About This File
This was one of my early projects. I had to measure resistance in pretty
wide range on a delicate circuit (subject was changing value and in range
from few ohms to more than 1MOhm).
Normally I would have used something like RTD transducer but I just couldn't
find anything suitable for that range (every transducer I could find was
stopping at some 8 or 10k i needed at least 100x more).
The time was running out so I had to put something together and fast.
This is it. Initially this circuit was using dedicated regulated
low voltage power supply but since I had spare analog input(s)
I replaced that power supply with voltage divider (Panel had existing
24VDC power supply) and simply had to monitor that voltage because
it was not regulated any more (the 24VDC was regulated but the output
of divider was changing depending on test conditions).
Although we like to avoid custom designs this turned out to be so simple
cheap easy to use and worked extremly well. It is so small that there
was no need for PCB two resistors ware connected directly to terminals.
Accuracy off course could not match the 8-digits of precission benchtop
multimeter but it was on pair with my multimeters which is more
than I could ask for. I didn't test it for significant ambient changes
(temperature and humitity).
There are several ways to do the math for this and in fact used aproach is
not the simplest or most efficient. But it is good example on how simple
DC circuits knowledge and very basic match can be used in practice.
R1 and R2 are not critical and don't need to be accurate either (just need to
produce voltage that fits selected range for used input card.
It is robust enough. In fact I was surprised to see results when R1/R2 ratio
was adjusted to produce only 1 Volt (what kind of ADC they have in their cards?).
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