[volt-nuts] Could you fall an HP 4-wire LCR meter into thinking you have a negative resistor?

[Bob Albert]

You can do a similar thing by connecting the measuring leads 'outside' the current source leads or reversing the measuring leads.  It's not a valid measurement, just the same as putting a signal into one pair of leads without connecting the other.  The instrument isn't measuring resistance; it's measuring voltage and you can put in any voltage you like and it will indicate what it thinks the resistance is if it were connected correctly.
Bob
 


     On Tuesday, September 1, 2015 6:47 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
   

 I just sent an email by mistake, failing to finish it before hitting
"send".

The recent discussion about creating inductor standards from a capacitor
and two resistors got me thinking. I suspect if you connected up an HP (or
similar) LCR meter in a way not intended, you could make it appear you have
made a negative resistor. I've not tried it, and some care would be needed,
as it has the potential to damage a meter if one did not use a bit of
common sense.

If you connect the current wires (Lcur and Hcur) to the primary of a
transformer, and measure the voltage at the secondary with Lpot and Hpot,
then one could probably get the meter to indicate a negative resistance. If
the wires to the secondary are are of  the correct phase (just revese them
if not), then the voltage measured on the meter would be out of 180 degrees
out of phase with the current injected. So one could appear to make a
negative resistor. Of course it is not really a negative resistor, and if
measured on a normal multimeter would would just measure the DC resistance
of the transformer.

One could need a bit of care to make sure the meter is not damaged, as one
has the possibility to develop a high voltage on the secondary of the
transformer.

Of course such a negative resistor would have no practical use, but I see
it an obvious extension of making inductors out of capacitors. Unlike the
synthesized inductor, this should be indicate something fairly close to a
pure negative resistor, rather than a very low Q inductor.

Dave
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