The ammeter measures the size of electrical current, in amps. Current flows through a circuit, so it must flow through the ammeter. That means connecting the ammeter in series by breaking into the circuit. The low resistance of the ammeter ensures that the circuit will operate normally when the ammeter is connected to the circuit.
Overview - Ammeter
An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. Electric currents are measured in amperes, hence the name. The word ammeter is commonly misspelled or mispronounced as "ampmeter" by some.
The earliest design is the D'Arsonval galvanometer. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The voltage drop across the coil is kept to a minimum to minimize resistance in any circuit into which the meter is inserted.
A galvanometer can burn out if its tiny delicate coil overheats. To measure larger currents, a resistor called a shunt is placed in parallel with the coil. Most of the current flows through the shunt, and only a small fraction flows through the meter. With this solution, arbitrarily large currents can be measured with a single meter. Traditionally, the meter used with a shunt reaches full-scale deflection when a voltage of 50mV is placed across its coil, so shunts are typically designed to produce a voltage drop of 50mV when carrying their full rated current.
Cruder ammeters simply use a moving piece of iron (or a magnet) that is acted-upon by the electromagnetic force of fixed coil of (usually heavy gauge wire). At very high current ratings, such an ammeter can actually just clamp on to an existing conductor (where the conductor acts as a single-turn coil); this later example is sometimes used in automotive applications where it clamps-on to the main battery wire to show the charging and discharging of the battery.
More modern ammeter designs use an analog to digital converter to measure the voltage across the shunt resistor. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor.
One problem with the use of an ammeter is the need for the meter to be inserted into the circuit and become part of it. In AC circuits, an inductive coupling adapter converts the magnetic field around a conductor into a small AC current that can be easily read by a meter. See clamp meter. In a similar way, accurate DC non-contact ammeters have been constructed using Hall effect magnetic field sensors.
A voltmeter measures potential difference, in volts, across 2 points. It is connected in parallel. It has a high internal resistance, so as not to affect the potential difference across the component or circuit being tested.
Overview - Voltmeter
A voltmeter is a measuring instrument for measuring the voltage between two points in an electric circuit.
Many voltmeters are in fact very high resistance ammeters. Hence the design of the instrument is identical to that of an ammeter except that one of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of electric current to operate.
The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
A voltmeter may also be realized using a potentiometer, which is a length of uniform resistance material (wire or carbon film, for instance) and a "wiper" that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer. The unknown voltage source may be connected to a current detector, which is in turn connected to the potentiometer's wiper, while the known voltage source is connected to an end terminal of the potentiometer. Then the wiper position is adjusted to change the potentiometer's effective resistance until a balance is obtained and no current is detected. At this time, record the potentiometer's wiper position. For example, if our potentiometer were a length of very long wire and our wiper were some sort of metal wand in constant with that wire, record the length of wire between the wiper and the end of the wiper that is in our circuit. Now replace the unknown voltage supply with the known voltage supply and repeat the procedure. The unknown voltage is then given by the product of the known voltage and the recorded used length of wire corresponding to the unknown voltage, divided by the recorded length of wire corresponding to the reference voltage.
The ohmmeter measures resistance, in ohms. The item to be tested must first be disconnected from its circuit. This is so any pressure in the circuit will not affect the readings of the meter.
Overview - Ohmmeter
An Ohmmeter is an electrical measuring instrument that measures electrical resistance, the opposition to the flow of an electric current.
The original design of an ohmmeter provided a small battery to apply a voltage to a resistance. It used a galvanometer to measure the electric current through the resistance. The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance decreased, the current through the meter would increase.
A more accurate type of ohmmeter has an electronic circuit that passes a constant current I through the resistance, and another circuit that measures the voltage V across the resistance. According to the following equation, derived from Ohm's Law, the value of the resistance R is given by:
R = V / I
For high-precision measurements the above types of meter are inadequate. This is because the meter's reading is the sum of the resistance of the measuring leads, the contact resistance's and the resistance being measured. To reduce this effect, a precision ohmmeter has four terminals, called Kelvin contacts. Two terminals carry the current from the meter, while the other two allow the meter to measure the voltage across the resistor. With this type of meter, any voltage drop due to the resistance of the first pair of leads and their contact resistance's is ignored by the meter. This four terminal measurement technique is called Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistance's.
Other measuring instruments including: Electronic test equipment
Practical electronics engineering and assembly requires the use of many different kinds of electronic test equipment, from the very simple and inexpensive to complex and sophisticated.
List of electronic test equipment:
Source: CDX Global & Wikipedia - en.wikipedia.org
