Relay trigger when voltage above 13v - Page 1
Forum section: electronics / general

author: bigjoncoop
author_rank: Regular Contributor
timestamp: April 29, 2024, 12:07:41 am
content: 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 - 14.6vTrigger Voltage: ≥13vOutput voltage to control relay: Not Important

author: Andy Chee
author_rank: Super Contributor
timestamp: April 29, 2024, 01:46:58 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: April 29, 2024, 02:44:07 am
content: 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.

author: SmallCog
author_rank: Frequent Contributor
timestamp: April 29, 2024, 02:51:17 am
content: Sounds, in function, similar to what a traditional dual battery controller would do.These are sometimes referred to as voltage sensing relays or VSR'sEven 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 otherseg:https://www.siliconchip.com.au/Issue/2019/July/Dual+Battery+Isolator+for+4WD%2C+RVs%2C+Caravans%2C+etcor this:https://www.siliconchip.com.au/Issue/2020/December/Dual+Battery+LifesaverSomeone may be able to lend you a copy of their issue of this magazine if required :-)

author: ledtester
author_rank: Super Contributor
timestamp: April 29, 2024, 04:37:36 am
content: Here is TI's app note on using the TL431 as a comparator:Using the TL431 for Undervoltage and Overvoltage Detectionhttps://www.ti.com/lit/an/slva987a/slva987a.pdfYou 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 Designhttps://www.ti.com/lit/ug/tidu020a/tidu020a.pdf

author: dietert1
author_rank: Super Contributor
timestamp: April 29, 2024, 06:24:10 am
content: Quote from: ledtester on April 29, 2024, 04:37:36 amI 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

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2024, 08:24:56 am
content: 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 29, 2024, 12:03:30 pm
content: Use a supply voltage monitoring chip, it will already have many features you want such as hysteresis and deglitching.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 29, 2024, 04:44:47 pm
content: Quote from: Zero999 on April 29, 2024, 08:24:56 amThe LM386 is basically a power op-amp, and would be fine to use for this, though the quiescent current might not be optimal. It'll certainly be better used as this than as an audio amplifier; it's horrible at that

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2024, 09:40:57 pm
content: Quote from: AVGresponding on April 29, 2024, 04:44:47 pmI don't think so. I would be difficult to use the LM386 as a comparator. The input voltage is limited to +/-0.4V, it's internally biased, so its output sits at half the supply voltage and the DC gain is only 20. No doubt it can be hacked into a comparator circuit, but it's much easier to just use the LM393.https://www.ti.com/lit/ds/symlink/lm386.pdfQuote from: Ian.M on April 29, 2024, 02:44:07 amLike this.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 30, 2024, 05:26:19 am
content: Quote from: Zero999 on April 29, 2024, 09:40:57 pmThe OP mentioned wanting to use an LM386 because they have lots of them. I always try to answer the question being asked, and not ignore it.The output biasing is trivially easy to incorporate as part of the design. The input limitations require more thought, the voltage divider values can't be as arbitrary as you could normally get away with. Aside from that, it's perfectly feasible, if not optimal.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2024, 07:01:30 am
content: Quote from: AVGresponding on April 30, 2024, 05:26:19 amUsing a part, just because you have loads of them, is normally a bad idea.QuoteAlright then, post a schematic. Show me how you would do it.