Re: Resistor vs transformer

On 9 Feb 2006 09:32:09 -0800, "Ancient_Hacker" <grg2@xxxxxxxxxxx>
wrote:


Weinberger Hans wrote:

Thanks for the advice. It certainly is the cheapest. I will check
when all the units are here . I think they will most likely be
SMPS inside, but even then your idea should still work .

Hans

Well, not exactly-- most SMPS don't have an input transformer, but
about 90% of the ones I've seen come with an input circuit that can be
switched from 120 to 240. For 120 volt input, they use a
voltage-doubler to generate about 300VDC peak. For 240V input, you
move a jumper that disables half of the doubler, turning 240VAC into
again about 300VDC.
<p>
Even if it doesnt come with a jumper, it's nearly trivial to disable
half of the voltage doubler.

So Hans check on the rectifier circuit if its SMPS . If you have 4
diodes there then "most likely" ... it can work on both 120/240 .Its
easy to do the correction even without a jumper there as the jumper
Ancient mentions is usually used as a connection between 2 connected
diodes (in the rectifier section) linked to a smoothing capacitor(s)
(which are in parallel to the diode connection) in case of 120V
operation. That backbiases the 2 series diodes throughout the whole
Voltage cycle. For 240V you would have to have them working for half
the cycle or in other words break the connection to the smoothing
capacitor(s).

lemonjuice

.

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