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.
* 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.
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.
If there is no control power for the circuit breaker, the load circuit relay will remain CLOSED allowing current to flow.
The Hall Effect type current sensor, IC 1, produces an output that is proportional to the AC or DC current flowing through load circuit.
The ACS712 Hall Effect current sensors used for this circuit can produce a positive or negative output voltage depending on the direction of current flow and can therefore be used for DC currents of either polarity.
Voltage Comparators IC 2A and IC 2B form a 'window' type voltage detector circuit. The output of one of the comparators will go LOW depending on whether the voltage at their input's is above or below the references set by the voltage divider formed by resistors R2, R3 and R4.
Voltage Comparator IC 2C is used as a basic comparator whose output goes HIGH when the voltage at its MINUS input is LOW. IC 2C will turn the green LED - D1, OFF when the circuit breaker is tripped. Comparator IC 2C is slaved to IC 2D.
LED - D1, also indicates that control power is applied to the circuit breaker.
Voltage Comparator IC 2D is used as a SET/RESET type Flip-Flop whose output goes LOW if the voltage at its PLUS input goes LOW. The output of the Flip-Flop will remain in a LOW state until reset by closing S1 or cycling the power OFF and ON.
IC 2D also activates the relay, RY 1, that opens the load circuit. When the relay is activated, the red LED - D2, will be ON, indicating that the circuit breaker is tripped.
Capacitor C3 ensures that the circuit breaker resets itself when control power is applied to the circuit.
Potentiometer R3 sets the trip current level for the circuit breaker. The setting range is 0.5 to 10 Amps AC and DC.
The circuit can also be built with a non adjustable trip current by replacing potentiometer R3 with a fixed value resistor. The value of the fixed resistor would be calculated to give the desired trip setting. ( Provison has been made on the circuitboard to mount a fixed value R3. )
The trip setting could also be made so asymetric using fixed resistors. For example; The forward trip current could be set at 5 amps while the reverse trip current point is set at 1 amp. This could be useful in battery powered or charging systems.
Push button switch S1 is used to RESET the circuit breaker after it has tripped.
An external switch can also be connected to the circuitboard at the holes marked 'A' and 'B'.
Capacitor, C4 allows the breaker to trip even if S1 is held closed during an overload condition.
Diodes D4 to D7, capacitors C5 and C6 and voltage regulator IC 3 form a 5 volt power supply that is needed by IC 1 and the rest of the control circuitry.
Two, parallel connected poles of an 8 amp mechanical relay are used to control the load circuit. Using a mechanical relay as the interrupt device means the circuit breaker causes no voltage loss in the load circuit.
The relay will not interfere with the signals from AC or DC based command control systems.
This circuit can be also 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.
DCC - This circuit is not designed for use with DCC systems due to spikes at the output of the current sensor IC when the current flow direction reverses. If used on a DCC system the trip setting should be increased by 10 to 15 percent to compensate for these spikes.
The trip current - voltage setting across R3 for DCC current is the same as that for a straight DC current.
A separate power supply should be used for DCC operation.
The circuit breaker's control circuitry needs a low voltage 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 diagralm 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 the relay is ON (Breaker is Tripped).
More than one circuit breaker can be operated from a single power supply if the supply has a capacity of 100 milliamps per breaker connected.
If there is no control power for the circuit breaker, the load circuit will remain closed.
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.
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.
The circuit breaker can be used for AC, Fullwave DC, Halfwave DC and Straight 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, Fullwave and Halfwave DC Amps is 1.414 times greater than the setting for Straight DC Amps. The difference in voltage levels is due to the PEAK voltages of sine waves as they apply to AC or DC circuits.
For Halfwave DC circuits an ammeter would indicate one half of the peak trip current setting. Again this is due to the nature of sine waves.
If the circuit breaker is used for straight DC, the maximum trip current of 10 Amps for this current should not be exceeded.
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.
Refer to the photograph above for the meter connection points.
Connect a low range DC voltmeter to the bottom lead of R3 (METER POS) and to the top lead of R4 (METER NEG).
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.
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.
Disconnect the voltmeter and press the RESET (S1) if the breaker tripped during adjustment.
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.
|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||-||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|
|2||-||R9, 11||-||470 OHM 1/4W||-||470QBK-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|
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.
23 January, 2020