[Boatanchors] Dropping Resistor Question

[Bob Groh]

Your comment about need for in-rush limiting is dead on.  You will/should
have some resistance in series with your solid state rectifier diodes - to
both control the peak charging current (i.e. into the filter caps) and to
partially compensate for the decreased voltage drop in new diodes (over the
old vacuum tube). You can use a zener diode and a resistor but I would
question if it is worth the bother.  Plus cutting down the peak charging
current is a really good thing - I would NOT use a zener but that is just
me.

On the choice between one resistor (shared between the two diodes) or two
resistors (one in series with each diode - assuming we are talking about a
full-wave rectifier) - both ideas work - just a question of power ratings
of the resistors. With two resistors, each sees 1/2 of the total power
lost.

If you have access to a simulator (e.g. LTSpice which is free), it is fun
to play around with various configurations and actually see the currents,
voltages and power in circuits like these.

73
Bob Groh, WA2CKY

On Fri, Oct 6, 2017 at 7:04 AM, Mark K3MSB <mark.k3msb at gmail.com> wrote:

> Now that’s an interesting suggestion.
>
>  For a 20 to 40V drop at 175 mA,  a few 21V 5W zeners should work nicely
> (due in part because I think I have a batch of them…..).
>
> Why zeners in series with each diode?  I would think placing the zeners
> after the two diodes are combined would work the same?
>
>  And just to make sure (as I don’t want to zorch anything),  for the
> positive LV supply the zener cathode is towards the tranny secondary,  and
> for negative bias supply the zener anode is towards the secondary.
>
> I would also think doing this would predicate one using in-rush limiting……
>
> 73 Mark K3MSB
>
>
>
> On Oct 5, 2017 4:51 PM, "David Stinson" <arc5 at ix.netcom.com> wrote:
>
> >
> >
> > -----Original Message-----
> > From: Jacques Fortin
> > Subject: Re: [Boatanchors] Dropping Resistor Question
> >
> > I once had a "solid stated" supply that gave me 195V at full load when
> the
> > "target" was 175V.
> > I just used two 20V, 5W zeners, and everything is fine since.
> >
> > 73, Jacques, VE2JFE
> >
> > --------------------
> >
> > You beat me to it, Jacques.  I was also going to
> > recommend Zeners, which will do job well.
> > D.S.
> >
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