[TowerTalk] Linear vs switcher wall warts

[Jim Brown]

On 8/15/2013 6:45 PM, Edward McCann wrote:
> Is there an easy way to determine whether subject wall wart is one or the other without opening the power block or wall wart or using a portable SW receiver to scan the device powered and unpowered and make a judgement?

Several clues.

1) Size/weight -- switchers are both smaller and lighter than linears 
for the same power.

2) Voltage regulation -- linears are rarely regulated, just a simple 
transformer, bridge rectifier, filter cap -- so if you measure the 
output voltage with no load connected, it will typically be 30-40% 
greater than the rated value.  There are exceptions -- I have one 5.1VDC 
linear and one 13VDC linear that are regulated, but both are fairly heavy.

3) Vintage -- new stuff is nearly always a switcher, old stuff is nearly 
always linear.

4) AC supplies are nearly always just a transformer.

My vintage Kenwood TH-F6A talkie has RX capability from below the AM 
band to nearly 1 GHz, and defaults to a built-in loopstick below 10 MHz. 
I tune it to around 2 MHz and around 10 MHz, hold it over the DUT and 
its wiring, and listen for hash.

Most devices that cause RFI conduct that trash to wires connected to it, 
and those wires radiate. So to judge whether DUT is a problem, you need 
to test with wires connected and probe for noise along the wires.

73, Jim K9YC