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News:

Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.

 

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A Simple Current Limiter

 

There are many applications where some form of current limiting is needed. There are a variety of ways to do this with circuitry. For example, here are two common methods.

http://www.radio-electronics.com/info/circuits/transistor_current_limiter/transistor_current_limiter.php

http://www.vidisonic.com/2008/07/10/current-limiting-circuit/

These are nice techniques, but if what you want is a simple, effective way to limit current without having to design a particular circuit implementation, here is a good way to do it.

Current Limit Macro



This macro uses an NTIOFV (Nonlinear Table I of V source) to limit the current to IMAX when the voltage across the device exceeds VMAX.

The default values are set to a common set for power systems.

The 1E15 ohm resistance serves as a convergence aide and can be eliminated if its fA of current leakage is a problem in your application. A high-valued resistance across a current source will often help convergence.

Here is a test circuit to be run in DC analysis to demonstrate the transfer function of the device.

Current Limit Test Circuit
Here is the DC analysis showing the current limiting transfer function. As you can see, the device clips currents in both positive and negative directions to +-IMAX.


DC Analysis of Current Limit Test Circuit


Here is a circuit to demonstrate the use of the current limiter.

Test Circuit for Current Limiting


X1 is the current limiter macro and the area encircled in green is the circuitry whose input current is being limited. The bottom waveform in the transient analysis of this circuit shows the current through V3 and thus through the X1 limiter.

Transient Analyis Showing Current Limiting

 
Download Fall 2012 Circuit Files
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