main index this page is updated regularly
last update: 20 Jul 2012
contacts:mail us here
test instruments services
Liu Hui analog panel frequency meter SM-72
We purchased this new analog panel frequency meter from Liu Hui, Nanjing, China.The meter is designed for 220 Volt, AC 50 Hz.

Once we have connected the frequency meter to wall socket power line (230V , 50 Hz), the pointer shot up over range. We have decided to connect the meter over regulated transformer to avoid damages and check where is a problem. Next photos show results.

The frequency panel meter is connected to regulated transformer and to digital multimeter. Digital multimeter is switched to AC 200 Volt range. The smallest adjustable voltage delivered from transformer is 1.3 Volt. The panel frequence meter show 45 Hz. However, the transformer has primar connector to wall socket 230 Volt at 50 Hz.

We increased secondary voltage from transformer to 4.1 Volt and frequency meter moved to 47 Hz.

At to 9.4 Volt from regulated transformer, the frequency meter show frequency 50 Hz.

At to 15.1 Volt from regulated transformer, the frequency meter show frequency 53 Hz.

At to 18.4 Volt from regulated transformer, the frequency meter is at end of scale, show frequency 55 Hz.

At 20.8 Volt from regulated transformer, the frequency meter is over range.

We opened the meter to locate a problem. Inside is small board.

Surprisingly the full circuit is made only with several diodes, capacitors and resistors.

Solder side of the board.

We have created a schematics.

After we claim the panel meter, Mr. Liu Hui sent as a replacement. This should be "100% working". Unlike, it was non-working, it was dead. The coil in the meter was damaged.

We have tested the electronic board from replacement with the first meter we have received earlier. The electronic board in the first panel meter has been replaced with the board from panel meter we have received as replacement. Unlike the frequency panel meter gives the same results as described above.

Beside all above, the power resistors R1 and R2 are heated to 60°C. Because the board is closed in plastic small cabinet, the lifetime of the panel meter cannot be long. The construction is wrong.