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Re: [EVDL] Zener regs: Can I get use 'em & still get a full charge?

Robert Johnston wrote:
> don't Zener diodes have a 0.7v "drain" before they conduct? So a 6.8v
> Zener wouldn't be conducting completely until 7.5v.

No, they don't work like that. The spec sheet for a zener shows the
voltage and current at a number of points. For the 1N5342B I use, the
specs are:

6.8v +/-5% at 175ma
6.65v at 17.5ma
5.2v at 0.01ma

The regs use two zeners, and a #43 lamp in series (plus a 10-ohm
resistor across the lamp). These have a nonlinear resistance, so the
resulting current vs. voltage looks like this:

13.5v at 1ma
13.75v at 10ma
14.0v at 100ma
14.2v at 200ma (lamp begins to glow)
15v at 500ma (lamp brightly lit)

The 6v version uses the same parts, but has just one zener in series
with the lamp, and two zener+lamp pairs in parallel. Thus it has about
half the voltage, and double the current.

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