RE: DRSSTC - I think I heard Cross Conduction...

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>Initially it seemed crazy to suggest putting 10k in series with the o/p of
the CT.

I know the circuit Sue was talking about. It is crazy and I have no idea how
or why it works. The "textbook" circuit is the same but with a low value
resistor (or better still, back-to-back parallel diodes) across the CT.
Without this "burden" network the CT can saturate and may put out crazy
spikes and unwanted phase shift and ringing.

The 10k resistor is just there to protect the chip in case the CT puts out
too much voltage. The resistance lowers the current enough that the chip's
anti-static clamping diodes can lop off the spikes.

A direct connection to the CT can deliver enough voltage and current to wipe
a 74HCxx logic chip off the face of the planet, especially if you leave out
the burden, when you can get several hundreds to thousands of volts. I'm not
surprised Sue reported the circuit going crazy without this resistor.
Wouldn't you go a bit crazy if you got hundred-volt spikes up one of your
inputs?

You should still use the protection resistor even with the burden.


Steve C.