Gas station without pumps

2015 July 18

DT-9205A voltmeter review

Filed under: Circuits course — gasstationwithoutpumps @ 13:42
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I recently bought a DT-9205A digital voltmeter for under $10, including free shipping from China, to see whether it would be usable for students in my electronics class for working at home.

There are some things to like about this meter, besides the price.  The digits of the display are 2.5cm (1″) high, making the meter easily readable from a distance, even without my glasses (my wife asked if it was a special multimeter for seniors). The meter doesn’t autorange, but has a knob to select several different ranges for its 3½-digit display:

V DC V AC A DC A AC Ω C
200mV 200mV 2mA 2mA 200Ω 20nF
2V 2V 20mA 20mA 2kΩ 200nF
20V 20V 200mA 200mA 20kΩ 2µF
200V 200V 200kΩ 20µF
1000V 750V 20A 20A 2MΩ 200µF
20MΩ
200MΩ

It also has a diode forward-voltage measurement (@1mA) that doubles as a continuity check and a bipolar transistor hFE measurement (base current ≈10µA, VCE≈2.8V). The diode tester does not get up to high enough voltages for testing the forward-voltage of LEDs, but seems to work OK with 171mV and 200mV for two different Schottky diodes and 600mV for ordinary rectifiers. The continuity checker does not beep with that large a voltage difference—they claim that the buzzer should sound if there is less than about 30Ω, but I get a threshold closer to 100Ω.  The 1mA current seems to be fairly accurate, though, so the measurement is about the same as using the 200Ω range on the ohmmeter.

The capacitance meter seems to be ok, but I don’t have any very accurate capacitances to check it with, and it doesn’t seem to work with electrolytic capacitors.

They claim a 10MΩ input impedance for both the DC voltage and the AC voltage, and about 1% accuracy on the voltages (varying slightly depending on the range). They claim a range of 40Hz to 400Hz for the AC voltage measurement, but we need more than that for the course, so I was curious what it was actually capable of, and what the actual input impedance of the meter was (including the parallel capacitance).

To measure the input impedance, I connected the voltmeter to my FG085 function generator with or without a series impedance, and recorded what the voltmeter reported (as I did in the Measuring voltmeter input impedance post).  I also used my BitScope oscilloscope to measure peak-to-peak voltages, which I scaled to RMS values, so I could see whether problems were with changes in the function generator output or in the voltmeter measurements.

For the 20V range with the FG085 set to provide a 10Vpp signal, I observed an interesting resonant peak:

The meter works ok up to  2kHz, but has a weird resonance around 50kHz.

The meter works ok up to 2kHz, but has a weird resonance around 50kHz.

Note: I don’t trust the absolute calibration of the BitScope enough to claim that there is any inaccuracy in the DT-9205A measurements at low frequencies—the meter is probably more accurate there than the digital oscilloscope. I can’t really check with PteroDAQ (where I’ve checked the calibration against the bench meters in the lab), because PteroDAQ is limited to 0V—3.3V (with the KL25Z boards), and I want to measure AC voltages that are centered at 0.

I made similar measurements with the 2V range and a nominal 2.5Vpp sine-wave input:

The 2V AC scale also has a resonant peak, but around 30kHz and not as big as on the 20V scale.  The measurements seem to be usable to about 3kHz.

The 2V AC scale also has a resonant peak, but around 30kHz and not as big as on the 20V scale. The measurements seem to be usable to about 3kHz.

The 2kHz–3kHz upper limit on the accuracy of the meter could be a limitation, but the only thing we use meter for at higher frequencies is relative measurements (measuring magnitude of impedance |Z| by measuring the voltage across the component and the voltage across a series resistance). I can estimate the impedance of the AC voltmeter input by treating the series resistor plus meter as a voltage divider, and assuming that the resistances are all big enough that the voltage from the function generator doesn’t change much by adding the series resistor.

The impedance measurements are not entirely consistent, getting estimates that vary by 20% in resistance and 50% in capacitance.

The impedance measurements are not entirely consistent, getting estimates that vary by 20% in resistance and 50% in capacitance.

It looks like I can use the ratios of voltages out to about 40kHz, ten times the frequency limit for using the voltages directly. That should be good enough for both the loudspeaker impedance lab and the electrode impedance lab, though it is not as good as the benchtop Agilent meters, which we can push to 2MHz for ratio measurements like this.

Unfortunately, the impedance estimates are not as consistent as I’d like, with the DC resistance varying from 11MΩ to 15MΩ and the capacitance from 21pF to 31pF. The variation in capacitance may just be due to different configuration of the test leads on my bench—I had not made a point of keeping the leads in the same place between changes of the series impedance, and I probably should have. The capacitance of the leads could easily vary by 10pF—indeed, most of the observed capacitance is probably from the leads.  For practical purposes, treating the meter impedance as no bigger than 10MΩ||25pF will probably be ok for most measurement work.

The variation in the estimated DC resistance is harder for me to explain, but it seems to be consistently higher than the 10MΩ spec. One problem with the 10MΩ input impedance is that the meter is very sensitive to capacitively coupled 60Hz interference—with a high output impedance (like the 3.3MΩ series resistor), the voltage reading depends on whether my hands are near the leads or not. I tried to be consistent in my body position and staying 30cm away from the leads while making the measurements.

The AC voltmeter takes a long time to settle—I believe it works by using a peak detector and low-pass filtering the peaks, and it seems to have quite a long time constant for the low-pass filter. This makes for more stable readings and the ability to measure AC voltages of lower frequencies, but it means waiting a while for the signals to settle after any change in the input. That tradeoff is good for what we use the meter for in the lab (measuring steady-state voltages), but not so good for people who want to measure transient signals.

Overall, the DT-9205A looks like a good buy for <$10, and I think I can make all the labs that use voltmeters work with this cheap meter.

 

1 Comment »

  1. […] the resistance with an ohmmeter. My Fluke multimeter is broken, so I’ve been using the DT-9205A multimeter that I reviewed earlier. I found out one reason that multimeter was cheap—one of the probes fell apart within a few weeks […]

    Pingback by Resistor assortment box | Gas station without pumps — 2015 November 27 @ 11:36 | Reply


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