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How to Measure Current with an Oscilloscope: A $10 Current Probe
The ability to measure current using an oscilloscope can be useful at times, for example when characterizing an unknown impedance or when designing current limiters. The only snag is, that an oscilloscope measures voltage versus time. So to measure a current with an oscilloscope, some kind of current probe is needed. Such probes are available commercially, but their prices typically start a bit shy of $1000 and extend well into the kilo-buck range, which places them well out of reach for the average hobbyist. Thankfully, there are other options.
Current Sensing Resistor
One possibility for current measurement is to add a little bit of resistance in series with the current under test. Using a 0.1 Ω resistor will result 0.1 V/A developed across the resistor. However, this works best if the current flows to ground. If the current sensing resistor cannot connect to ground, a differential measurement must be performed, either by measuring the voltage difference across the current sense resistor with two probes and calculating the difference, or by using a differential probe. Unfortunately, differential probes are also rather expensive with prices starting at around $1k and extending past $5k.
Current Sensing Transformers
A quick search on Mouser or Digikey reveals hundreds of transformers intended for current sensing applications. One attractive option is the Triad Magnetics CST-1030 (Mouser P/N: 553-CST-1030), which is available for less than $10. Another option is available from Sparkfun: Sparkfun SEN-11005.
According to Triad Magnetics, the CST-1030 exhibits a secondary voltage of 0.0977 V/A when loaded with a 100 Ω resistor. Should you wish for a more convenient 0.1 V/A conversion factor, use 102 Ω. A 250 mW, ±1% tolerance metal film resistor will work well here. Or, should you, like I, have a full EIA E-12 ±1% resistor kit available, you will find that the parallel combination of 120 Ω and 680 Ω will result in 102 Ω as well.
The CST-1030 will sense currents up to 30 A RMS. I fitted mine with a BNC connector for easy connection to test equipment.
The current transformer is easy to use. Simply feed a wire with the current that needs to be measured through the hole in the transformer, and measure the output of the transformer. Although, this transformer is intended for use at power line frequency (50-60 Hz), it is surprisingly wideband.
As the CST-1030 is a transformer, not a Hall-effect sensor, it naturally cannot measure a DC current. However, as the measurement below shows, it does come quite close.
I conducted this measurement by using an LM3886-based audio amplifier and an Audio Precision APx525 to provide a sinusoidal voltage into a 4 Ω resistive load. For each point in the measurement, the output amplitude of the amplifier was adjusted to ensure the desired test current flowed in the load. The load current was measured using the CST-1030 simply by feeding the Speaker+ output wire through the transformer.
The bandwidth of the CST-1030 is wider than the measurement capability of the APx525, so to find the -3 dB points, I used a function generator and an HP 34401A multimeter. At a test current of 250 mA, the CST-1030 showed a bandwidth of 3.2 Hz – 330 kHz, which is certainly plenty for measurements within the audio band.
Increasing the Sensitivity
The only drawback of the current sensing transformer appears to be that it requires a somewhat substantial current in order to work. As seen in the measurement, the bandwidth of the current probe does start to suffer once the measured current is decreased to 100 mA. This can be mitigated to some extent by increasing the number of turns of the primary winding as illustrated below.
With five turns on the primary (as shown above), the sensitivity of the current transformer is increased fivefold, thus, the transformer delivers 0.5 V/A. As shown in the measurement below, this increases the bandwidth of the transformer at lower frequencies when measuring lower currents.
Measuring Current with an Oscilloscope
I built this current probe for measuring the inrush current of a toroidal power transformer. I simply allowed one of the primary wires to go through the hole in the current sensing transformer and measured the voltage at the output of the transformer with an oscilloscope. The result is seen below.
As the current probe provides 0.1 V/A, the vertical scale in above measurement is 50 A/division.
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