Features and functions of multimeters
Multimeters describe them as the tape of the new millennium. What exactly is a digital multimeter? What are all sets utilisations? Which model do you need for your work? Which top multimeters is best suited to the environment in which you work?
All the answers to these questions are in this small application note. Nothing escapes the speed of technological change: embark increasingly computer cars, electric motors are controlled by electronic controls … We find integrated circuits in all applications – from the coffee machine up at spaceship. The recent boom in telecommunications with mobile phones, pagers, and the Internet connections, has further increased the pressure on electronics technicians. The installation, maintenance, and repair of this often complex equipment require diagnostic tools is precise, practical and complete.
So let’s start by seeing what a digital multimeter. One can compare this instrument with an electronic rule for measuring electrical quantities. While it can be enriched with all sorts of other features, a digital multimeter, therefore, perform primarily measures volts, ohms, and amps. Examples of measures presented in this brochure were made with digital multimeters. Other types of multimeters may have a different operation and characteristics of those used here. However, examples and usage tips that give this brochure apply to most instruments.
How to measure voltages
1 Select VOLT AC (V), VOLT DC () or 300 mV — as appropriate.
2. Connect the black test probe to the COM input and the red test probe to the input V.
3. If the meter has a manual adjustment range, first select the highest range so as not to overload the input.
4. Touch the circuit with hand tips and sides of a load or power and shown in the diagram (parallel connection).
5. View the reading, although noting the unit of measurement used.
Note: For DC measurements, the polarity (±) of the circuit using the red test probe to the positive side and the black tip to the negative side or circuit ground. If inversion with a DMM with automatic polarity, the instrument simply displays a negative value. This type of false maneuver against risk by damaging an analog meter.
- 1/1000 V = 1 mV
- 1000 V = 1 kV
Use the tips of special high voltage keys to the interventions T.V. and CRTs – on which the voltage up to 40 kV. Warning: these touch points are not intended for electrical distribution applications where high voltage is accompanied by high energy. They are intended for use in low power applications.
A common mistake is to forget the test leads into the current measurement data when making a voltage measurement, causing a short circuit between the live two-run homer voltage measurement – then the current through the low resistor value, called current shunt, using the multimeter to measure the intensity. In the absence of appropriate safeguards, the digital multimeter is then traversed by a current of very high energy can damage the instrument seriously as measured circuit and especially hurt the operator. Of extremely high-energy currents can indeed occur and the high voltage industrial circuits (240 volts or more).
This is why a digital multimeter absolutely must possess current measuring inputs protected by fuses of sufficient capacity for the regular channel. Multimeters without protective fuse on their current mapping data should therefore certainly not be used on electrical circuits high energy (> 240 V at). As for those who are equipped with a fuse, the latter must be of sufficient capacity to handle any error on a high energy circuit. In other words, the maximum voltage protection fuse must be greater than the maximum voltage that the instrument will have to be measured. And a fuse 20A / 250V will not provide the necessary protection performed on this type of circuit; it will choose a multimeter with a fuse 20A / 600V.
Common mistakes to avoid
1. Contact with an alternative source when the test leads are connected to the current measuring input
2. Contact with an alternative source when function switch is resistance measurement method
3. Exposure to high voltage transient
4. Exceeding maximum permitted (voltage data and current)
The different types of protection circuits
1. Protection with automatic weapons
Some meters are equipped with circuitry which detects overload conditions protecting the instrument until disappearance of the risk. Upon removal of the bad connection, the meter is ready for operation. This type of circuit is used to protect the function of measuring a resistance against overcharging.
2. Protection without automatic weapons
Some multimeters are equipped with systems protecting them in the case of overload, but not allowing an immediate return to service the instrument once the damage avoided. The operator must then perform an operation on the meter, such as changing a fuse.
Here are a safety multimeter absolutely must have:
1. current measuring inputs protected by fuses.
2. Using large breaking capacity fuses (600 V or more)
3. Protection against high voltages (500 V or more) resistance measurement method
4. Freedom from voltage transients (6 kV or more)
5. secure test lead design: finger guard, insulated terminals
6. Approval / Accreditation by independent agencies (UL, CSA)
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