How to Select a Current Monitor

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1Obtain a copy of the specification sheet that lists the standard current monitors. These monitors are able to measure pulses, transients and continuous signals. They do not measure steady-state dc current.

 

2The first four columns of the specification sheet give model number, output sensitivity and physical dimension information. The next four columns list the specs that apply to measuring pulses, and are referred to as time domain parameters. The final four columns list the specs that apply to measuring continuous sine-wave currents and are referred to as frequency domain parameters.

 

3Determine if the application is to measure a pulse or continuous signal. If the signal is more complex, and rms, peak, dc or other limiting values are hard to determine, refer to Pearson Electronics Application Notes, or call our engineering department.

  Pulse Signals
  • For a single pulse, estimate the maximum peak current (amps) and the maximum pulse length (seconds).
  • Multiply these together to obtain the current-time product (amp-seconds). Choose a model with adequate peak current and current-time product to cover the requirement of the pulse to be viewed.
  • For repetitive pulses, consult our app Notes and be sure that average dc level (zero frequency component) does not exceed specifications.
  • The rise-time of the pulse to be viewed should be longer than the rating of the current monitor to avoid excessive overshoot and ringing.
  • For a rectangular pulse the deviation from a perfectly flat top is given by the droop rate. Multiplying the pulse length by the droop rate will yield the percentage deviation from the flat-top value at the end of the pulse.
 

Continuous Signals

  • Determine the approximate maximum sine-wave amplitude, I (amps), and the approximate minimum frequency, f (Hz).
  • Compute I/f (amps/Hz). To use a given model, this should not exceed the spec sheet value.
  • Compute the maximum rms current. To use a given model, this should not exceed the spec sheet value.
  • Determine if the frequency over which the monitor is to be used is within the range of the low and high 3 dB points. The monitor will lose its accuracy outside of this range.

  

4If there are several monitors that satisfy the above criteria, a selection based on voltage output (sensitivity) and size can now be made. In the third column is given the output volts per primary amp to be measured. For example, a 1 milliamp signal will produce a 1 millivolt output for a monitor that has a 1 volt/amp output.

 

5An often asked question concerns terminating the output of the monitor with a resistor. Since the monitor can be modeled as a voltage source in series with 50 ohms, the addition of an external terminating resistance will decrease the output of the unit. For example, a 50 ohm external termination would reduce the output to one-half.

 

 

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