[time-nuts] Low noise power supply recommendations

J. Forster jfor at quik.com
Wed Jun 15 13:51:37 UTC 2011

> 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.


> 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.



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