In the circuit design, 4-20mA generating circuit is commonly used. There are many design schemes on the Internet, and of course there are ready-made chips (such as AD694, which I have used before, and these chips are generally compared). Regardless of whether it is a circuit or a chip, most of them are not isolated from the MCU, and at the same time, there are also no isolation between several 4-20mA circuits. This kind of stability on the industrial site is easily affected.

By analyzing other people's isolation card circuits, a better and less expensive design is obtained.

The core circuit is as follows:

The left side of the optocoupler is isolated from the right side. The left side is generally controlled by a microcontroller, CPLD, DSP, etc., V_ref on the 4th pin of the optocoupler is a 5V reference source (such as TL431), and the voltage of the 3rd pin of the op amp is determined by the 3rd pin of the optocoupler The output PWM and the adjustable resistor R57 are composed of two parts. The following two sets of RC low-pass filtering get the DC voltage, divided by R55 is the output current.

The circuit has several points:

1. This part of the adjustable resistor R57 is mainly used to improve the benchmark. For example, the original current output is at least 0mA, which can be increased to about 4mA, which simplifies the output accuracy.

2. Each group of 4-20mA circuits are isolated, but the two grounds between adjacent 4-20mA circuits need to be connected to a varistor. If they are not connected, they will interfere with each other, and I do n’t have specific reasons. Clearly, I discovered it during the original test. If you know the reason, hope to inform.

3. Isolation of power supply can be directly purchased off-the-shelf DCDC isolation chip, 1W is enough, I used Jinshengyang A2412S-1WR2, it seems to be more than a dozen pieces, if you consider the cost you can use a transformer, while outputting multi-channel isolated power, I am on the transformer The design is unfamiliar, so I won't talk about it here.