[ARC5] More on receiver current drain.

[Dennis Monticelli]

I think we're talking two 5W bullet style zeners here, which are hard to
heat sink.

One circuit trick that works well is to make a simple zener multiplier out
of a power transistor, two resistors and a single zener.  Take an NPN power
transistor and put the zener in its emitter leg.  Build a resistor divider
that draws roughly 10% of the max current you wish to shunt.  Connect the
top of that divider to the collector, the bottom to the bottom of the zener,
and the "center tap" to the base.  You can even replace the resistor divider
with a pot if you wish.  The circuit will act as a shunt at a voltage
determined by the zener voltage times the resistor divider ratio plus one.

For example, if you happened to have a 50V zener in your junkbox and wanted
to shunt 50mA at 150V then you would create a divider with a 2:1 ratio.  The
current pulled by the divider would be 5mA so the bottom resistor would be
10K and the top resistor 20K.  The total heat dissipated by the
shunt is 7.5W of which 10% ends up in the two resistors, 30% in the zener
and 60% in the power transistor.  You can swing the dissipation more toward
the transistor by choosing a lower voltage zener and upping the divider
ratio to attain the same 150V shunt voltage. I recommend that you not use a
ratio of more than 10 or the accuracy will suffer.

Perhaps this little circuit will be on help to others that wish to build
power shunts with solid state devices.

Dennis AE6C

On Sat, Jan 8, 2011 at 6:54 PM, jmfranke <jmfranke at cox.net> wrote:

> 250V times .013A = 3.25W
>
> 120V times .013A = 1.56W
>
> 130V times .013A = 1.69W
>
> 1.56W + 1.69W = 3.25W
>
> Without a heat sink, the Zener diodes would get hot.
>
> John  WA4WDL
>
> --------------------------------------------------
> From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
> Sent: Saturday, January 08, 2011 9:44 PM
> To: <Arc5 at mailman.qth.net>
> Subject: [ARC5] More on receiver current drain.
>
> > Well, Mouser got my Zeners to me today.
> >
> > Paired up one 120 V and one 130 V and hung them across the output. Total
> > current drain at 250 VDC output was 13 mA and voltage dropped was 69
> > volts.
> >
> > The output voltage started slowly climbing past 250 VDC, and kept going,
> > while the Zeners got too hot to touch within a few minutes.
> >
> > Total power dissipated in 10 watts worth of Zeners was less than 1 watt
> > (0.897).
> >
> > The only thing I suspect is that one of the two diodes in the rectifier
> is
> > bad...
> >
> > :-(
> >
> > Ken W7EKB
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