I've got a wifi device that works off of a 5v 2a wall wart. I need to walk around with it to get some signal strength ratings, so I am making it a portable power supply. However, the device won't boot with anything I've tried except for the wall wart, and it is baffling me.
The wall wart uses a 3.5mm/1.3mm power jack (reads 5.3V open), and I have a USB -> 3.5mm/1.3mm cord I am using with my attempts to power the device.
Here is what I've tried:
- Using a usb power bank rated for 5v/2A output.
- Using various batteries (up to a 12v SLA to make sure power or amps wasn't the issue) into two separate, known good, buck converter modules that brought the voltage to 5v, both rated for >3amps.
- The above with/without a large decoupling capacitor attached to handle any startup current.
Here is what I see:
Wall wart: Works fine, all LEDs on and steady.
Everything else: Does not initialize. Network LED blinking rapidly. (Which indicates a failure to initialize.) Another display led appearing to flicker at ~5-10Hz, like it is briefly losing power. Ammeter never shows more than ~200ma draw.
Here is where it get's weird: I built a test point just prior to the device to monitor the voltage via oscilloscope. I had assumed from the flickering (above) that my supply was dropping below 5V intermittently. However, the scope proved that completely wrong. The scope data (below) boggles me (left side is the wall-wart, the right side is my portable PSU (12V SLA –>Buck Converter –> 5v output).
(FWIW, both supplies were showing as ~5.2V on a DMM. The portable has a ripple of +/- .1V. The wall wart swings so wildly that it occasionally is negative. If I lower the sampling rate, the spikes get lower in magnitude, but are always way larger than anything on my portable supply.)
So the working supply has terrible voltage regulation. The supply far better regulation is the one that doesn't work.
What would cause this? What can I try to make my supply work?
Best Answer
Ok, that 120hz voltage wave means a 'full-wave' rectifier (with no effective capacitor) is being used.
Most likely there is a capacitor inside the device that's actually storing power at the max peak voltage of that waveform.
The device is then (likely) feeding a resistive/linear Vreg from that elevated voltage.
That Vreg is likely dropping your 'real' 5VDC power supply too low to run the circuit's electronics.
DANGEROUS ADVICE: You may have to carefully raise the output voltage of your power supply in order to compensate for the drop.
Unfortunately, without opening the wifi device & doing at least enough reverse-engineering to figure out what Vmax your device is storing pre-regulator (and preferrably finding the output voltage after it, too) I can't really tell you what voltage to try, or exactly how unsafe any specific voltage would be for your device.