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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).

oscilloscope data visualization

(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?

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  • \$\begingroup\$ I would either have a look at the wallwart internals and check if it is constructed to do something fancy, or try it with a lab power supply and monitor internal voltages after regulation within the wifi device \$\endgroup\$ – PlasmaHH Feb 18 '16 at 16:09
  • \$\begingroup\$ It appears that the wall wart is not a switch mode type, but a s̶i̶m̶p̶l̶e̶ cheap unregulated version. This means that the WiFi device most likely has an internal regulator. Can you add a picture of the power supply's label and what model the WiFi device is? \$\endgroup\$ – 3871968 Feb 18 '16 at 16:13
  • \$\begingroup\$ The device is the previous generation of this IP camera: vstarcam.com/VStarcam-C37A-1-3MegaPixel-IP-ip-camera-151.html The wall wart is labeled as DC 5V - 2000ma and is just a basic, low cost, modern wall wart (can't get a picture until this evening). \$\endgroup\$ – SvdSinner Feb 18 '16 at 16:23
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    \$\begingroup\$ That wall-wart signal on your scope looks like a clipped (probably clipped @ scope's max range) version of a single-diode rectifier setup, with the neg. spikes likely coming from output loading spikes (almost like your module has an internal SMPS or some other highly inductive load). Can you get a frequency count from that wall wart feed on the scope? \$\endgroup\$ – Robherc KV5ROB Feb 18 '16 at 16:53
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    \$\begingroup\$ What is the power connector? Is it a 3-poles 3.5mm or just a 2-poles? \$\endgroup\$ – frarugi87 Feb 18 '16 at 17:29
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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.

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  • \$\begingroup\$ Pictures of the internals of the camera would help. \$\endgroup\$ – 3871968 Feb 18 '16 at 17:14
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Likely what is happening is that the Wifi device is drawing more current than 2amps in some cases, and then feeding current back to Vreg in others. Notice the ripple frequency on both supplies seems consistent. I'm assuming your wall wart is not the switching type; that would explain why it's responding so differently from your portable supply. To confirm this you might check the open circuit voltage of the wall wart to see what it says (if it's higher than the load voltage then it's an unregulated xfmr supply).

To solve your problem you likely need to emulate the wild voltage swings on your portable supply, or more specifically the ability to handle them. I would try putting a good size capacitor on the output of the portable supply. That will provide a source and sink for whatever the device needs.

If you want to check what is happening current-wise, put a sense resistor on the negative line on your device and put the scope on that. That will allow you to see the changes in current draw.

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The issue appears to be that USB power ports rated for 2A won't provide the 2A of current the device needs, or the USB cable I used was lossy enough to keep the 2A from being delivered. I don't know why I couldn't spot voltage drops on my oscilloscope, but I assume it was not happening often enough to capture it in my traces.

However, when I took a power connector with nothing but 30mm of lead wires and hooked it directly to my bench PSU +5V port, the camera worked as expected.

Of course, this brings up my next question of: How can I get 5v 2A from a 5V 2A usb charger? My guess is that cable must do/communicate something to enable current beyond 500ma (USB spec), and my power plug wasn't doing it.

However, that is a very different question, and I will post it separately.

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  • \$\begingroup\$ No enumeration or communication is needed, most power banks will put out as much current as possible based on its boost regulator and load. Cheap ones are ~700 max, some are 1A, some say 2 A but wont. Also keep in mind voltage drop under load, or over current protection. A decent power bank is required. \$\endgroup\$ – Passerby Nov 16 '16 at 19:28

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