[time-nuts] Low noise power supply recommendations

[Will Matney]

John,

They used Lucite rods, I shouldn't have said tubes. The neon bulbs are
about at the center of the PC board, and the CDS cells are at the edge, so
to get the light to them, they used the rods. I think they did the same
thing on the 845AB too, but I'm not looking at the manual right now to see
for sure, as I'm not sure the 844 and 845 use the same PC board, just the
same circuitry. The 844 was sold to other manufacturers, and it came in a
plain aluminum box, with the switch shaft sticking out the front. The meter
and pots were shipped seperate to be mounted in the customers equipment. HP
did the same with the 419, and tacked another letter on the model number,
as well as Keithly, which I know uses the same PC board in both versions.
Keithly didn't use this type of chopper though (using LED's or lamps).

I am going to locate the post on the HP forum and read about it. My guess
is that the LED's brightness helped kill the CDS cells. I was thinking
about trying an orange or yellow LED here, and dimming the LED with the
series resistor, trying to make it as dim as the neon bulb, but I don't
know if a LED can be dimmed down that low. On the old HP 412 VTVM, the
incandescent bulbs in it were not very bright, and they were using CDS
cells with them. Another instance of this, if I recall, was that Tektronix
used a chopper like this in one of their older scope calibrators.

Thanks,

Will

*********** REPLY SEPARATOR  ***********

On 6/15/2011 at 6:51 AM J. Forster wrote:

>> John,
>>
>> I will check it out, and may do some experiments myself on these. Also,
I
>> will take a look at the photo-FET's, as I had forgotten about those.
>>
>> What has me wondering is how neon bulbs act in the circuit, their low
>> brightness, and their drop out times, as I think the on voltage is
around
>> 90 volts or so,
>
>More like 70. They need a higher voltage to turn on (strike).
>
>> but the square wave going to them is around 100-115 volts
>> if I recall. I thought about using a simple 10:1 resistive divider, then
>> using a series resistor from that junction going to the LED, the same as
>> for a 10-15 volt supply.
>
>Neons run at very low currents.
>
>> The neon bulbs light goes through two Lucite tubes
>> to the CDS cells, and it couldn't be too bright by the time it reached
>> them.
>
>Tubes or rods (as a light guide)?
>
>> I also thought about using a new form of chopper, as Paul mentions, but
>> making it fit and work could get complicated. ESI quit using the HP 419
in
>> the last models of their 801 DC supply and detector-null meter, and
>> started
>> using a Keithly 155. I either figured it was over this very thing, or HP
>> dropped the 419 from its line. An engineer at Vishay told me that they
>> quit
>> using the Fluke over this neon problem, and went to the Keithly in the
>> last
>> versions of this bridge.
>
>OK.
>
>> I'm going to be using the bridge not only as it was intended, but to do
>> other null measurements, as I added a circuit to use the meter circuit
>> seperatly from the bridge.
>>
>> Thanks,
>>
>> Will
>
>Good luck. BTW, the HP thread referred to degradation of the CdS cells.
>You should read it. It is possible to makle a very high Z chopper with
>CdS. I'm not so sure about other ways.
>
>-John
>
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