[volt-nuts] Neon bulbs for HP 3420B or 419A chopper circuit

[Pete Lancashire]

I've seen 95V but nothing as high as 125V.

The high brightness ones are usually 95V

http://www.farnell.com/datasheets/57560.pdf


-pete


On Thu, Nov 16, 2017 at 6:38 AM, David C. Partridge <
david.partridge at perdrix.co.uk> wrote:

> In an earlier post I quoted the following description of the photo-chopper
> circuit from the 419A manual
>
> 4-43.   Assume that DS1 lights when the input is applied to T2.  Capacitor
> C1 charges until the oscillator switches the input, and DS1 goes off.
>  When
> the oscillator switches again, the charge on C1 insures that DS2 fires, and
> DS1 stays off.   This cycle continues with DS1 and DS2 firing as long as
> there is output from the oscillator.  CR1 and CR2 prevent the capacitor
> from
> discharging through R1 and R2
>
> That description also applies to the 3420B chopper.
>
> For this to work as described, George Einst says that the striking
> characteristics on the neons are critical, which I totally believe given
> what's happening in mine.
>
> AFAICT, *both* neons are striking on every +ve portion of the square wave
> drive signal, so the flip-flop behaviour doesn't happen.
>
> I checked on the curve tracer, and both neons strike at almost exactly 70V.
> Both are also totally clear which suggests to me that they are not the
> originals.
>
> Does anyone have any idea how much the strike voltages would need to differ
> for this to work, and does anyone have any suitable neons - the only ones I
> have are these shorter ones and they strike at about 125V.
>
> These are the longer bodied neons (glass about 5/8" long).
>
> Thank you
> Dave Partridge
>
>
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