Current Actuated Switch (CAS)

Our Two-Terminal CAS is used to activate another process in response to a customer specified current (up to 100 amps). We use back-to-back MOSFET's mounted inside the sensor as the switching element. When the AC current being monitored reaches or exceeds the switch setting, these MOSFET's will conduct (within 50mS) to activate another AC or DC control circuit connected to that switch. No power is drawn from the controlled circuit to operate this device. There is no "leakage" current in the off state. Continious current through closed output contacts can be up to 1.2 amps (larger amounts for short periods). Sensitivity of the switch can be enhanced by passing multiple turns through the current sensing hole.

The AC or DC control circuit connected to the CAS will remain on at any moderate current above the switch setting (with no damage to the switch), and then turn off as the current falls below 10% of the turn on setting.

If the CAS is set to turn on at 50 amps it will stay on until the current drops back to 45 amps or less, at 100 amps it will turn off at 90 amps. This hysteresis is required to prevent chattering between the on and off state. Standard SRT policy on all of our CAS's is to use a hysteresis of 10%, but this can be changed higher or lower to accommodate customer requirements.

SRT offers two Current Actuated Switches packaged in our smaller, more popular "C" case. Similar to our Two-Terminal CAS above, but with no special calibrations for setting the switch point. Instead they are fixed to switch at either 3 amps or 8 amps. However, sensitivity may be enhanced by passing multiple turns through the current sensing hole so the switch can close with as little as 100mA.

If you want the 3 amp CAS to activate at 1.5 amps, take the wire being monitored and wrap it through the sensor hole 2 times (i.e drawing at left). To switch at 0.5 amps wrap 6 windings and so on.

We also manufacture a C-case CAS which uses a darlington transistor (open collector) as the switch element for the switching of DC control circuits. The output contacts can handle up to 60 volts peak DC at 300 mA peak.

The flux surrounding the wire being monitored supplies the power to all of our current actuated switches. They have consistent switching repeatability and are easy to retrofit into an existing system, or they can be implemented into a variety of new systems.

Applications can include:

We also have downloadable pdf files describing in more detail the CAS internals and how they could be installed for a varity of useful applications.

Engineers at SRT are available to discuss any questions you might have on any of our products. Often we are asked to customize or prepare special calibrations on our sensors to suit customer needs. Please contact us at (800) 447-6805 and ask for Dayle Smith or Doug McNutt, or FAX us at (719) 634-2601, or send E-mail to

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