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Wednesday, 11 March 2009

Fast Acting Electronic Circuit Breakers

Fast Acting Electronic Circuit Breakers

The circuit breakers on this page make use of the Zetex - ZXCT1009 "High Side Current Monitor' integrated circuit. The circuits have variable trip current settings and are designed for low current applications.

Zetex - ZXCT1009 "High Side Current Monitor'

The current sensing IC is placed in the supply side of the circuit ahead of the isolation device. This allows the common line of the circuit to remain free of voltage dropping resistances.

For information on the operation of the ZXCT1009 current monitor itself, please refer to the manufacturers data sheet at - ZETEX - ZXCT1009 Data Sheet (PDF)


Two circuit breaker designs are shown; The first has its power isolation device on the Low or common side of the circuit while the second has it isolation device on the High or supply side of the circuit. Both designs use MOSFET devices to isolate the suppiy from the load.

The overload detection and trip system is completely electronic and therefore has a very fast response time (1/120th of a second or less for a full wave DC supply). This allows the load to be disconnected almost instantaneously when an over current condition is detected.



- Low Side Isolation Circuit Breaker -

This circuit has its isolation device, Q1, on the Low or common side of the circuit. The disadvantage of this method is that the breaker cannot easily be used in typical circuit configurations that share a low side or negative common.

The method is simple to implement however and a wide range of output devices are available. The transistor used to test this circuit was a IRL520 MOSFET


Low Side Circuit Breaker

Circuit Controls and Indicators

  • S1 resets the circuit after and overload trip has occurred.

  • ROUT - ADJ adjusts the tripping current level.

  • LED - D2 will turn OFF when an overload has occurred and the circuit has tripped.





- High Side Isolation Circuit Breaker -

This circuit has its isolation device, IC 3, on the High or Supply side of the circuit. This allows the circuit breaker to be more easily used in typical circuit configurations that share a low side or negative common.

IC 3 is a PROFET® BTS409L1 - 'Smart High Side power Switch' from INFINEON.


High Side Circuit Breaker

Circuit Controls and Indicators

  • S1 resets the circuit after and overload trip has occurred.

  • ROUT - ADJ adjusts the tripping current level.

  • LED - D2 will turn OFF when an overload has occurred and the circuit has tripped.

LED - D3 is optional and is connected to the internal diagnostics system of the BTS409L1 and will turn ON to indicate various fault conditions. These are listed in the BTS409L1 data sheet.



Very - Basic Operation Of Both Circuit Breakers

  1. As the current through RSENSE increases the voltage across resistors ROUT - MAX and ROUT - ADJ increases proportionally.

  2. When the voltage at the MINUS input of IC 2A rises above the voltage at the PLUS input its OUTPUT will go LOW.

  3. When the OUTPUT of IC 2A goes LOW it causes the Flip-Flop formed IC 2B to change states, producing a LOW condition at it's OUTPUT.

  4. Depending on the circuit used, a LOW at the output of IC 2B causes the Gate of Q1 or the Input Terminal of IC 3 to go LOW. This will cause Q1 or IC 3 to turn OFF and open the circuit.



Circuit Breaker Page Notes

  • The maximum Input to Output voltage of the ZXCT1009 is 20 Volts. There are ways to increase this voltage but these are not shown on this page.

  • With the component values shown the calculated adjustable trip current range is between 0.23 and 3 amps.

  • A heat sink may be required for transistor Q1 or IC 3 at higher current levels.

  • Capacitor C3 provides a short delay time so that the circuit automatically resets itself when power is applied.

  • Capacitor C4 prevents the circuit's protection from being disabled if S1 is held closed. This means that when S1 is held closed the circuit will reset and then will quickly trip again if an overload is present. If C4 is not used the circuit breaker cannot trip as long as S1 is held closed. This may be desirable in certain situations and might be considered as an option.

  • Both Q1 and IC 3 have internal diodes to protect them from being reversed biased. This could happen if the power transformer's fuse failed while the breaker was connected to a circuit with a large filter capacitor. In this case reverse current would be shunted around the isolation devices and into the circuit breaker's power supply until the circuit had been discharged.

  • Diode D1 and capacitor C1 provide a short term sub-power supply for the control circuitry in case there is a drop in the voltage of the main supplies voltage during a dead short at the output of the breaker.

  • For more information on voltage comparators please refer to this page; Voltage Comparator Information And Circuits.

  • There are other devices from Zetex and other manufactures that could also be used in this type of circuit.

  • The ZXCT1009 used has a SOT-23 surface mount style case that is quite small. To aid in using this device a "PROTO-BOARD ADAPTER FOR SOT-23-5" listed as DigiKey part number 33205CA-ND. is recommended. (The distance between the pins is 0.100in or 2.54mm.)

PROTO-BOARD ADAPTER FOR SOT-23



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