# How do I convert a digital panel meter that reads 2.4 V to 30 V to read 200 mV full scale?

Monitor DC voltage between 2.4 V and 30 V with this simple 2-wire voltmeter:

• ±1% tolerance
• Working current: <30mA
• Refresh rate: 100ms
• Auto ranging 0.01V precision
• Size: 30x11.5x9mm
• Weight: 3g
• Wire Length: 15cm
• Operating Temp: -10℃ - 65℃
• DC only, not for use with AC
• Voltage above 40V will damage the unit

Any help you can give would be appreciated I know I can use my DMM on 200 mV scale but like use nice LED displays for $3.00 on eBay that purchased. But I have no idea how to do this using this type of meter. How do I convert a digital panel meter that reads 2.4 V to 30 V to read 200 mV full scale so I can use it in millivolt test unit I made? ## 2 Answers Notice the first sentence: "Monitor DC voltage between 2.4V and 30V with this simple 2-wire voltmeter" The fact that it is a two-wire voltmeter means that it is powered by the voltage it is measuring - which is a minimum of 2.4 volts. So using this meter by itself is simply not going to work for 200 millivolts. There is a potential solution which will allow you to use the meter, though. You can make a x100 amplifier with its own power supply. Then 200 millivolts will produce 20 volts at the meter, and the digits (but not the decimal point) will be correct. The combination meter/amplifier will successfully measure 24 millivolts to 300 millivolts, assuming you can get an amplifier which will put out 30 volts for 0.3 volts in. The cheap eBay meters typically use the internal ADC in an inexpensive single-chip MCU rather than a meter chip, so they can't read negative and they can't directly read low voltages like 200mV with full resolution. The two wire type is also powered from the input signal, so it is quite unsuitable for use on an input that is that low. While it might be possible to add components and hack such a meter, you'd be far better off to just buy a suitable meter with separate supply and 0-199.9mV range, which you can do for around the$10 range.

In any case, in some applications you might need to add an isolated DC-DC converter module for the power supply to get accurate readings (or any readings at all) depending on the relationship of your available power supply to the input signal.