V G wrote:
> I want to be able to generate a low current isolated supply (for driving =
my
> solid state relay MOSFETs) gate-to-source.
When designing an isolated power supply your first task is to define the=20
isolation parameters.

How much leakage is acceptable?
What frequencies do you need to isolate at? DC? 50Hz? higher? (in=20
particular remember that a sudden step change is like a high freqency=20
signal)
What are the consequences of an isolation failure


>  I googled and found this:
> http://www.maxim-ic.com/app-notes/index.mvp/id/1932
>
> 1. How does this circuit work? I'm guessing the 74HC14s give a sharp sign=
al
> to U1 whose outputs oscillate signals into high pass filters C1 and C2.=20
Right, the electronics genertes a high frequency AC waveform (not sure=20
exactly how high off the top of my head) which passes easilly through=20
the 0.1uf capacitors.

> At
> that point, I'm guessing the signal looks more like a sine wave=20
Probablly closer to a sqaurewave than a sinewave but it doesn't really=20
matter.

> which passes
> through the full wave rectifier comped of those 4 diodes and the capacito=
r.
> Tell me if I'm wrong.
>  =20
Right

Now the downside of this circuit is that while it provides good=20
isolation against steady DC voltages it's not good at providing=20
isolation against step changes or AC voltages.

> 2. Is there a simpler way to generate an isolated supply?
If you want good isolation you really need to look at transformer based=20
soloutions and since transformers don't pass DC a transformer based=20
soloution is likely to be roughly similar in complexity.
--=20
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