Current / Voltage monitor - Page 1
Forum section: electronics / projects

author: mayor
author_rank: Regular Contributor
timestamp: February 28, 2016, 01:07:07 am
content: Hello,I am working on a small project, which aims to display voltage and current for a given source and load. It's still a work in progress, but before going too far, I would appreciate comments on the overall design, if you would care to give them. Are there glaring omissions or errors?Thanks in advance!For quick reference, INA152 is a difference amplifier, LMP7702 is a dual precision opamp, ADR5040 is a 2.048 voltage reference.Schematic:https://easyeda.com/export_nRL3XewOI/Board1.pdf?type=1&penWidthIncrease=0&version=3.2.8&_t=1456621157223.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: February 28, 2016, 01:10:19 am
content: Personally, I prefer to receive a write-up what the circuit is supposed to do rather having to reverse engineer things...

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 01:12:30 am
content: HiWhere do the 3.3 and 5V supplies come from? What is the power up sequence? How does this thing self protect (or is that required?) ?Bob

author: mayor
author_rank: Regular Contributor
timestamp: February 28, 2016, 01:48:39 am
content: 5V is from USB (Power on lower left), 3.3 is from the fixed output regulator (RT9058). As for protection, besides the fuses and one schottky diode, what is a general good practice?As for a description:The device take in power (from a wall wart, batteries, whatever), pass it through a shunt resistor (0.25ohm), and output it to the load. It is itself powered by USB 5V.The working principle is that a 250mV drop occurs for each amp of current through the shunt, and that is produced by the INA152 differential amp, which itself is fed into an opamp, mostly to limit the output to 3.3V. Input voltage is also fed directly into a second opamp and is divided by 4 (3M and 1M resistor divider). The outputs of the opamp are fed back to the microcontroller, which will interpret the voltages through ADCs, and display the result. I've made provisions for a 2.048 voltage reference (the ADCs are 10-bit, so an easy division by 2) should more accuracy be needed.

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 02:23:03 am
content: Quote from: mayor on February 28, 2016, 01:48:39 amHiWhat about the case where you have the wall wart hooked up but no USB? (Cable falls out, computer turns off, who knows ...)Bob

author: mayor
author_rank: Regular Contributor
timestamp: February 28, 2016, 03:19:53 am
content: Well, as far as I can tell, nothing of import happens. The load is powered, but the current monitor is not, since the opamp rails would effectively be 0V and GND, no 3V3 anywhere since no 5V.

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 03:17:03 pm
content: Quote from: mayor on February 28, 2016, 03:19:53 amHiSo what happens when you put voltage on the op-amp inputs and have no supply hooked up?Bob

author: mayor
author_rank: Regular Contributor
timestamp: February 29, 2016, 05:45:55 am
content: Not sure where you're leading me...! How _does_ an opamp behave without rails? What would happen on this case?

author: uncle_bob
author_rank: Unknown
timestamp: March 02, 2016, 01:51:17 am
content: Quote from: mayor on February 29, 2016, 05:45:55 amHiA normal op-amp blows up when you put voltage on the inputs with no supply voltage.Bob