohms and 5 watts) for which I am using an LM358P operational amplifier. If I do not connect the load, that is, empty, in both inputs of the operational (both the inverter and the non-inverter) the voltage is the same and the LED turns on. If I connect the load, I measure a voltage drop on both sides of the shunt (and therefore on both inputs of the operational) of 20 millivolts, but the LED does not turn off. If I invert the inputs, it also does not turn off, that is, it always stays on although the two inputs of the operational are detecting unequal voltages.
I've tried a thousand and one combinations and I can't see where the fault is, even changing the LM358P for a new one. I only get it to work as I want if I put a shunt resistor of 4 omhios or more, because then the voltage drop is several volts, but then the circuit loses all sense.
My question is: Is the operation not supposed to detect voltage variations of a few millivolts? Why, even if it is a difference between tens of millivolt inputs, the operation does not react? Surely there is some clear error, but I don't know how to see it, really. I appreciate any guidance.
Thank you all. Happy New Year to all!
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Electronics and Circuit Design
Good afternoon.
First of all, sorry for the unfortunate diagram I've done, but I still think it's understood.
My problem is the following. I try to measure if current flows through a shunt resistor (0.2 ohms and 5 watts) for which I am using an LM358P operational amplifier. If I do not connect the load, that is, empty, in both inputs of the operational (both the inverter and the non-inverter) the voltage is the same and the LED turns on. If I connect the load, I measure a voltage drop on both sides of the shunt (and therefore on both inputs of the operational) of 20 millivolts, but the LED does not turn off. If I invert the inputs, it also does not turn off, that is, it always stays on although the two inputs of the operational are detecting unequal voltages.
I've tried a thousand and one combinations and I can't see where the fault is, even changing the LM358P for a new one. I only get it to work as I want if I put a shunt resistor of 4 omhios or more, because then the voltage drop is several volts, but then the circuit loses all sense.
My question is: Is the operation not supposed to detect voltage variations of a few millivolts? Why, even if it is a difference between tens of millivolt inputs, the operation does not react? Surely there is some clear error, but I don't know how to see it, really. I appreciate any guidance.
Thank you all. Happy New Year to all!
Intensity measurement through shunt resistance with operational
hace 6 años
hace 6 años
Good afternoon.
First of all, sorry for the unfortunate diagram I've done, but I still think it's understood.
My problem is the following. I try to measure if current flows through a shunt resistor (0.2 ohms and 5 watts) for which I am using an LM358P operational amplifier. If I do not connect the load, that is, empty, in both inputs of the operational (both the inverter and the non-inverter) the voltage is the same and the LED turns on. If I connect the load, I measure a voltage drop on both sides of the shunt (and therefore on both inputs of the operational) of 20 millivolts, but the LED does not turn off. If I invert the inputs, it also does not turn off, that is, it always stays on although the two inputs of the operational are detecting unequal voltages.
I've tried a thousand and one combinations and I can't see where the fault is, even changing the LM358P for a new one. I only get it to work as I want if I put a shunt resistor of 4 omhios or more, because then the voltage drop is several volts, but then the circuit loses all sense.
My question is: Is the operation not supposed to detect voltage variations of a few millivolts? Why, even if it is a difference between tens of millivolt inputs, the operation does not react? Surely there is some clear error, but I don't know how to see it, really. I appreciate any guidance.
Thank you all. Happy New Year to all!
hace 6 años
Interesting observation, Jorgeal. I would swear that I did measure it and it didn't change the output
In any case, tomorrow I look again. Do you see the diagram correctly?
hace 6 años
Juite, the idea is to capture the small voltage drop that also causes the small shunt resistance, and send this difference to the comparator. Then send the output to the led. The scheme apart from bungling I see that it is wrong, the cathode of the LED must go to ground, not to the load. Sorry for the mistake.
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