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[burakkucat]

. . . my “open circuit” measurement was of course a little white lie as my DVM itself drew a small current so not really open circuit.

Isn't there an oscilloscope in the seventh layer grotto?

[sevenlayermuddle]

Isn't there an oscilloscope in the seventh layer grotto?

There is indeed...

https://forum.kitz.co.uk/index.php/topic,24212.msg407595.html#msg407595


Is there some experiment you would like me to conduct?

[burakkucat]

I was thinking about your statement confession that the DMM presented a (tiny) load to the switching PSU and any possible way the off-load voltage could be measured . . .

I expect that the input stage of an oscilloscope would posess a greater impedance than any DMM. 

[tiffy]

I expect that the input stage of an oscilloscope would posess a greater impedance than any DMM.

With a decent quality DMM such as Fluke don't think there would be that much difference in input impedance certainly by comparison to an older CRO, development of components such as IG FET's changed the goal posts.

I still own an old analogue AVO 8 Mk.4, 20K.Ohms/Volt input impedance, perfectly fine in it's time for trouble shooting thermionic valve equipment and still gives a better indication of low frequency oscillations than a DMM in lieu of a scope.
Also handy for the old "wet finger" test on transistors.

Apologies, I digress !

[burakkucat]

With a decent quality DMM such as Fluke don't think there would be that much difference in input impedance certainly by comparison to an older CRO, development of components such as IG FET's changed the goal posts.

Thank you. 

[sevenlayermuddle]

My current favourite DVM was purchased from Maplin mid noughties, probably paid £10-£20.   I have lost the formal specification sheet but have established, by experimentation with resistor networks, that the impedance appears to be 10M ohms - on the low voltage DC ranges at least.

My little (toy) oscilloscope discussed in linked thread above is documented to have an impedance of 1M ohm.   

Observed behaviour is consistent with above.  A recent and ongoing project involves detection of very small currents from an LED that is deployed “inside out” as a photodiode.   By loading the LED with a high value resistor, the Maplin DVM can be used to measure the photovoltaic current, even under indoor lighting conditions.  Providing I make allowance for the 10M ohm load of the DVM itself, by measuring the voltage on the meter’s lowest scale, I can calculate the actual photovoltaic current.   The little oscilloscope is pretty useless in comparison - presenting simply too high a load to allow meaningful measurements to be obtained.