[TowerTalk] Linear vs switcher wall warts

[Roger (K8RI) on TT]

On 8/15/2013 11:10 PM, Jim Brown wrote:
> 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?
>

I have boxes of the thing...mostly older, much older. Unfortunately most 
have connectors for specific devices which are long gone.  I've started 
labeling then as to the device the come with.

73

Roger (K8RI)
> 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
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