The circuit on this page is for a fast¹ acting electronic circuit breaker for use with fixed and variable, low voltage AC and DC systems.

  The circuit breaker is meant for use with high current AC or DC model railroad throttles and control systems used by the larger scales. It will also work for other high current, low voltage AC or DC applications.

  For model railroad use the primary function of this circuit breaker is to prevent damage to locomotives and equipment by quickly removing the power to the track during short circuits that are often caused by derailments.

  The circuit uses an Allegro® Microsystems, Inc. - ACS712ELCTR-20A-T, Hall Effect, Linear Current Sensor devices to measure the current flow and provide electrical isolation between the load and the control circuits.

  The circuit breaker's trip setting range is adjustable from: 0.5 to 10 amps². The maximum load voltage for this circuit is 24 volts AC or DC.

  1 - According to the datasheet for the relay used in this circuit, longest expected time to open the contacts is 15 milliseconds which translates to about 1/66^thof a second. The electronic sensing and control portion of this circuit breaker is almost instantaneous.

  2 - This circuit can be made with a 5 amp maximum current rating by using an ACS712ELCTR-5A-T current sensor and changing the values of R2 and R4 to 4,420 ohms.

  The Hall Effect current sensor IC is surface mounted on the copper side of the circuitboard. Its position is indicated by the 'X' in the photograph.

  Potentiometer R3 sets the trip current level. Push button S1 resets the breaker after it has tripped.

  The holes marked 'A' and 'B' can be used to connect and external RESET switch if needed.

  The next photograph shows the current sensor IC mounted on the copper side of the circuitboard. (The image has been flipped so that it agrees with the picture of the full circuitboard.)

  Also shown is two methods of bolstering the copper on the circuitboard for high current applications. The section of copper on the left has been built-up with extra solder. The sections of copper on the right have been supplemented with short sections of wire soldered to the terminals and IC pins.

1.   A Hall Effect type current sensor, IC 1, produces an output that is directly proportional to the PEAK AC or DC current flowing through the IC.

     The ACS712 Hall Effect current sensor used for this circuit can produce a positive or negative output voltage depending on the direction of current flow and can be used for DC or AC currents as both current flow directions are measured.

     The ACS712 Hall Effect current sensors produce output voltages that indicate PEAK current flow conditions. Therefore, setting the trip point will depend on whether the current being measured is AC, Fullwave DC or Straight/Regulated DC. See the Setting The Trip Current section of this page for more information.

2.   Voltage Comparators, IC 2A and B form a 'window' type detector circuit. The output of one of the comparators will go LOW if the the voltage at their input's is above or below voltages that are determined by the voltage divider formed by resistors R2, R3 and R4.

3.   Capacitor, C3 ensures that the circuit breaker resets when power is applied to the circuit.

4.   Switch, S1 is used to RESET the circuit breaker after it has been tripped. An external switch can also be connected to the circuitboard via the holes on the circuitboard labeled 'A' and 'B'.

5.   Diodes D1 to D4, capacitors C4 and C5 and voltage regulator VR 1 form a 5 volt power supply that is needed for IC 1 and the rest of the control circuitry.

6.   Two poles of an 8 amp (16 amps combined), 5 volt mechanical relay are used to control the load circuit, allowing the circuit breaker to operate without voltage loss and without regard for the polarity of the load.

7.   Transistors Q1 and Q2 form a crude, hybrid SCR that has two gates. (Similar Circuit)

     A LOW input at the base of Q1 will trigger the hybrid SCR which will activate the relay RY 1 and OPEN the load side of the circuit.

     A LOW (ground) input at the base of Q2 will turn the SCR - OFF and release relay RY 1. The relay will then CLOSE the load side of the circuit.

8.   If there is no control power for the circuit breaker, Relay RY 1 will remain CLOSED, allowing current to flow in the load circuit.

9.   The relay will not interfere with the signals from AC or DC command control systems.

10.   Green LED - D7 indicates that control power is applied to the circuit breaker.

11.   Red LED - D5 indicates that the circuit breaker has tripped.

12.   This circuit has not been designed for use with DCC systems but it should work if capacitor C2 is removed from the circuit.

•   The circuit breaker's control circuitry needs an AC or DC power supply. This power can come from the load circuit in a fixed voltage system or from a separate supply, such as a plug-in type transformer, for a variable voltage load. See the diagram below for examples.

•   The voltage of the supply for the control circuitry must not exceed 16 Volts AC or 24 Volts DC. The current requirement for the breaker circuit is 100 milliamps when it is tripped (the relay is ON).

•   More than one circuit breaker can be operated from a single power supply if the supply has a capacity of 100 milliamps per breaker.

•   If there is no power to the circuit breaker, the load circuit will remain closed.

  NOTE: The terminal labeled N on the schematic and control power diagram is not connected to the circuit breaker itself. The N terminal does not need to be connected for the breaker to work but can be used as a junction point in the return side of the load circuit if needed.

  The trip current setting of the circuit breaker is controlled by potentiometer, R3. The voltage across R3 sets the forward and reverse current trip points for the circuit breaker.

  Resistors R2 and R4 and R3 form an adjustable voltage divider with two output levels.

  A voltmeter connected across R3 at the leads of R2 and R4 is used to adjust the trip current setting based on information in the table on the following diagram.

  As the resistance of R3 is increased, the voltage difference between the PLUS input of IC 2A and the MINUS input of IC 2B increases. This increases the trip current level of the circuit in both the forward and reverse directions of flow.

•   The circuit breaker can be used for AC, Fullwave DC and filtered DC circuits but different voltages across potentiometer R3 are needed to set the trip point for straight DC compared to AC or Fullwave DC.

•   The voltage setting needed for AC RMS Amps and Fullwave DC Amps is 1.414 times greater than the setting for Straight DC Amps. The difference in voltage levels is due to the nature of sine waves as they apply to AC circuits.

  If the circuit breaker is used for straight DC, the maximum trip current of 10 amps for this current should not be exceeded.

  The diagram and its table give the calculated trip current for a particular voltage across R3.

  The photograph indicates where to connect a voltmeter to adjust the trip current's voltage setting for the circuit breaker.

  Resistors R2 and R4 are specified as 1% tolerance as this minimizes any difference between the forward and reverse current flow trip points. Matched 5% resistors could also be used for R3 and R4 but there is a better selection of fixed values available for 1% resistors.

  Refer to the photograph above for the meter connection points.

1.   Connect a low range DC voltmeter to the right lead of R3 (METER +) and the top lead of R4 (METER -).

2.   Apply the 'Control Power' to the circuit breaker.

     If the load's power supply is being used to provide 'Control Power' to the circuit breaker, the load current must be less than the desired trip level during adjustment. Or, the load's power supply does not have to be ON if a separate power supply is being used for control power to the breaker.

3.   Adjust potentiometer R3 until the meter reads the voltage that corresponds to the desired trip current level for AC, Fullwave DC or Straight DC as is appropriate in the table above.

4.   Disconnect the voltmeter and press the RESET (S1) if the breaker tripped during adjustment.

  The RESET button must not be held closed if the circuit breaker trips. Holding the RESET closed will cause the relay to chatter if an overload condition is still present.

  The RESET switch does not override the trip circuitry and the relay could be damaged if it is allowed to chatter.

  If there is an overload, the fault should be cleared before resetting the circuit breaker.

  See next section for an Anti-Chatter reset circuit.

  To prevent the relay from 'chattering' if an overload ocurrs while the EXTERNAL RESET switch (S2) is closed, capacitor (C6) and resistor (R10) are added to the circuit.

  If the breaker trips while the EXTERNAL RESET switch is closed, C6 will quickly charge and block the flow of current though S2 and prevent the circuit from being reset.

  When S2 is open C6 will discharge through R10. The breaker can again be RESET in about 5 seconds.

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Circuitboards And Parts Prices

OPTION 1 - Circuitboards Only

  1 - Circuit Breaker circuitboard without parts is 12.00 dollars US., plus postage.

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OPTION 2 - Circuitboard With The Current Sensor Installed

  1 - Circuit Breaker circuitboard with only the ACS712 current sensor installed is 18.00 dollars US., plus postage.

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OPTION 3 - Circuitboard With The Current Sensor Installed and Kit with all parts

  1 - Kit - Circuit Breaker circuitboard with the ACS712 current sensor installed and with all other parts is 30.00 dollars US., plus postage.

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OPTION 4 - Assembled and Tested Circuitboard

  1 - Assembled - Circuit Breaker circuitboard is 33.00 dollars US., plus postage.

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 If you are interested in a printed circuitboard, please send an email to this address: rpaisley4@cogeco.ca

  Your message will be answered as soon as possible.

  Circuit Breaker circuitboards sold:  14

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

10 October, 2009