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

[David C. Partridge]

Errm  - I'm looking for around 65-75V range so I end up near but not exactly the same as the 70V ones that are already there.

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Pete Lancashire
Sent: 16 November 2017 18:24
To: Discussion of precise voltage measurement
Cc: hp_agilent_equipment at yahoogroups.com
Subject: Re: [volt-nuts] Neon bulbs for HP 3420B or 419A chopper circuit

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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