Inductance tester , continued

 I tried previously to measure the inductance of a coil using the same method as for capacitors :

take 2 points in the charge/discharge curve and compute the L value

After a pulse on the coil, the voltage across it is (1-exp(-t*R/L)), so if we have t, R and 2 points we can compute L.

The problem with inductance is the curve is very very small, like ~ 1us, way too fast for the ADC, and using very small R will give high current.

Recently i saw a youtube video from DiodeGoneWild,  he showed his own inductance meter

It's 100% pure analog, no MCU.

How does it work ?

It sends GND/VCC squares to the coil do trigger charges/discharges.

A schmidt trigger creates a nice square when coil voltage is between 1/3 and 2/3 vcc.

The duration of the square gives the inductance.

Measuring a small-ish duration is not easy to do with analog circuits so that's how it does it :

since a PWM is charging/discharging the coil and generates squares, the output squares are integrated though a 2nd order low pass filter.

The average voltage  = (duration of the square)/(duration of the PWM)


Ok, since we do software, how can we do ?

First, instead of selecting RC+inverters  as a PWM source we can use one timer of the MCU

That's for free and will give good accuracy ( max we need is ~ 100 kHz or so)

Second, we can do the same trick i.e. integrates the squares and measure the average value after integration. It would be all automati.  Maybe we can do better.

My idea is to have a timer counting the duration of the pulse. Even if the pulse is small (~ 1 us), if the MCU is fast enough that should give us something acceptable in terms of accuracy.

We can adjust the PWM until we are in the "ideal" spot to get good results.


The first try with my homemade square generator was not great for  several reasons :

  1. The square generator is too soft to raise/lower the output. That prevents the voltage across the coil to get high.
  2. The schmidt triggers i have are CD4050s , powered by 5v. They seem to be a bit slow for that usage.
Ideally, it would be a small mod on the STM32 component tester, just adding a few (inverting) schmidt trigger gate and that's it.
NB, the same principle can also be used for capacitors.




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