DMM Peakmeter MS8248S

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This is a DMM with most common function and fully automatic, i.e. it has no range and function switch.
How well do it work and are there limitations?

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It arrived in a DMM box with a image of the meter without brand.

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It included the DMM, a pair of probes, a thermocoupler, spare fuses, a bag and a manual.

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Everything fits in the bag.

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The probes has removable tip covers. The CAT rating is marked on the tip and will change when the cover is removed.

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With tip cover: CAT III-1000V, CAT IV-600V, without tip cover: CAT II-1000V (This is the usual marking on probes).

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The plug is fully shrouded and a bit shorter than standard probes.

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The meter includes a cheap thermocoupler.

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The tilting bale is stable enough to use the buttons.

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Display

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The meter shows all segments during power on.

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Meter is waiting for something to measure.

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

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When using the NCV detector the display will show NCV and the green leds on top will show the strength (There are only 3 steps, see tear-down), there are also sound effect from the buzzer when a electric field is detected.



Functions

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Buttons:

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This slider switch moves a cover between termocoupler/transistor-tester and input terminals. This prevents having anything in the holes when measuring voltage, this is good for safety.



Input

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Due to the auto selection the meter will apply a pulsed low voltage on terminals.



Measurements
1uF

A look at the capacity measurement waveform.

10kOhm

With auto mode selection, the meter always tries the different modes, i.e. ohm is only active some of the time. The above is measured with a 10kOhm resistor across the ohm input.

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The mF range gives wrong value when measuring above 4000uF
Combined AC/DC voltage shows neither value, but something near the combined value.



Limits

Due to the automatic selection there must be a minimum value on the input to be detected, this is the rated values:
Current: Minimum 5mA
DC Voltage: Minimum 0.2V
AC Voltage: Minimum 0.5V
Capacitance: Minimum 1nF



Tear down

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Four screws and the back could be removed.

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As usual the circuit board is shaped to fit the box.

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8 small screws and I could remove the circuit board and part of the front of the multimeter

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The shutters are mounted on the front, but tied to the circuit board with two screws.

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On this side of the circuit board is the switches and they are real switches. There is also the socket for the transistor tester and the connector for the thermocoupler. For good thermocoupler response a temperature sensor is needed very close to this socket, I cannot see any on either side of the circuit board.
The scale for NCV is only 3 leds and not 6 as the enclose looks like.

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The meter has two sets of input protection PTC's (PTC1..PTC4), one of the sets goes to the 10Mohm input divider (R26+R27), the other goes to a diode (D2) and a big transistor (Q6: BSP300: 800V FET transistor), it is probably used to switch in the ohm range.
The current range has the usual diode protection (D3..D7), with a transistor (Q5) to switch in the shunt (R0).
The handle all the automatic range switching this meter has a couple of analog switches (U5, U7, U8: SGM4582). There is two dual OpAmps (U3, U4, A42S/LM158).
Some of the transistors looks like protection pairs (Q7, Q8 & Q9, Q10), there is also the switches for backlight (Q2), flashlight (Q3) and buzzer (Q1). The NCV scale is controlled from some transistors at the top (Q15, Q16, Q17) near the top. Near the flashlight is a voltage regulator (U2: BL8503).
The NCV antenna (RF1) is a piece of metal sticking out at the top.
But I do not like the Spark gab (SG1) across the input terminals, it is fine for handling small transients, but larger ones will blow it up. To be safe it must be mounted on the other side of the PTC's.

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Conclusion

The automatic function/ranging is a novelty feature, but not that useful, because it limits the available ranges significantly. The protection looks fairly good, except SG1 is mounted in the wrong location, this makes me doubt the CAT rating.
It is nice with a real thermocoupler connector, but it is lacking temperature compensation, this is probably mounted inside the multimeter IC. The NCV scale is a bit of a cheat with 6 markings, but only 3 leds.
Also remember never to use this meter if it shows low battery!

Comparing to a normal auto ranging meter the only real automation on this meter is automatic voltage/ohm/continuity (No diode test), everything else requires moving a plug or pressing a button at the right time.



Notes

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