[aprssig] Emergency Power for Ham stuff

[KF4LVZ]

On 2016-01-22 10:47, Robert Bruninga via aprssig wrote:
>>> But every AC/DC switcher is just a DC/DC supply with a rectifier in
>>> front of it....
> 
>> Bob, you should know better than to use the word "every".
> 
> That modifier applied to the the complete noun of the sentence: "AC/DC
> switcher..."  the key word being "switcher".  I am not aware of any
> switching powersupply anywhere that does its DC/DC switching directly from
> 60 Hz AC alternating current.

That's why I said to go search Google for schematics.  A switching
supply means only that it uses a high frequency oscillator in a charge
pump or buck/boost configuration.  In many cases one side of the AC line
is the reference for the DC outputs (usually the neutral).  On some
laptop power supplies, no ground wire on the AC side is supplied.  These
two wire supplies can be reversed leaving the DC reference ground tied
to a high voltage.

To aid your search here's a couple examples of what would be low cost
switching power supplies:

http://www.discovercircuits.com/DAJ-Schematics/Non-Isolated.jpg

http://www.eeweb.com/company-blog/power_integrations/non-isolated-buck-led-driver-for-gu10-lamps

They're both switching supplies but neither is isolated.

Compare to an ATX power supply that is also switched but uses flyback
transformers in the circuit to isolate the inputs from the outputs:

http://www.next.gr/uploads/6/atx-power-supply-schematic-circuit-dbl494-smps.png

Or this simple 12 volt switching supply also with a flyback transformer:

http://powersupply33.com/wp-content/uploads/2011/08/Switching-power-supply-12-V-2-A.jpg

Note in both cases there is a direct connection back to the neutral even
though there are transformers in the circuit.


It is already known that many products from various overseas suppliers
have cut corners to increase profits on cheap hardware (there was a very
recent case about Apple power supplies for Macbooks that lacked all the
critical safety features).  It's entirely possible for non-brand name
laptop power supplies to have the same issue, especially when they are
fully sealed and difficult to examine (just like the Apple supplies).


> 
>> There are plenty of cheap power supplies out there that will not fit your
>> "every"
> 
> I agree.  But they are not switchers...

Yes they are, see above.

> 
>>  I've had plenty of switching supplies that were most certainly not
>> isolated.
> 
> I agree also.  I was only saying of the 5 sealed laptop switching power
> supplies that I tested, all of them were isolated.  I agree, most open frame
> and modern supplies are not isolated.  But many of the consumer sealed
> "laptop" type supplies are.

Small sample size, no guarantees especially with some of the
non-grounded types as I mentioned above.