From: Terry Pinnell
Subject: Re: zero-power toggle circuit; was, how to master electronics
Date: Tue, 12 Nov 2002 16:54:51 +0000
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NNTP-Posting-Date: Tue, 12 Nov 2002 16:55:06 +0000 (UTC)
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Winfield Hill wrote:
> A very nice explanation of how the circuit works, thanks!
> You're right of course. I'm sorry - that's what can happen
> when a design passes through several iterations on an ASCII
> computer screen; I should stick to paper for my doodlings!
> Anyway, I overlooked updating the value of R2, it should be
> much smaller. In fact one could eliminate that resistor
> entirely except for the fact it's not nice to blast the base
> of a transistor with many amps for a microsecond! If we
> choose say 330 ohms, that should suffice, yeilding a 330us
> time constant and a peak base current of 20 to 50mA. This
> is easy on the transistor and insures that the FET's required
> turn-on gate charge is provided. The 2n4401 can be expected
> to provide at least 400mA of charging current to the FET gate
> for at least 100us, that's over 40000nC of charge. A modern
> IRF540 FET turns fully on with less than 30nC, the original
> version might require 50nC of gate charge. Larger FETs mayf
> require up to 250nC, so they can be nicely accommodated too.
> I'd like to increase R1 to 470k as well.
> Perhaps Terry can try these new values in his model. Enjoy!
That sorted it, Win. With R1 = 470 K, it worked fine for R2 in a range
330 R (the lowest I tried) to 1K4 K. Toggling failed at 1K5.
Hobbyist, West Sussex, UK