This is an isolated DC current sense relay from .3-5A using ACS712-5 (5) Hall current sensor. The advantage is
these have a near zero voltage burden and can be placed in the negative or positive lead. Suitable for pump
monitoring and other loads. Higher currents with added shunt. Can cost less than $4 to build.
The first TL431 acts like a 5V zener regulator for the ACS712 chip. This could be replaced with a 4.7 or 5.1
zener if the voltage divider is adjusted slightly. A 7805 5V regulator is the preferred method since it will
lower the standby current and give the widest operating voltage range. The second TL431 starts conducting
between 2.5 and 3.5 volts, approximately 185mv per amp above 2.5V. Pot adjusts this additional voltage setpoint.
The TL431 is an ideal match for the ACS712 since its null output is 2.5V or half the supply input. Only a simple
voltage divider is needed to match the 1V range.An external shunt can be added for higher currents. There are
20 and 30 amp versions I prefer not to use because of the dicey connectors and board foil. Circuit has a
standby current of 20ma and the ON current of the relay. Capacitor prevents false trips. The 12V
relay should be at least 500 ohms resistance, preferably higher to keep less than 25mA. Relays wit only NO
contacts tend to have the highest resistance coils as strong springs are not needed
If a PNP transistor is added as a high side driver to the relay it would be easy to make this latch in an
overload situation. An opto isolator can be used for digital inputs. The ON voltage across the 431 only drops
to about 1.8V at saturation.
Parts List
(1) ACS712-5
(2) LM431 TL431 adjustable zener, 7805 alternate
(1) 12V relay 500 ohms or greater coil resistance
(1) red LED indicator
(1) 1-4.7uF, electrolytics to prevent false trips
(1) any general purpose diode, 1N4001 type
(1) 390 ohm resistor
(2) 2.2K resistor
(1) 47K ohm resistor
(1) 20K pot
