Sunday, 17 March 2013
Monday, 11 March 2013
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.
Tuesday, 5 March 2013
The ones in Newtownards and Southport are on 2.1GHz 3G (3 network) and were received in the Seascale area this afternoon.
Cell 20764 is probably in the Southport area too, as it has the same LAC of 22 (Lancashire area).
2G phone networks from the Isle Of Man are often stronger than the UK ones along this coastline, but receiving Northern Ireland on 2.1GHz is a new one for me.
Friday, 1 March 2013
AnyTone ST-118 portable CB radio http://www.aliexpress.com/item/AnyTone-ST-118-portable-CB-radio/723098383.html
A 16 channel PC programmable 25-31 MHz handheld with CTCSS. These look like they are meant for business use but who would use that band with handhelds instead of PMR446/PBR frequencies? I can't find any mention of them online except from Chinese sellers.