Relay trigger when voltage above 13v

[bigjoncoop]

Trying to create a simple circuit that will trigger a relay only when the monitored voltage is above 13v.

(the output voltage of the circuit is not important. Just needs to be 0v when the monitored voltage is below 13v)



I was able to find a couple of threads on this, but there was no clear answer in the end...



I was originally thinking of using a LM386 and a TL431 as a voltage reference since I have alot of these components. But was unable to find a detailed schematic on how to do so...

I do have a couple of low voltage disconnect modules I could use if all else fails connect a Buck converter on the output to trigger a 5 volt relay..

Specs:

Monitored Voltage Range: 12v - 14.6v

Trigger Voltage: ≥13v

Output voltage to control relay: Not Important

[Andy Chee]

I think you’re after a hysteresis circuit i.e. turn on relay as voltage rises above 13V, turn off relay as voltage drops below 12V.

Alternatively, perhaps you’re after a monostable “reset” circuit i.e. turn on relay as voltage rises above 13V, relay remains on until power is removed.

[Ian.M]

TL431, driving the base of a PNP transistor to switch the supply to the relay.  You'll need appropriate base resistors, an anti-parallel diode across the relay coil, and possibly a high value feedback resistor from the collector of the PNP to the TL431 REF pin to add a little hysteresis.  Make the feedback resistor much lower, and once triggered it will latch on, till you momentarily either remove the supply, or ground the REF pin.

[SmallCog]

Sounds, in function, similar to what a traditional dual battery controller would do.

These are sometimes referred to as voltage sensing relays or VSR's

Even if your project is not for this purpose, you may find some useful information in magazine articles for similar projects - it's certainly been a topic covered in our local magazines here and likely will have in others

eg: https://www.siliconchip.com.au/Issue/2019/July/Dual+Battery+Isolator+for+4WD%2C+RVs%2C+Caravans%2C+etc

or this: https://www.siliconchip.com.au/Issue/2020/December/Dual+Battery+Lifesaver

Someone may be able to lend you a copy of their issue of this magazine if required :-)

[ledtester]

Here is TI's app note on using the TL431 as a comparator:

Using the TL431 for Undervoltage and Overvoltage Detection
https://www.ti.com/lit/an/slva987a/slva987a.pdf

You might find it easier to use a real comparator - eg. a LM339 / 393 / etc.

Here is TI's app note on designing a comparator circuit with hysteresis:

Comparator with Hysteresis Reference Design
https://www.ti.com/lit/ug/tidu020a/tidu020a.pdf

[dietert1]

Here is TI's app note on using the TL431 as a comparator:

Using the TL431 for Undervoltage and Overvoltage Detection
https://www.ti.com/lit/an/slva987a/slva987a.pdf
..

I would replace the bipolar transistor of Figure 6 in that app note by a p-channel mosfet in order get rid of the relay. The voltage sense divider causes a small current drain even below threshold.

Regards, Dieter

[Zero999]

The LM386 is an audio amplifier IC. I think you meant the LM393.

Plenty of good ideas have been given above. If you want an off the shelf solution, then you could use a voltage monitoring relay.