On 13 Nov 2008 at 10:59, Xiaofan Chen wrote:
> http://forum.microchip.com/tm.aspx?m=379514
> http://coledd.com/electronics/sony_vaio_power_supply_problem/
> 110V AC on the USB port, that is kind of dangerous.

Yes, an increasingly common problem with un-grounded switch mode power 
supplies. I've come across this on notebooks and general purpose switch mode 
"wallwart" type power supplies. Talked about it here some time ago.

Internally such power supplies have a small capacitor between the low voltage dc -
ve output and high voltage rectified mains -ve side of the reservoir capacitor. Low 
enough in value to not exceed the 0.5 ~ 1mA ground current (body current?) 
allowed by various standards, yet effective at decoupling the EMI from the high 
frequency switching.

In practice this means up to 115V AC no load (with a 230V AC supply) on the low 
voltage DC side with respect to ground. Dangerous? No and yes. It loads down 
easily, and current under about 1mA is considered electrically safe. But it does 
cause problems. It can cause visible sparking while connecting to grounded 
equipment, and can sometimes be felt by the user. Leakage currents from several 
interconnected appliances can also be cummulative, resulting in a potentially 
dangerous current.

I don't like it. I talked to an electricity safety authority but didn't get very far with it. 

-- 
Brent Brown, Electronic Design Solutions
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Hamilton 3200, New Zealand
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