Cropico Electronic Cell Power Supply

[enut11]

I have a Cropico Electronic Cell that outputs 1.00000v and 1.01861v using 18 D-cells for power. It takes about 30 min to warm up to spec. Rather than waste battery power I want to use a mains power supply during the warm-up phase and switch over to battery power for testing. I have an idea to isolate the battery supply with a diode so as not to charge the dry cells when mains power is on.

Question: will a diode in series with the dry cell supply impact on noise levels at the cell output?
enut11

[enut11]

I have not tried the isolation diode yet but I did power the CROPICO from a 24v switchmode power supply for about 30 min and plotted the output. I then switched to dry cell power and the noise difference is easy to see (after sample #400).
What's more remarkable is that there was (almost) no output noise on battery power. The HP3456A was set to sample at 5 sec intervals and at 100PLC.

I also recorded the last 10 min on dry power using the statistical function on my Keysight 34461A with the following results.

Ave 0.9998947
Min 0.999891
Max 0.999896
SD 0.0000004

The Cropico 1v is amazingly stable and relatively noise free on battery power.
enut11

PS
See @lowimpedance teardown of the CROPICO at
https://www.eevblog.com/forum/metrology/cropico-esc1-electronic-standard-cell-a-look-inside/

[Vgkid]

What are you using to log your 3456a, I still need to get around to messing with my prologix (other than using it with my counter).

[IconicPCB]

neither the number of samples nor number of powerlne cycles will be much good with a switch mode powersupply.
make up a linear regulator and interpose between the instrument and the switcher.

Instead of the diode use a POWER FET connected to provide the diode function.

[enut11]

@Vgkid
@Bud wrote the macro for logging HP3456A via Agilent 82357B GPIB to USB into Excel.
https://www.eevblog.com/forum/metrology/data-logger-for-legacy-hp-instruments/

Based on an idea here:
https://www.eevblog.com/forum/metrology/3458a-logging-via-excel-macro/

[Vgkid]

Thanks

[manganin]

I have an idea to isolate the battery supply with a diode so as not to charge the dry cells when mains power is on.

Question: will a diode in series with the dry cell supply impact on noise levels at the cell output?

Many (if not most) of these have been modified for mains operation. All you need is a two diode OR circuit as shown in the picture. The external supply voltage should be higher than a fully charged battery. There is plenty of battery voltage, no need to worry about the diode drop. Check with an oscillosope if the original electrolytic capacitor needs to be replaced.

In case of a switching supply an additional linear regulator stage (e.g. LM317) would be a good idea.

Always disconnect the external supply (cable!) before wiring your test setup. The leakage of some poor power supplies can be high enough to damage your equipment. At least it ruins the measurement.

[enut11]

Partial schematic for CROPICO Electronic Cell plus theory of operation (courtesy of @lowimpedance). Interesting that the primary reference zener/transistor is 6.5v and this is divided down to 1v and 1.018v.
I wonder if it would be possible to boost the 6.5v to 10v with a low noise op amp.
enut11