Amplifying 1mV DC for current sensing - Page 1
Forum section: electronics / beginners

author: gnif
author_rank: Unknown
timestamp: February 08, 2013, 03:16:44 am
content: Hi All,This is my first time posting here, so please be kind. I am designing a bench PSU using a STM32 micro (I know, overpowered, but I have a few on hand) that will produce a variable output from 0-24VDC, with current limiting up to 10A. The reason for this project is that my current bench PSU was one my dad built about 30 years ago from recycled components (parts were expensive back then) and is not capable of producing > 1A, and is quite hard to set exacting voltages on it.Also, I should note that I am more of a software guy and enjoy programming the micro more then circuit design, and as such my circuit skills are rather limited.So far I have built a voltage divider that is capable of being calibrated via a trimpot and is fed into the ADC of the micro, with a 3.6v zener diode to protect the input. I have also scrapped an old Chinese multimeter for it's current shunt, which through testing seems to produce 1mV for every 30mA.The problem is that this voltage is too low and I need to increase it's dynamic range back to 0-3.3VDC. I have played with various op amps (first time user of them) to perform this which works nicely except for they all need around 10mV or more DC offset before they begin to operate. I tried providing the op amp with a +/- 5V supply which also overcame this, except I have no idea how to offset the negative voltages back into the positive range for the MCU.All I need is a simple circuit that performs a voltage amplification of the voltage difference across the shunt that can either be fed into the MCU directly or is high enough for an op amp to perform the final amplification.Note: I do realise there are op amps that have input offset voltages of < 1mA, but I live in an area that procuring parts is rather hard. On hand I currently have most standard transistors and a few basic op amps, including those that will operate on a single supply voltage.Any help on the matter would be greatly appreciated.

author: Psi
author_rank: Super Contributor
timestamp: February 08, 2013, 04:08:44 am
content: Ideally you should use a high side current sense IC.It's a chip intended for doing just this.You wire it up to a each side of a resistor (your shunt) and it will output an amplified analog voltage, with respect to ground, which is proportional to the current through the resistor.The chip takes care of the gain necessary to get a usable signal.They come in many types with different gains so you would need to find one that suits your application.Or, doing it the cheap way, if your micro has a differential ADC with selectable gain and you don't mind low side current sensing you could just use that.

author: gnif
author_rank: Unknown
timestamp: February 08, 2013, 06:15:54 am
content: Thanks for the reply.My MCU does not have selectable gain so I can not use that option. As for a IC that performs the function, I would rather learn how to do this to better educate myself.I believe I found a solution... I am using two LM358 op-amps... one running at +/- 5V, the other at +3.3V. The first one is configured with a gain of 100 where the voltage is fed into it, this produces a +/- 5V output. The output is fed into the input of this circut:http://www.daycounter.com/Circuits/OpAmp-Level-Shifter/OpAmp-Level-Shifter.phtmlThis seems to perform what I am trying to accomplish. If anyone can see any problems here please let me know.