Fluke 45 current readings significantly inacurrate - Page 1
Forum section: electronics / testgear

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 09, 2017, 09:21:01 pm
content: I recently acquired a Fluke 45 dual display meter from eBay and I am generally pleased with the purchase given the price I paid for it. The case is a bit damaged (which I expected and was clearly shown in the photos) but the display is nice and bright and meter seems to work well reading DC and AC voltage as well as resistance and frequency. However, I have noticed that there is a problem reading current when using the 100mA range.To test I used a PSU as a power source set to 9v. I placed a 100ohm resistor in series with the meter on the 100mA range to read the current, i.e GND - meter - resistor - 9v. I could also monitor the voltage and current readings on the PSU. The readings were as follows:R = 100.35V(meter) = 9.012V(psu) = 9.00vI(meter) = 80.50mAI(psu) = 82mANow this is off by about 9.5mA. To compare I tried the same with my Fluke 179 and GW-Instek GDM-8145 and got as follows:Fluke 179:R = 100.2V(meter) = 9.01V(psu) = 9.00vI(meter) = 88.4mAI(psu) = 90mAGW-Instek GDM-8145R = 100.06V(meter) = 9.005V(psu) = 9.00vI(meter) = 88.79mAI(psu) = 90mATo eliminate some weird behaviour of the PSU I conducted a similar test using a used 1.5v batery cell. The battery voltage was read as 1.4224 and the current across the same resistor was again low at 12.508mA. A current reading of approximately 14.174mA would have been expected.Clearly some resistance is still being imposed added by the meter, presumably the shunt, but nowhere near as much and the reading is much closer to the mark. By contrast the same test using the 10A range seems to be accurate and returns a reading of exactly 0.090A. So why is the Fluke 45 reading so low on the 100mA range? Is this a problem with the shunt resistor? I can't cannot accept that a Fluke (or indeed any DMM) should be this far off otherwise?Just to confuse matters, AC current readings seem to be somewhat on the high side. A small transformer was used to supply AC voltage and the RMS voltage measured across this with just the meter as a load was 17.782v. The current read across the same 100.35ohm resistor was 0.182mA for an expected reading of approximately 0.177mA.

author: edavid
author_rank: Super Contributor
timestamp: September 09, 2017, 09:25:45 pm
content: It's in the manual:http://www.mdc.umn.edu/facility/files/electrical/Elec%20Manuals/Fluke%2045%20Dual%20Display%20Multimeter.pdfSpecifications on page 109Burden voltage on 100mA range = 1.4V -> shunt resistor is 14 ohms.9V/114ohms = 79mA

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 09, 2017, 10:19:34 pm
content: Thanks for pointing that out and it would certainly seem to explain it, although my reading is still 1.5mA out. Of course in practice I suppose its a little more complex than that as the meter itself will present an impedance across the shunt and there are the probe leads and clips to consider. Still, 79mA is certainly much closer to my readings. By contrast the 10A shunt would be only about 0.025ohms which would barely make an impact on the reading which would explain why the reading on the 10A range was spot on. I can't see it stated for the Fluke 179, but for the GDM-8145, the burden voltage on the 200mA range is 0.3v so the shunt is 0.3/0.2 = 1.5ohms which makes a big difference. So 9v/101.5mA = 0.886 which is pretty close to the figure recorded. Generally speaking, I have observed that current readings do tend to vary between different meters which perhaps makes sense if this were explained by the different burden voltage/shunt resistance of each make/model of meter. However, doesn't this rather defeat the point of an accurate 4/5 digit DMM of providing precise measurements? Of course, knowing the burden voltage of the meter at the particular range, one can make the appropriate calculations and make corrections to the readings, but shouldn't the meter iself compensate for this somehow? I think this excercise has helped me to appreciate why burden voltage matters.Interestingly, when using the PSU current limiting, the meter gives much closer results. For example setting the PSU to limit at 74mA and measuring the current across the output gave a reading of 72.87mA on the meter. Since I cannot be certain of the accuracy of the PSU current display, I am not surprised by a slight variation of a mA or two. In fact I find it re-assuring that the reading is quite close to where it should be.

author: Damianos
author_rank: Frequent Contributor
timestamp: September 10, 2017, 12:30:14 pm
content: To compare the ammeters connect them in series so as for them to measure the exact same current.To characterize any of them, supply it with a well known current and measure with a voltmeter the voltage drop on its' terminals, at various points of each range. Don't be surprised if that has not a linear behavior, have in mind that may be some components there (like a fuse) that have not pure Ohmic properties and the resistance varies due to various current (temperature) values. This maybe is referred somewhere in the manual.BTW: as a "current source" is characterized one that has voltage that tends to infinity and internal resistance that tends also to infinity. Therefore by increasing the voltage and the value of the resistor, that is used to set the current, the load (ammeter) will "disturb" less the setup.The above are valid for DC and "stable" measurements, in AC we have to cope with capacitances and for higher frequencies also with inductances that make the things a little more complicated.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 11, 2017, 02:36:31 pm
content: Quote from: Damianos on September 10, 2017, 12:30:14 pmThanks for your comment. Yes, I came to realise that the shunt resistance was rather high in proportion to the 100ohm resistor I had chosen, giving an error 14% which is very high. If I had chosen a 10k resistor, the error would have been only 0.14% instead of 14%. I did actually repeat the excercise with a 10k resistor and a 30v source. The results were:R = 9.963kV = 30.000 (yes I managed to get it spot on!)I = 3.009Calculating the expected current I get 3.0111mA and allowing for the extra 14ohms of shunt resistance gives 30/9977 so approx. 3.0069 so the reading I got of 3.009 is pretty accurate under the circumstances which is re-assuring.