I recently came across an interesting way to monitor current, but was not certain exactly how it works. Likewise, I was hoping that someone might be able to explain how the device measures current.
The object shown in green is a High Voltage Electrostatic Precipitator capable of putting out a maximum of 30 kV and 10 mA (however, the designer stated that the load will usually only draw 1-3 mA).
Going from left to right, the first voltmeter is used for measuring the current off of the positive lead (the other voltmeter uses a voltage divider to find the voltage, but that section is irrelevant to the question). What is interesting is that the device is, in fact, a voltmeter rather than an ammeter. The voltmeter is set to roughly 3000 mV DC.
What's more interesting is that the readings (in mV) provided by the left-most voltmeter provide the exact number of milliamps that the load (shown as the box below the ballast resistor) is consuming.
Likewise, I was curious why this is and what math may be behind this effect?
Best Answer
The 10 ohm resistor is simple acting as a current to voltage converter since it is in series with the load. By Ohm's law, the voltage across the resistor in volts is simply 10 times the current through the resistor in amperes. However, the conversion factor is such that the current in milliamperes will be 1/10 the voltage in millivolts since the value of the resistor is 10 0hms.