HP 3420B DC Differential Voltmeter/ Ratiometer

[@victorman2227]

Oh wow those huge battery banks to power it, awesome! These kind of instruments are fun, dialing in the just the right amount makes for a nice feeling.

[@KeritechElectronics]

Nice! I wonder if it could be modded to make a calibrator by adding an output terminal for the reference voltage source, but that'd probably throw it out of adjustment.

[@garyramsey4275]

Yes, I’ve wondered the very same thing. I do have the manual, with schematics and everything. I may look into it.

[@JayWye52]

@@garyramsey4275 actually,it could be used as a "meter calibrator",just connect the meter(DUT) across the measurement terminals,leave it on 1x sensitivity,and dial in the voltage to 90% of full range on your device under test. You just cannot draw a lot of current from the HP unit.(i was a PMEL tech in USAF.) a 10M input Z DMM won't load it. probably not a 1M cheapo Harbor Freight DMM,either.

[@terrygains8327]

Thank you Gary for another excellent video. :) Maybe you'll also be able to review the HP 740B DC Standard/Differential Voltmeter? It's very similar to the HP 3420B DC Differential Voltmeter but has a nixie display. Have not been able to find the correct test leads for my HP 740B so may have to improvise a solution. Kind regards.

[@garyramsey4275]

@@terrygains8327 Yes, I do have a 740B that I started working on over a year ago, but I haven’t finished repairs on that instrument yet. I had to fabricate replacement input and output cables for mine, which work fine, but there are still other problems with it. It only “sort of” works. I have a feeling there are issues with the photochoppers in it (there are 2 of them.) It will probably take me some time to get it fully operational again, but of course I’ll make a video (or several) of that unit.

[@zdzisiek1979]

Ciekawy przyrząd :) To kalibrator DCV ? Ciekawa budowa. 11:40 to czarne dwa pakiety to co to jest ? Pozdrawiam

[@victorman2227]

The black thingies are battery banks 4x24V to provide isolated power for lower noise. Impressive.

[@garyramsey4275]

It is not exactly a calibrator. The meter indicates the difference between the internal reference voltage set by the dials, and the dc voltage applied to the input terminals.

[@zdzisiek1979]

@@garyramsey4275 Ok myślałem że kalibrator bo tak wygląda. Nie znam języka angielskiego to nie wiedziałem. Ładny sprzęt.

[@Stelios.Posantzis]

I've always wondered what a ratiometer was! I've had one for decades without knowing what it was for. Now I have to find out where I've stored it and dig it up! Thanks! I'm very thrilled to have kept it now! By the way, why is it called a ratiometer? I was never able to find any information on ratiometers on the web until now.

[@garyramsey4275]

@@Stelios.Posantzis I didn’t actually demonstrate the ratiometer part of this instrument. The ratiometer uses both inputs A and B, and indicates the ratio B/A, thus the quantity measured is not a voltage, but a fractional ratio. It’s another way of comparing an unknown voltage to an external standard reference voltage.

[@Stelios.Posantzis]

@@garyramsey4275 A-ha! Any chance of a short follow up then? In what situations would you use this function? Another thing: what can you do if you want to measure a high voltage? Is it possible to measure it directly with a ratiometer? Thanks for the quick reply.

[@garyramsey4275]

@@Stelios.Posantzis I may make a follow up video once I get my HP 740B DC Standard up and fully functional. The ratiometer function was generally used to compare an unknown voltage to a standard 1 volt. The ratiometer function can measure ratios of up to 1000 I.e. comparing 1kV to a standard 1V.

[@Stelios.Posantzis]

@@garyramsey4275 So it's not just for measuring very low voltages, excellent! Thanks again!

[@JayWye52]

using a differential voltage measurement,at null,you draw no current from the DUT,thus your meter impedance is effectively infinite.