DMM Mestek DM90S

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This is a automatic meter with a limited amount of ranges.

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I got the meter in a cardboard box with a drawing of the meter

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It included the DMM, a pair of probes and a manual.

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The probes are rated for 600V CAT III and CAT II without the tip cover.

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The shrouded plug is the short variety.

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Using the switches requires holding the meter.

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Display

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The above picture shows all the segments on the display, not all of them are used.

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Display when meter is idle, it will show four bars and wait for a value.

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When measuring it will show the value and range.

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The NCV shows bars when nothing is detected and will use a number from 1-3 to show field strength.

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A bug in the meter, it shows LIVE while measuring a 6uF capacitor. I did not get it to show LIVE by connecting either of the input terminals to live mains voltage. According to the manual it is used to indicate when voltage is feed into these terminals, but it do not detect 10V DC/AC.



Functions

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Input

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Measurements 1uF

A look at the capacitance measurement waveform.

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The voltage input is only 900kOhm input impedance.

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Voltage input uses negative voltage to measure resistance. In this picture the input is loaded with 10Mohm from the oscilloscope.

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The diode test input is also pulsing, but with a higher voltage to detect a diode.

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Meter cannot handle a combination of DC and AC when DC is highest.



Tear down

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I had to remove four screws to open the meter.

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The detection of diode/capacity input is done with a split terminal, as can be seen here.

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Getting the circuit board out was 6 screws and I had to unsolder one wire.

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The voltage input has the usually 3 input paths: a high impedance (R46 & R47: 2x5Mohm), a medium impedance (R41, R42, R43: 3x300kOhm) and a low impedance (PTC1, D1, U14 & U15: Electronic relay). It looks like the ohm circuit may be partially implemented with discrete components. There is a transistor to open for the circuit (Q10) and four transistors (Q7, Q7, Q8, Q9) to select four resistors (R32, R33, R34, R35: 1kOhm, 10kOhm, 100kOhm, 1Mohm).
The circuit has a couple of opAmps (U9, U11, R12, U16: A42S: LMV358) and some multiplexer (U4: SGM48752: 2x4Mux) (U6: 3157: 74LVC1G3157: 2 to 1) (U7: 3167: maybe PI5A3167C).
The diode/capacity input uses two PTC's (PTC2 & PTC3) and MOV's (MOV1 & MOV2) for protection. MOV2 do probably not do much, because it is parallel with a transistor pair(Q1 & Q2), before it goes to U7. The other path splits into 3 (R21, R22, R24-R25-R26: 100kOhm, 100kOhm, 3x300kOhm). The sense part has its own resistors (R58, R59, 60, R61: 300kOhm, 300kOhm, 4.999Mohm, 4.999Mohm).
Everything ends up in the multimeter IC (U1), with the display driver in another chip (U3: HT1621).

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On this side is only the pads for the LCD display and buttons, together with the PTC resistors.
The leds for the front are firing through holes in the circuit board from the other side.
The button pads are not correctly marked with functions.

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Conclusion

This meter has a fairly low CAT rating, but only one of the inputs are marked to comply with it, that is not correct.
The amount of ranges and functions on this meter is fairly low and the idea to avoid a range switch and instead have to move the input plug between terminals is not something I approve of (Except for current).



Notes

The multimeter was supplied by banggood.com for review.

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