Changes to support 220 volts

[brghena]

According to Sam DeBruin:

One of my recent goals has been to determine why PowerBlade doesn’t work on 220v, even though it definitely should. I recently ran a test on the 220v source, and although PowerBlade boots and sends packets, the values for voltage and power are wrong. I determined that the voltage AC wave is actually saturating the amplifiers at 220v, which leads to the error.

I also found the cause. If you do the amplifier math (I use KCL) the voltage measurement circuit is actually wrong. The GND symbol at (-2.3 -1.6) on page 5 should actually be VCC. I can take a picture of my math if you want, but its pretty simple to do the calculation yourself. The GND symbol adds a +Vcc/2 term to the output equation, which leads to saturation at high voltages.

The message: in future revisions change that GND to VCC. i.e. RI2 goes from the common terminal to VCC rather than GND. I would also recommend turning down the gain slightly, although that is less important.

We should fix this before producing any additional PowerBlades.