DMM Mastech MS8212A

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This is a pen style DMM with voltage, ohms and current.

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The meter arrived in a typically Mastech box with an image of the meter and specifications on the box.

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Inside the box was a pouch and a manual.

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In the pouch was the meter and a single probe, together with alligator clips.

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Because the probe are without any tip protection and the meter is a bit long for the pouch, the probe tip can fairly easily poke through the pouch.

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The probe are branded Mastech and rated for 10A and up to 1000V

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The tip cover must be on for CAT III or CAT IV rating. This tip cover is screw on/off.

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The meter also has a tip cover, but here the rating is limited to CAT II/III and 600V to match the meter.
This tip cover is screw on/off.

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Both meter and probe can be equipped with alligator clips, these are screw on types.

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The front has a hole for a led, but it is not mounted on this model.

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Display

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Typical display during usage, it will show the number and what measurement is selected.

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At the front are 3 leds, a red/green set for H/L display in LOGIC mode and a red one for NCV detection.



Functions

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Buttons: Rotary switch:
Input

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One probe is integrated in the meter, the other is a loose test wire. As usual it is a bit problematic to share current and voltage on the same input.



Measurements
DMMInputVoltageSweepmVDC

Input impedance in mV DC range.

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High DC voltages in AC ranges can confuse the meter.
The meter can only show max, not min.


Tear down

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I had to remove 5 screws to open the meter, four where self tappers the last had a real thread.

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The circuit board is made to fit the box exactly and is mounted with clips.

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The current sense input use a 250mA fuse and then a shunt resistor (R20: 1ohm) with two diodes across (D3 & D4). The ohm input has two PTC's (PTC1 & PTC2) in series and two transistors (Q1 & Q2) to clamp any voltage. The voltage input has two 10Mohm paths (R52, R53 & R21A, R21: 4x5Mohm). One of the paths are just a load on the input (R52 & R53). The other parts near the input is the LOGIC handling (IC1: LM358, OpAmp), the sense for that goes through a resistor (R14A: 470kOhm), there is also a couple of transistor to drive the leds. The NCV must be the other transistors and the chip (U4: marked 22T4).
At the other end of the meter is the multimeter chip (IC1) and two trimpots (VR1 & VR2)

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This side has the pads for switches, range switch and lcd, there is also the 3 leds and the NCV antenna.

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Conclusion

With a 250V fuse the meter do not live up to a 600V CAT rating, but safety looks acceptable for occasion home mains work.
The meter is a bit low on ranges with no capacity, frequency or uA, but the calibration on the meter is very good on all ranges. The logic function is for 5V logic and cannot catch fast pulses.




Notes

This meter may exist with many different names on it and small variations in functions.

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