For some time now we’ve been experiencing strange problems with the USB system inside the cube.  Somewhere along the USB path something drops the connection, either the Arduino or the Hub or the Linux box. We first tried swapping out USB cables or Hubs. We suspected the Arduinos were drawing more power then the hubs could provide so we separately power them with power supplies. Finally we started suspecting the Dimduino boxes them selves were at fault.

Nothing seemed to provide any particular relief. Oddly the frequency of the disconnections was not isotropic and certain boards tended to cause disconnections much more frequently (every few minutes) then others. Swapping out the dimduino boxes or the arduinos didn’t affect this though, which was suspicious. Finally it dawned on me that the PowerSupplies that power the Lights must be at fault because that was the only element we hadn’t yet tried to change. Sure enough, analysis of the freuquency of the dropped connections with whether or not the lightbox was powered by a new style power supply (an IP67 compliant, waterproof switching powersupply) or an ATX powersupply was conclusive. The new switching powersupplies were in fact at fault. Removing them all and powering everything off ATX PSUs fixed all the disconnection problems overnight.

So far so good. Now, however the cube was running on a single PSU per box, i.e. our redundancy was compromised (we only just had enough ATXes to power each box with one PSU). Thus I’ve been trying to figure out what the issue is with these new PSUs and if there’s a cheap fix.

I hooked them up to my oscilloscope and it was pretty clear that these new power supplies are switching power supplies but it appears of poor quality. Unlike an ATX which gives perfectly smooth power there’s a lot of high frequency noise on the output.

Measuring Vout and V1 (Fig A) shows some interesting patterns: B) Vout has a 83KHz beat of about 2V magnitude and a faster superimposed frequency in the Mhz range decaying exponentially like a damped oscillator after each 83Khz beat.

C) V1, between 0V output and Ground, is clearly carrying a 60Hz mains hum and a faster ripple on top of that, which resembles the ripple in B). The 60Hz oscillation has a large magnitude (like 60V or so) but clearly very high impedance (you dont get zapped when you touch it and barely any current flows if you connect 0V to Ground, but the 60Hz oscillation on both 0V and 12V disappears completely).

These oscillations make the whole cube circuitry swing like hell, and sends out a ton of EMI and RF everywhere. I can pick up the Mhz signal using just a piece of wire in the air. No wonder the USB was struggling.

The two solutions I arrived at, empirically are shown in D) and E).
One involves a resistor and a capacitor and the other has an additional direct resistor.
Both smooth out Vout ( B) ) quite a lot, though not completely and V1 also looks quite a bit better but also not perfect. E) i think works a weeny bit better, but its all comparable. Some spike voltages remain on V1 that I just can’t get rid of it seems. Vout is now pretty smooth. Adding another cap between Out+ and Out- doesnt help to remove any residual noise. Nor does adding Y caps for the +12V line (from +12V to GND ).

I found a nice article on Switching PSUs and EMI which suggests a more elaborate but straightforward circuit with additional caps and particularly in-series inductors. Makes sense, the inductors will further resists the AC going forward. I’m tempted to try it but maybe what I cooked up is sufficient (it certainly appears to be, empirically speaking).

Also I talked to a knowledgeable person at ORDcamp this weekend who suggested using higher voltage caps should not only make things safer but also help absorb more of the 60Hz hum and the RF.

If all of this doesn’t help the next step is clearly to add in series inductors but this is a slightly bigger undertaking and begs the question if it wouldn’t be better to just move to a different PSU. Would be a shame though to toss the 14  $70 PSUs that in all other ways are absolutely perfect.