Skip to main content

Measuring current draw with LTC4150 + ESP-12E

My LTC4150 Coulomb counter has finally arrived!

For testing, I hooked up the unit to the spare ESP-12E I have lying around:


All the jumpers on the LTC4150 are soldered (SJ1 = interrupt-driven counting; SJ2, SJ3 = 3.3V circuit).

The code for driving the Coulomb counter is as follows:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#include <Time.h>
#include <TimeLib.h>

const int BATTERY_CAPS = 2300;
const byte INT_PIN = D1;
const float INT_TO_COULUMB = 0.614439;

bool trigger = false, init_done = false;
unsigned long total_time = 0, total_interrupts = 0;
volatile unsigned long num_interrupts = 0;
volatile unsigned long time1 = 0, time2 = 0;

void debug(const char *format, ...) {
  char buf[256];
  va_list ap;
  va_start(ap, format);
  vsnprintf(buf, sizeof(buf), format, ap);
  va_end(ap);
  Serial.println(buf);
}

void handleInterrupt() {
  if (time1 == 0) {
    time1 = millis();
    init_done = true;
  } else {
    num_interrupts++;
    time2 = millis();
    trigger = true;
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(INT_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(INT_PIN), handleInterrupt, FALLING);
} 
 
void loop() {
  if (init_done) {
    Serial.println();
    init_done = false;
  }

  if (trigger) {
  cli();
    trigger = false;
    unsigned long interval = time2 - time1;
    unsigned long ni = num_interrupts;
    time1 = time2;
  num_interrupts = 0;
  sei();
  total_time += interval;
  total_interrupts += ni;
    float ma = (ni * INT_TO_COULUMB) / (interval / 1000.0) * 1000.0;
    float ma_avg = (total_interrupts * INT_TO_COULUMB) / (total_time / 1000.0) * 1000.0;
    float lifetime = BATTERY_CAPS / ma_avg / 24.0;
    debug("\ninterval = %ldms; num_interrupts = %ld; ma = %fmA; ma_avg = %fmA; lifetime = %f days",
      interval, num_interrupts, ma, ma_avg, lifetime);
    time1 = time2; 
  }
  else {
    Serial.print(time1 == 0 ? '#' : '.');
  }
  delay(10*1000);
}

When the load is a 1K resistor, expected current draw is ~4V/1K = ~4mA. The following output was observed:

interval = 150862ms; num_interrupts = 1; ma = 4.072855mA; ma_avg = 4.072855mA; lifetime = 26.598871 days
interval = 150775ms; num_interrupts = 2; ma = 4.075205mA; ma_avg = 4.074029mA; lifetime = 26.591200 days
interval = 150829ms; num_interrupts = 3; ma = 4.073746mA; ma_avg = 4.073935mA; lifetime = 26.591818 days
interval = 150710ms; num_interrupts = 4; ma = 4.076962mA; ma_avg = 4.074691mA; lifetime = 26.586882 days
interval = 150850ms; num_interrupts = 5; ma = 4.073179mA; ma_avg = 4.074389mA; lifetime = 26.588854 days

When the load is a 47K resistor, expected current draw is ~4V/47K = ~0.08mA:

interval = 8380827ms; num_interrupts = 1; ma = 0.073315mA; ma_avg = 0.073315mA; lifetime = 1477.645142 days
interval = 8124328ms; num_interrupts = 2; ma = 0.075630mA; ma_avg = 0.074454mA; lifetime = 1455.033325 days
interval = 7995918ms; num_interrupts = 3; ma = 0.076844mA; ma_avg = 0.075234mA; lifetime = 1439.949219 days

I aborted the test after 3 readings because the current draw is so low it was taking too long to get 5 readings. But I think the readings are consistent enough to conclude that the power meter circuit gives reasonably accurate readings.

Comments

Popular posts from this blog

Adding "Stereo Mixer" to Windows 7 with Conexant sound card

This procedure worked for my laptop (Thinkpad E530) with a Conexant 20671 sound card, but I suspect it will work for other sound cards in the Conexant family. I was playing with CamStudio to do a video capture of a Flash-based cartoon so that I can put it on the WDTV media player and play it on the big screen in the living room for my kids. The video capture worked brilliantly, but to do a sound capture, I needed to do some hacking. Apparently, there was this recording device called "Stereo Mixer" that was pretty standard in the Windows XP days. This allowed you to capture whatever was played to the speaker in all its digital glory. Then under pressure from various organizations on the dark side of the force, Microsoft and soundcard makers starting disabling this wonderful feature from Windows Vista onwards. So after much Googling around, I found out that for most sound cards, the hardware feature is still there, just not enabled on the software side. Unfortunately, to

Hacking a USB-C to slim tip adapter cable to charge the Thinkpad T450s

This hack is inspired by this post . A year ago, I bought an adapter cable for my wife's Thinkpad X1 Carbon (2nd Gen) that allows her to power her laptop with a 60W-capable portable battery (20V x 3A). A USB-C cable goes from the battery into the adapter, which converts it to the slim tip output required by the laptop. Everything works out of the box, so I didn't give much thought about it. Recently, I decided to buy a similar cable for my Thinkpad T450s. I know technically it should work because the T450s can go as low as 45W (20V x 2.25A) in terms of charging (though I have the 65W charger - 20V x 3.25A).  I went with another adapter cable because it was cheaper and also I prefer the single cable design. So imagine my surprise when the cable came and I plugged it into my laptop and it didn't work! The power manager just cycle in and out of charging mode before giving up with an error message saying there is not enough power. After much research and reading the Thinkwiki

Using Google Dashboard to manage your Android device backup

I used to use AppBrain/Fast Web Install to keep track of which apps I have installed on my phone, and to make it easier to reinstall those apps when the phone gets wiped or replaced. But AppBrain had been going down the tubes, and Fast Web Install had always been a hit-and-miss affair. Android's own "backup to the cloud" system had previously been even more unusable. There isn't a place where you can see what has been backed up. And when you setup a new phone with your Google account, you just have to wait and pray that your favorite apps will be restored to the phone. Typically all the stars have to be aligned just right for this to happen. More often than not, after waiting for an hour or so and nothing happens, you just curse under your breath and proceed to install your favorites apps manually via the Play Store. But I just looked again recently and was pleasantly surprised that things are much more civilized now. Firstly there is a place now where you can loo