Adjustable - Fast Circuit Breaker

(0.5 to 10 Amps AC or DC Range)

  The circuit on this page is for a fast acting, electromechanical circuit breaker for use with low voltage AC and DC systems. The protected voltage source can be fixed or variable.

  The circuit breaker was designed to protect high current AC and DC model railroad throttles and control systems typically used by large scale trains but it will also work for other high current, low voltage applications.

  The trip setting is adjustable from 0.5 to 10 Amps AC or DC.

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

Circuit Breaker Features

  1. Fast operation - Approximately 1/60th* of a second.
  2. Adjustable trip current setting - 0.5 to 10 Amps AC and DC.
  3. No voltage loss in the load circuit.
  4. Power and Tripped indicator LEDs.
  5. Low power consumption when reset.
  6. Variable, Fixed and Asymetric trip current options are possible.

*  According to the datasheet for the relay used for this circuit breaker, the longest expected time to open the contacts is 15 milliseconds or about 1/66 th of a second. The electronic sensing and control portion of the circuit breaker is instantaneous.

Circuit Breaker Limitations

  1. May need a separate power supply for variable voltage systems.
  2. Load circuit will close if the control power is removed.

  This circuit is not designed for use with DCC systems but will work if the trip current setting is increased by 10 to 15 percent when compared to DC systems.

Fast Circuit Breaker Notes

Fast Circuit Breaker Schematic (2010)

Control Power For The Circuit Breaker

  NOTE: The terminal labeled N on the diagrams is not connected to the breaker's circuitry and does not need to be connected for the breaker to work. The N terminal can be used as a junction point in the return side of the load circuit if needed.


Trip Current

  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 R3 and R4 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 data in the table of 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.

AC Vs. DC Trip Settings

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

Trip Level Voltage Across R3

  The table in the next diagram gives the trip current for a given voltage across R3.

  Resistors R2 and R4 are 1% tolerance to minimize any difference between the forward and reverse current flow trip points. Matched - 5% resistors could be used for R2 and R4 but there is a greater selection of values available for 1% resistors.

Trip Current Setting Procedure

  Refer to the photograph above for the meter connection points.

  1.   Connect a low range DC voltmeter to the bottom lead of R3 (METER POS) and to the top lead of R4 (METER NEG).

  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 current flow should be less than the desired trip level during adjustment.

      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 Straight DC or for AC and Fullwave DC as is appropriate in the table above.

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

Current Sensor Mounting

  The next photgraph shows the ACS712 current sensor IC mounted on the copper side of the circuitboard. (The image has been flipped so that position of the IC it agrees with the full picture of the circuitboard.)

  Also shown is a method of bolstering the copper of the circuitboard for the high currents the circuit breaker might have to carry. The copper has been suplemented with wire soldered to the terminals and IC pins.

Fast Circuit Breaker Parts List

Qty   Circuit Part Number   Part Description   Digi-Key Number
1 - IC 1 - Hall Effect Sensor - 620-1190-1-ND
2 - IC 2 - Quad Comparator - LM339NFS-ND
1 - IC 3 - 5 Volt Regulator - TO-220 - LM7805CT-ND
1 - Q1 - 2N3906 - 2N3906FS-ND
1 - D1 - Green LED 3mm - 67-1396-ND
1 - D2 - Red LED 3mm - 67-1402-ND
6 - D3, 4, 5, 6, 7 - 1N4001 - 1N4001FSCT-ND
- - - - - - -
3 - R1, 5, 7, 8 - 10K 1/4W - 1.0MQBK-ND
2 - R2, 4 - 3.74K 1/4W - 1% - 3.74KXBK-ND
1 - R3 - 10K Potentiometer - 3352T-103LF-ND
1 - R6 - 3.3K 1/4W - 3.3KQBK-ND
2 - R9, 11 - 470 OHM 1/4W - 470QBK-ND
1 - R10 - 100K 1/4W - 100KQBK-ND
- - - - - - -
1 - C1 - 0.1uF - 495-1147-ND
1 - C2 - 0.001uF - 495-1132-ND
1 - C3 - 2.2uF/50V - P5175-ND
1 - C4 - 10uF/50V - P 5178-ND
1 - C5 - 1.0uF/50V - P5174-ND
1 - C5 - 330uF/35V - P5167-ND
- - - - - - -
1 - S1 - N.O. Push Button Switch - SW400-ND
1 - RY 1 - DPDT Relay - 8 Amp - PB295-ND
1 - Heat Sink - Heat Sink TO-220 .375"   HS106-ND
1 - Terminal Block - 2 Position, 3.5mm. - ED2635-ND
1 - Terminal Block - 3 Position, 5.0mm. - ED1602-ND

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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  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.

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23 January, 2020