MA-149 – Automotive DMM

Preamble

A friend was given one. The screen was shot, but repaired using the old silver foil technique. However, the meter refused to give any readings.

It is difficult to find the User guide (in English). It seems to be a Spanish device… even though it is made in China.

Links

PTC Links

PTC resistance should be around 1 kΩ ( 890 Ω is ok – according to this post). The two PTCs in the MA-149, each with a resistance of 1.46 kΩ.

From https://lygte-info.dk Multimeter protection and safety

To keep the input voltage limited meters uses this circuit, the resistor and PTC values may vary a little but is about 1kOhm (Some meters may use two PTC’s in series without the resistor and cheaper meters may leave out the 1kOhm resistor).

The two MOV’s will keep the maximum voltage limited (Their voltage will also vary, depending on meter) and if the high voltage pulse is long the PTC will increase resistance to reduce current (Some meters use spark-gabs instead of MOV’s). This mean the internal design (10Mohm resistor and range switch) must be able to handle about 2000V, then the meter is safe. On some meters the 10Mohm input resistor is not protected by this circuit and with a good resistor it do not need to be.

Specifications

User guide

 

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