Current meter, electronics

Another month, another board version.  This time the updated edition of the USB current meter.  And not just one, but 20 boards; kindly made by Seeed Studios.  White (of course!), with rounded corners, about the size of a USB drive.  Once assembled, it is pleasingly solid to hold.  These boards are meant to serve as demonstration samples: I can populate them, show them to people, and mail them all over the world.  In addition, building more than one unit adds confidence that the circuit functions as expected.

The only major change on this revision is the use of an ZXCT1010 to measure the output current – my previous LM358-based effort struggled at low currents, and required a larger sense voltage than I felt comfortable with.  The ZXCT1010 is suitably sensitive, highly linear and requires very little additional components.  (I did, however, spend a day debugging an unexpectedly high transimpedance, before I realised that I had been referring to the datasheet of the ZXCT1110.)

In addition to the change in current sensing, I moved the components closer together and shortened the board to be less than 50mm long.  The LED also moved to be next to the switch.  Although the footprint for the programming header is still present, I plan on leaving it unpopulated, and just connecting using pogo pins.

v0.2 bringup

electronics, usbpwr

I’ve started soldering the v0.2 board, testing and porting the v0.1 software as I go along.  The LED, button, and ADC channels all moved, which required adjustment of a couple of header files.  Two minor wire mods were required, but so far everything tested is working.  There is a tiny bit of coupling between the PWM signal and the ADC feedback signal, but I suspect a well placed capacitor will clean the line up.

From here, I have to do the following:

  1. Populate the auxiliary transformer and generate a sane voltage rail for the opamps on the secondary side.
  2. Populate the feedback circuitry.
  3. Adjust the ADC driver to use the new configuration.
  4. Calibrate.

And then I can test my primary side current monitoring.

Layout rev 0.2

electronics, usbpwr

Layout for rev 0.2 is complete, it’s PCBtrain time again.  I should get the boards before the bank holiday season starts next week.

Key changes to rev 0.2:

  • Improved voltage and current monitoring on the isolated side
  • Fixed Atmega PWM pin.
  • Improved filtering on voltage rails – there is space for play here, with an excessive number of cap footprints.
  • Different inductor – higher peak current, smaller package, lower price. Also lower isolation.
  • Size is now 75mm x 50mm (was 60mm x 50mm)

Layout rev 0.2