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Old 31st Jul 2021, 5:36 pm   #76
Radio Wrangler
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Default Re: HP 180A Oscilloscope.

Just be a little careful with the choice of zener diode. Some types have their voltage specified at currents like 5mA, 10mA and 20mA. One of these running at 2mA will be well below the knee in its curve, so the accuracy and temperature stability will be a lot worse than spec.

Discharge tubes like neons and VR150/30s exhibit hysteresis between striking and extinguishing potentials. Zeners don't and circuits like the 'amplified zener' pointed to above don't, so they don't need as much care as the neon circuits to not make a relaxation oscillator of the application circuit.

That said, the zener amplifier circuit can create some negative resistance and RF oscillation. Be ready with ferrite beads just in case, though the high-ish resistance drive to the base of the darlington transistor ought to help keep it tame. Just make sure there isn't capacitance on the node in the middle of the potential divider/base.

David
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