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Sunday, 7 April 2019

Probes for Multimeter

Probes for Multimeter:


Any test instrument requires test probes for measurement. Generally all multimeters be it analog or digital will be supplied with a pair of probes for general use of length approximately over 1 m, in Red and Black colors, provided with proper connector on one side and test points on the other s die The black lead will be connected to the common or earth or ground socket on the instrument. The red lead will be connected to the positive socket or to the socket of the appropriate range of quantity measured. This arrangement may not be sufficient lithe instrument is to be utilized to its complete capacity.

A multimeter can be used to measure voltages and currents beyond the maximum range of design, using additional probes. Further measurement of very high voltage such as that in colour television receivers, measurement of temperature, measurement of high frequency voltages, etc. require additional probes. These are supplied only on optional basis, and are designed by the Manufacturer for the particular type of multimeter. They come under accessories for the instrument. Some of the important probes and their applications and typical specifications will be given hereunder.

(a) Long Reach Safety Probe: 

These probes are made with high insulating material, with extra length. the test pins will normally be encased in the plastic sleeve at the tip. A spring loaded button is to be used to bring the test pins out, and to establish contact with the component lead. The probes thus can be clamped at the required place on the component leads for testing and monitoring.

(b) Crocodile Clips: 

A pair of crocodile clips will be supplied, one of a small size and the other a large size. They will fit in to the multimeter test probes on the test pin side, and can be used when the lead requires a hold at a component or bus for monitoring the signal.

(c) High Voltage Probe: 

Generally the highest voltage range that will be provided in the multimeters is 1 kV. When a multimeter is to be employed for measurement of EHT voltage in television sets, a high voltage probe becomes a must.

Commonly available high voltage probes consist of a series resistor network that forms a voltage divider. The attenuation ratio is 100:1. This arrangement is sufficient to measure voltages up to 10 kV. When voltages of the order of 30 to 40 kV are to be measured, this arrangement cannot be used. This is because of the stray conduction paths setup by the resistors.

For such high voltage measurements capacitor type probes are employed. They provide an attenuation of the order of 1000:1. The probe contains two or more capacitors with suitable value of capacitance, that produces the required voltage reduction, and matching required to the input impedance of the meter.

Special high voltage probes are available for EHT measurement and servicing. These contain a meter built in the probe itself and can be used to measure up to 30 kV. These special probes will be provided with a separate earth lead, for connection to the ground while measuring EHT.



The specifications of a high voltage probe that can be used with a digital multimeter are given here under:

Specifications:

Input Resistance : 1000 ohm
Division ratio : 106:1 ( into 10 M ohm)
Range : 1 kV to 40 kV
D.C. Accuracy : Ratio accuracy (% of FSD) - 3% up to 25 kV, 5% above 25 kV.
A.C. accuracy : 5 % over and above D.C accuracy.
Total length of the cable : 2 m
This high voltage probe can be used to measure high voltages up to 40 kV. It can also be used for measurement of A.C. up to 100 Hz.

(d) High Frequency Probe: 

This probe can be used to measure wide range of high frequency voltage. Optionally 100:1 divider will also be provided to extend the range of the meter.

Specifications :

Frequency Range: 50 kHz to 400 MHz.
Frequency Response: ± 7 dB over the entire frequency range

Voltage Range and Accuracy:

Voltage Range
Accuracy
50 kHz to 200 MHz
200 MHz to 400 MHz
0.1 V to 0.5 V
±2 dB
± 2.7 dB
0.5 V to 10 V
± 1.2 dB
± 1.6 dB

Maximum D.C. blocking: 300 V

(e) Current Transformer : 

In order to extend the alternating current range a current transformer can be used. It is a step down transformer, with different taps provided for required ranges. The commonly available ranges are 100 mA, 1 A, 2.5 A and 10 A.

(f) Temperature Probe : 

This probe can be used to measure temperature varying from 0°C to +800°C. The sensing element is a thermocouple. The attachment is of plug in type. The sensing thermocouple can be detached for change with another thermocouple. The probe can be used to measure temperature of electrical and electronic components.

(g) Frequency Transducer Probe :

This probe is used for measurement of frequency at audio and power frequencies. Usually it will be provided with two ranges, one range is for line frequencies and the other for the audio frequency range. The probe will be a plug in type. The ranges usually available are from 20 Hz to 200 Hz. and 200 Hz. to 20 kHz.

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