EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: watchmaker on November 20, 2023, 05:08:10 pm
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I have two 8840 Afs. Why do I have to clear the last voltage readings by shorting the leads? It slowly counts down but does not hold the exact value nor go to zero. Just an inconvenience. The Fluke 45 does not do this.
Display clears "correctly" on resistance.
Regards,
Dewey
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Many voltmeters have a fixed resistance, often 10M, across the inputs. This quickly bleeds off bias currents and stray voltages thus you get a nice near-zero reading with open leads. Your 884x meters don't have this and thus have a very, very high input impedance--so high you really can't call it resistance because it is dominated by other effects. The very small internal bias currents and any external voltages charge up the tiny amount of input capacitance and then you see whatever that is reflected on the display. The reason this is good and not bad is that when you do an actual measurement, this type of meter puts a much smaller load on the circuit. Take a 9V battery and short it with 2 5M or 10M resistors in series, then measure the voltage across each of the resistors separately. With your 8840A/AF, the two will add up to the same amount as the battery voltage (which is correct) while your Fluke 45 will just be all wrong.
The high impedance feature on these meters is only on the ranges of 10V (or 19.9999V range if you want to call it that) and lower. Manually range up to the 100V range and the input impedance becomes 10M just like any other meter and the "problem" goes away.
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Interesting. Thank you for the detailed explanation which I could in fact follow.
Regards,
Dewey