©2011, Bob Putnak. This post examines the performance (directly related to the input impedance) of low-cost meters; specifically, I explore a common multipurpose Colluck PM-128E DPM (digital panel meter) and a bargain-priced Cen-Tech #98025 multimeter.
Limitations in the design of these low-cost meters can severely affect measurement accuracy. First of all, I prove that the input impedance of the PM-128E is 1-megohm, not the 100-megohms or 10-megohms that is specified by the manufacturer and most vendors that sell this DPM. Second, I demonstrate that the input impedance of the Cen-Tech #98025 multimeter is also 1-megohm. The conclusion is that either meter will not accurately measure high-impedance circuits, and both perform poorly at measuring low AC voltages. They can be suitable for other types of measurements, though.
First a little background –“Input Impedance” as it pertains to a meter — is the load that the meter places upon the circuit being measured. Ideally, a perfect meter would have no loading effect, but all meters have some loading effect on the circuit they are measuring. For example, early analog VOM’s had an input impedance of 1000 ohms per volt, which meant that when the meter was set on the 500v range, the input impedance was 500k ohms. This input impedance (sometimes called ‘meter sensitivity’) is the exact same as placing a 500k resistor across the circuit. Newer analog VOM’s had an input impedance of 20,000 ohms per volt; therefore using our 500v range as the example, the 20,000 ohms/v meter would only load the circuit at 10-megohms. VTVM’s (vacuum-tube voltmeters) and TVM’s (transistorized voltmeters) commonly had a fixed loading effect of 11-megohms or 22-megohms, regardless of measurement range. Most quality modern DMM (digital multimeters) have a fixed input impedance between 10-megohms to 11-megohm. The higher the input impedance resistance, the more accurate the measurement. Input impedance is a serious issue when measuring high impedance circuits.