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How can an oscilloscope measure current ?

2
Engineer trapped inside a Marketing Person, love my job as it allows me to not only use the latest Tektronix products, but also see how other Engineers use them to solve their daily jobs to be done

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November 4, 2019 16:40

Hi, What if I use a 1x voltage probe and set the impedance of the oscilloscope to 50Ohm? would this give an accurate indication of the current flow through the component?

0 Votes

November 6, 2019 15:19

@Asanka If you are talking about using the scope's internal 50 Ohm termination mode resistor as the current sense resistor, there are several reasons not to do so. First, it would only work when you have the scope hooked up and in the correct termination mode, completing the circuit. Second, it requires the shunt (50 ohm) have one side at ground potential, versus a differential probes with an external shunt, or coil or hall-effect based sensors. The third reason has to do with the choice of fifty ohm as a shunt resistor. You normally want a shunt for current measurement to be as low a resistance as possible while still generating enough voltage-proportional-to-current to be measured with sufficient resolution and noise rejection. You want this resistance to be low for two reasons. The larger the resistance relative to the impedance of the circuit being measured, the more the reading will be affected by the presence of the shunt, vs operation of the circuit without a shunt. For example, suppose you are measuring waveforms on a relay coil of a 5 volt relay, which typically has a 100 ohm coil impedance and nominal 50 ma coil current. Using 50 ohms as a current shunt would result in the net impedance being 150 ohms, the nominal current dropping to 33ma, and the voltage across the relay coil dropping from 5V to 3.3V. That might be low enough to be below the pull-in voltage of the relay, making the circuit you are testing non-functional during the current measurement (as well as giving you a reading only 2/3 that of the current in normal operation). Another problem is that the current through the shunt will heat up shunt resistor, and if high enough, damage or destroy the shunt resistor. If you check the manual for your oscilloscope, you will find it lists a maximum signal input voltage when using the 50 ohm termination mode. For one common scope, the limit is 5V RMS, corresponding to a half watt power rating (100 ma current rating) on the 50 ohm termination resistor. You really don't want an expensive scope repair because you misjudged the current on a circuit you are measuring! This by itself is a good reason not to use the scope's internal termination as a current sense resistor in power circuits.

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