It was fun to look at a revived LED backlight module, salvaged from a LG laptop display panel model LP133WF2(SP)(A1). It was controlled from a breadboard Arduino, and powered by my bench power supply. I’m still unsure what input voltage it was originally designed for, but it seemed to run well at 5V. When I turned brightness up to maximum, the bench power supply reported 1A of current draw. As a 5W LED light, it does feel approximately in the same ballpark as the 7W LED bulbs serving as 60W incandescent bulb replacements. But with the key and very valuable difference of the fact its light is evenly distributed across a much large area for a softer more diffuse light.
While I’m at it, I measured the electrical behavior of these LED strings. This is mostly for reference since the TPS61187 chip handles adjusting these voltage values to keep electricity flowing at the target current. When it sees a very minimal PWM signal, I measure the voltage drop from anode to ground to be roughly 15V and the panel is not visibly illuminated at this low level. When I turn the duty cycle up high enough to see just a little bit of visible illumination, the voltage differential has climbed to 24V. At max power, I measured about 28V. This was all generated by the onboard boost converter from a 5V input signal. In my experience white LEDs drop roughly 2.7-3V at full power, so these values are consistent with parallel LED strings of either nine or ten LEDs per string.,
Since this circuit seemed to run at 5V, I thought it would be fun to convert this to run on USB. The Arduino Nano was designed to run on 5V and already had a handy USB jack, and most portable USB power banks can supply 5V@1A or at least they claim to. When I hooked up the wires, it was able to illuminate up to a certain level. But beyond that level (roughly 1/4 to 1/3 brightness) the lights started flickering in a classic sign of power instability. Oops. What went wrong?
Whenever I see potential sign of power instability, my first reaction is always to perform the Big Honkin’ Capacitor test. Find the biggest capacitor I have handy, connect it across the power input terminals, and see if that solves the problem. In this case, the big capacitor failed to soothe the system.
Digging into schematics for an official Arduino Nano, I see that the VUSB line is not directly connected to the +5V output pin. There are a few components in the way, relating to power control and regulation. The Arduino Nano could be powered via its VIN pin. Following Arduino Uno barrel jack precedence, the input voltage is usually recommended to be 9V. When this happens, there’s a diode presumably to make sure that 9V does not feed back into the USB +V line. There are also several capacitors in parallel with VUSB but they should help rather than interfere with any instability.
Mystified as to why I couldn’t power the backlight via this Arduino Nano’s USB jack, I wanted to isolate the problem. See if the problem lies within the Arduino Nano or with my USB power bank. I took a USB cable and cut off its a damaged micro-B connector. Splaying out the wires, I found VUSB and GND wires, and I connected that to the Arduino Nano circuit. With this arrangement, my backlight module is happy all the way up to full brightness with no flickering problem.
Something about this particular (non-genuine) Arduino Nano module is causing interference, and I don’t understand why, but at least I have a workaround. That’s enough for me to ignore this weirdness today and proceed with my backlight project, even if there was a temporary setback.