Need a simple voltage monitor? The TL431 amplified zener is a three terminal solution. Think of it as a 2.5V reference connected to an opamp. The first circuit turns on a LED anytime the voltage is over the calculated value. The TL431 starts conducting anytime the voltage at the resistive divider exceeds 2.5 volts. Take the case of wanting to switch at 12.5V. If R1 is 15K, R2 will be four times that or 60K (circuit A). Using a pot is more convenient but it can tale a lot of testing to set the voltage. A handy trick is to temporarily move
the sense resistor to point X. You can measure the voltage at point X and adjust the potto the voltage you want it to trip at. Then just move the sense resistor back to the voltage to be monitored. The data sheet will have examples of these circuits and calculations.
The LED current limiting resistor can be replaced with a sensitive relay to control (circuit B). The TL431can handle up to 100ma. A typical small 12V cube relay is about 400 ohms or 30ma. Problem is the difference between on and off is less than 5mV. That can lead to a lot of relay chatter as the voltage rises when the load is removed. Powering the relay off the switched contact prevents
the relay from engaging again. A second TL431 relay circuit could be used to engage the relayagain when the voltage reaches normal again.
Another solution would be to add positive feedback from the relay switched contacts (circuit C). Because resistors greater than a meg are less common. I fed the feedback into a resistor in the lower leg of the voltage divider. If you have 3- 8 meg ohm resistors, you could feed that directly into the input. All this can be calculated and you will ge a lot of strange value resistors. Most of us build from what is in the parts box, thus all the series parallel resistors. If looking for a fun winter project, subbing resistors and testing can consume many hours. Just one of the reasons I prefer using a micro when something has to be done in a hurry.
The feedback circuit has the following results as built. Pot turned to the highest position, dropout 11.80 volts and pull in 12.56 volts. Pot turned to the lowest position, dropout 12.99 volts and pull in 13.91 volts.
The TL431 will never conduct lower than 2.5 volts and a LED will drop about 1.7 volts in series. That will reduce the voltage about 4.2 volts. with a 14V supply that leaves just 9.8V on the relay. Most small 12V relays will easily operate on 70% of rated voltage for pull in. Drop out is usually below 25%. The TL431 should not be used when the voltage exceeds 36V. A 24V relay and 25-32V zener in series can be used to drop the voltage in 48V systems. Use a 10K resistor to insure enough load on the zener.
