The answer to the circuit to transmit temperature readings over a 4-20ma loop ends with the comment "The green LED is just to drop the voltage because the LM358 output can't get closer to the power rail."
However, in the schematic GND for the LM358 (pin 4) is connected to the anode of the green LED.
Wouldn't it make more sense to have it connected to the cathode of the green LED?
Then the inputs to the op amps would be operating in a voltage range that would be ~2 volts smaller than the supply range of the op amp.
Or am I missing something?
question about 4-20ma RTD circuit in June Q&A
Re: question about 4-20ma RTD circuit in June Q&A
Yeah, nav, I understand where you're coming from. But I'm not knowledgeable enough with the current loop circuit
to give you a decent answer. I'm a bit surprised no one else has responded yet. The fact that U1 is also connected
at the LED anode is one clue to me. That and the first sentence of the last paragraph, "The current through R4 plus
the bias current of U1 and U2 totals about 2 mA and is part of the 4 mA minimum current.", means that all that
current flows through the LED. Which leads me to believe that is how Mr. Kincaid intended it.
to give you a decent answer. I'm a bit surprised no one else has responded yet. The fact that U1 is also connected
at the LED anode is one clue to me. That and the first sentence of the last paragraph, "The current through R4 plus
the bias current of U1 and U2 totals about 2 mA and is part of the 4 mA minimum current.", means that all that
current flows through the LED. Which leads me to believe that is how Mr. Kincaid intended it.
Who is online
Users browsing this forum: No registered users and 97 guests