Monday, 11 March 2013

Switch Mode PSUs

Page on leakage current from the outputs of cheap switch mode PSUs
I started looking into this after getting a small "shock" from the metal case of the iPhone 4S, using 2 different chargers. This was with both a very cheap (about £1 including delivery) and what I believe is an official Belkin USB charger.
I measured around 100V AC between the 0V DC line of the USB cable and mains earth for both. Other chargers had some AC voltage but much lower. Because the cheap charger was so badly made, with solder almost bridging the isolation gap between high and low voltage sides, I thought that the leakage was down to poor insulation. But after the Belkin did the same thing, I looked online a bit more and found that it is probably down to the 2.2nF capacitor that is often connected between the high voltage rectified mains and the 0V DC output line, for interference suppression.
When I measured the leakage current to earth from the output, through a 100K resistor (in case there was actually a solder bridge!), I was getting about 150 microamps AC current from both chargers. This type of equipment is allowed up to 250 microamps of AC leakage to the output, so perfectly legal chargers can still give a bit of an electrical buzz if the USB powered device has exposed metal. The problem is when your phone case touches some other wiring like the input to audio equipment, and it gets the hundreds of volts from a charged capacitor.
Leakage currents might start getting dangerous if you have a lot of unearthed equipment connected together (multiple TVs and set top boxes daisy chained or plugged into the same unearthed amp). I measured over 300 microamps from the USB socket of a TV that was also plugged into the RF output of a SKY+ HD box. Unplugging the coax brought it down to 150.

1 comment:

Unknown said...

Hi Simon very informative all your reports! Thank you, Karl.