Lessons Learned: Don't Buy Instrumentation Amps With Onboard Potentiometers

2018-04-25 - By Jaswinder Kaur Johal, Anami Thakur, Satinder Kaur, Robert Elder

Introduction

The purpose of this article is to review the findings of a student-led project where the goal was to make use of the CJMCU-333 circuit, containing an on-board INA-333 instrumentation amplifier to measure the current draw of an ESP8266. In the end, accurate current draw measurements of the ESP8266 module were not collected, however useful information and experience was gained.

CJMCU 333 INA 333 Instrumentation Amplifier

One of the primary conclusions of the project was that the choice of instrumentation amplifier board, in this case the CJMCU-333 circuit, made it difficult to get accurate readings and test the validity of the measurements. For the CJMCU-333 circuit, the potentiometer is used to control the gain of the amplifier, and even the smallest touch could throw off the consistency of the readings.

Throughout the project, another source of problems was constant lose connections. Even after soldering, there were cases were results were not repeatable.

During the experimentation, there was also a case where the CJMCU-333 circuit appeared to short out. Magic smoke appeared, and afterward the amplifier would no longer produces values anywhere near what it did previously. The precise condition that made this occur was not known, but the students hypothesize that they may have accidentally shorted Vout with 5V. Further experimentation was performed on a second version of the exact same model circuit without incident.

Flashing The ESP8266

The students found this ESP8266 flashing circuit diagram useful. Here is their circuit layout:

For flashing software, the students used 'Termite' V3.4.4, an easy to use and easy to configure RS232 terminal. After installing the software, the students started Termite and clicked the settings button to change the setting according to the COM Port and USB to TTL converter and got the following window:

The students got an "OK" when entering the command "AT" and the results below:

Next, the students used the EXPRESSIF firmware download tool:

Here is what it looks like when the module is flashed:

Measuring Current

Here is the circuit that the students used for taking simple measurements of the current through a resistor:

Unfortunately, because of the potentiometer, the precise value of the gain on the amplifier was not known when collecting these measurements. Here are some of the measurements that the students collects using the amplifier:

S.No.	Resistance	Voltage(V)	Current(I)
1	100KΩ	2.53	0.03
2	56KΩ	2.53	0.06
3	27KΩ	2.55	0.12
4	1KΩ	2.78	3.30
5	560Ω	2.95	5.85
6	270Ω	3.45	11.86
7	180Ω	4.79	17.46
8	82Ω	4.97	39.2
9	33Ω	4.89	92.4
10	15Ω	4.73	185.9

Resources

The students also found the following links useful in their analysis:

Conclusion

The students concluded that a better choice of amplifier would have been one that allowed for a more fixed gain control through the used of an externally resistor. If this work was to be repeated, the students suggested using an amplifier such as the AD620 instead.

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