Assorted snips from the last 24 hours of the regulator discussion...
On Tue, 29 Jan 2002 11:49:58 -0000 "Steve Thompson" <g8gsq@qsl.net>
writes:
<snip>
> A couple of thoughts. MOVs are widely used to clamp screens in the
event of
> a flashover. At 10s amps, the clamping voltage will be in the regions
of
> 600V for a 375V MOV. Many designs survive this ok, but I wonder how
many
> times it's luck rather than design. There can be hidden side effects -
for
> example, if the supply has a series element using a Nchannel FET,
there's a
> big parasitic diode which will carry the 600V from source to drain.
> Similarly, a bipolar series element will suffer reverse emitter base
> breakdown and make 590ishV available at the base.
>
The FET is self protecting in this instance. The surge will be
conducted back to the screen supply filter cap which will likely not
charge to that the above mentioned potential, given the limited duration
of the flashover, with any reasonable impedance in the rest of the screen
circuit.
And a bipolar should have diode externally connected (CE) for
protection in this event similar to IC 3 terminal regulators that may
experience reverse voltage.
On Tue, 29 Jan 2002 04:59:10 -0800 rich <2@mail.vcnet.com> writes:
<snip>
> - To ice the cake, one still needs a similar 400v zener string to
protect
> the FET from Drain/Source breakdown. As I recall 500V C-E FETs go for
a
> bit more than $1. Such a device also needs a heatsink.
A bit -- $1.06 for a suitable part from the ubiquitous Digi-key --
they may not be the most economic supplier.
If you are encasing your amp in the traditional metal, rather than
plywood, then any convenient spot on the side of the cabinet will suffice
for the heatsink.
> >Get fancy, add a trimpot
> >for voltage adjust so you have "something to turn" and you have a $3
> >regulator.
> >
> - And, as an added bonus, one has a high current-gain IC in an RF
> environment.
I always keep the IC's and such outside of the RF shielded area. No
need to invite trouble!
On Tue, 29 Jan 2002 13:51:34 +0000 "Ian White, G3SEK" <G3SEK@ifwtech.com>
writes:
<snip>
> There doesn't seem to be any real need for an active series element to
> feed an active MOSFET shunt regulator. The loop gain in the shunt part
> is already very high, so the series feed can be just a plain resistor.
The only reason I use(d) one is to reduce the dissipation in the shunt
regulator.
73 & Good morning,
Marv WC6W
*
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