Let's make a ESR meter : Principle (part 2)


Overview

The complete schematic is as follows 


It it made of 4 parts :

  • Sinusoidal signal generation
  • Resistance divider (impedance actually)
  • Amplification
  • Peak detection

Sine generation

The sine is generated using a timer controlled dma feeding the PWM to have a sine envelop on a high frequency signal (see previous post).
A simple low pass filter will only keep the sine signal and it then goes through the  transformer.
That transformer has been  salvaged from an old phone charger.
It has a 1:20 ratio. So the input signal is ~ 3.3 v peak to peak with ~ 100mA max current, the output is 165 mv peak-to-peak BUT have a 2A max current.

Since the voltage is so low, that helps for in-circuit measurement, the voltage is not enough to trigger a diode/ NP junction or most mosfets.

Resistance Divider / Amplification / peak detection

The output of the divider is Z/(R9+Z) x VccTransfromer
Z is the total impedance of the capacitor at ~ 50 khz. 

It is assumed the cap is high enough so that the ESR part is much bigger than the capacitance at that frequency. That means a capacitor of some uFarad at least.

The output is then multipled by 21 and a simple peak detector is used to get the peak value (most of it)
and it is then fed to an ADC entry of the MCU.

Now the fun part begins, how do we get the ESR from the value read by the ADC ?
(not as simple as it seems)




Comments

Post a Comment

Popular posts from this blog

Component tester with STM32 : Part 1 ADC, Resistor

Image
I was always  fascinated by the so call "transistor testers" that identify and report pin out, features of pretty much whatever you throw at it. They are based on Atmel 328p chip, the same as arduino nano. So why not do the same thing with a STM32/Bluepill ? The STM32 is faster, has a better ADC accuracy, more memory, can do float etc... and is not more expensive. Warning : I'm doing this for fun & to learn, it might contain error & plain misunderstanding So, i watched a couple of video on the general principle on how they work. This is not a "port" but a rewrite from ~ scratch. When something is weird, i check what the original one does for comparison. Measuring resistance So first thing first :  Resistance & Capacitance. These 2 will unlock the other ones. So how do we measure resistance ? What we do is a resistor divider with a know resistance value: Measure = Vcc*(RtoTest/(RtoTest+Rknown)) As far as the ADC goes, it means ADC=...
Read more

Fixing the INA3221

Image
It 's not too complicated to change the board we have seen  previously . You 'll need to cut some tracks, i did it with a dremel and i'm not very handy, so you'll do better. Red =Cut tracks Blue = Add wire 1- You'll need to separate the bottom connector from the ground, cut the 3 connectors BOTH SIDES. 2- Remove the small red area below the middle resistor to isolate each channels from each other Last, connect right side of each connector to the now isolated pad. Mine looks like this after the operation (ugly i know): Be wary of the small track on the left connector that leads to the left side of channel3 resistor. I cut it while isolating the right pad. Now it works, we can go back to the charger.
Read more

INA3221, weird wiring

Image
Continuing my "smart" battery charger, i've finally received the INA3221 module. It is the purple one you can find on ebay for ~ 5 bucks I soldered a couple of pins to be able to connect it so that i could test it and something immediately caught my attention. It is supposed to be a high side current/voltage sensor, typically in a setup like that : So you can measure the voltage supplied to the load, and the current going through it. Now, let's have a look at the module, look at the top of the following pic, where the sense connectors are. CH3/GND/CH2/GND/CH1/GND ? Wait ? What ? Yes, all the IN- pins are connected to ground, so it is a bottom current only sensor. Additionally, the - side of the shunts are ALSO connected to the ground, no only the connectors. The actual layout is like that : That raises two problems :  The voltage is meaningless as we are only measuring Current x ShuntValue, not the supply volt...
Read more