Power the load only when car supply voltage is above 14 volts - Page 1
Forum section: electronics / projects

author: jancelot
author_rank: Regular Contributor
timestamp: February 19, 2017, 03:04:29 pm
content: I want to connect to my car a portable fridge/cooler/refrigerator/pizza heated delivery bag. But only when the car is running, when the voltage on the cigarette lighter receptacle is about 14.21 volts, so limit below 14 volts or so.This heated grips for motorcycles do it automatically below 12.5 volts, see at 4min 33s:Any commercial product available or easy solution? The limiter should be between the cigarette lighter and the load.

author: Ian.M
author_rank: Super Contributor
timestamp: February 19, 2017, 03:20:38 pm
content: You want a voltage sensitive relay - commonly available from boating and caravanning suppliers. You could hack a Velleman MK138 thermostat kit, replacing its sensor with a resistor to +12V so it acts as a voltage comparator, but its output relay is only rated for 3A.

author: Refrigerator
author_rank: Super Contributor
timestamp: February 19, 2017, 03:35:04 pm
content: Four resistors, a TL431 and a P-MOS is all you need to do the trick.I can doodle a schematic if you want.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2017, 05:09:48 pm
content: Quote from: Refrigerator on February 19, 2017, 03:35:04 pmI'll save you the bother.Note to original poster: you need some hysteresis, otherwise it'll oscillate. Voltage > 14V, load turns on, causing the voltage to immediately fall below 14V, causing the load to turn off, voltage rises to above 14V, load turns on and the cycle repeats indefinitely. R3 provides the hysteresis by feeding some of the output voltage, back to the potential divider.

author: jancelot
author_rank: Regular Contributor
timestamp: February 19, 2017, 05:30:41 pm
content: Quote from: Hero999 on February 19, 2017, 05:09:48 pmOk but what happens when I power off the car, will automatically disconnect the load? My battery voltage is 12.8v maximum with car off. A portable refrigerator like the Mobicool MB25 or Mobicool S28 use a peltier cell 3A@12V.I wish there were a commercial product already available at sites like banggood, ebay, dealextreme, amazon...

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2017, 06:18:29 pm
content: Of course it will disconnect the load, when the voltage falls below the set point.Most cars disconnect the cigarette lighter socket, when the power is turned off, so 1mA or so of current used in standby shouldn't be a problem.If you want lower power consumption, change the components to the following:U1 = ATL431R1 = 680kR2 = 150kR3 = 10MR4 = 220kThe standby current will now be around 70µA, when the battery sits idle.

author: Ton
author_rank: Regular Contributor
timestamp: February 19, 2017, 06:59:32 pm
content: I think the correct search term is Battery Guard or Battery Protector.see what i found on amazonhttps://www.amazon.co.uk/12v-Battery-Discharge-Protector-Guard-x/dp/B004AUJ002/ref=sr_1_fkmr1_1?ie=UTF8&qid=1487530257&sr=8-1-fkmr1&keywords=12V+Battery+guard+saver+carI have never used this product nor do I know the company behind, but on the surface it looks decent - might even be same circuit with a trim pot addedBR ton :-)

author: jancelot
author_rank: Regular Contributor
timestamp: February 19, 2017, 07:07:22 pm
content: Quote from: Hero999 on February 19, 2017, 06:18:29 pmMy car is a volkswagen and I can use the cigarette lighter even with car engine off and doors locked. In fact I have an Anker powerbank of 3350 mAh and I leave it charging overnight a lot of times.How to I set the breaking point?Quote from: Ton on February 19, 2017, 06:59:32 pmThanks a lot, I'll look further into this.

author: Ian.M
author_rank: Super Contributor
timestamp: February 19, 2017, 07:08:40 pm
content: One thing to be aware of: A TL431 is only rated for operation up to 36V, and an IRF7343 has a max Vgs rating of +/-20V, so without extra protection circuits to clamp transients etc. on the input voltage to under 20V, the simple TL431 + MOSFET circuit wont be reliable in an automotive application.

author: BravoV
author_rank: Super Contributor
timestamp: February 19, 2017, 07:26:19 pm
content: Quote from: Ian.M on February 19, 2017, 07:08:40 pm+1, especially in car electronics, I've learned my lesson well, hard lesson though thru fried circuits.Put TVS to clamp down the voltage rail.

author: Benta
author_rank: Super Contributor
timestamp: February 19, 2017, 08:17:50 pm
content: QuoteTo the cigarette lighter plug? These are normally fused at 20 A, I hope that's enough for your appliance.

author: calexanian
author_rank: Super Contributor
timestamp: February 20, 2017, 02:10:56 am
content: The "Proper" way to do this would either be with a comparator, or a micro. You could also cheat with a relay and a LDO regulator.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 20, 2017, 06:35:07 am
content: Quote from: calexanian on February 20, 2017, 02:10:56 amNo. It's to use an voltage monitoring relay. You can buy one, and you will know it works and it costs you 0 development hours.

author: David Hess
author_rank: Super Contributor
timestamp: February 20, 2017, 03:09:19 pm
content: Quote from: BravoV on February 19, 2017, 07:26:19 pmThis is a bad idea; clamping a low impedance source adds another failure mode.Clamp the MOSFET gate to source voltage and use a shunt regulator (resistor and zener, whatever) to protect the rest of the circuit.Note that if the TL431 is properly configured, it can protect itself because it *is* a shunt regulator.The tricky part of a circuit like this is not getting it to work; it is getting it to work under adverse conditions. Some hysteresis should be used to keep it from oscillating and I would also include a time delay and soft starting.

author: calexanian
author_rank: Super Contributor
timestamp: February 20, 2017, 07:47:40 pm
content: Quote from: Jeroen3 on February 20, 2017, 06:35:07 amWell excuse me for trying to encourage some educational activities.

author: Refrigerator
author_rank: Super Contributor
timestamp: February 20, 2017, 10:06:19 pm
content: Whatever we say, i think it's OP's decision how he's going to implement his solution.If he's not interested in experiments and bodgy prototypes then it would be best for him to look for an off-the-shelf part.I, personally, like a challenge, that's why i do stuff myself and encourage others to at least try it themselves.

author: jancelot
author_rank: Regular Contributor
timestamp: February 21, 2017, 05:19:11 pm
content: Quote from: Ton on February 19, 2017, 06:59:32 pmFinally bought the "BATTERY GUARD- BATTERY DISCHARGE PROTECTOR - (M148A)" from ebay userswitchedkwb. When it arrives will try to test it properly.

author: jancelot
author_rank: Regular Contributor
timestamp: March 09, 2018, 07:11:42 pm
content: Tested the M148A basic operation, but couldn't try under load yet. The on-off switching is operated through hysteresis of 0.8 volts. Personally, I've set it to 13 and 13.8 volts operating point.When the voltage is very low, for example 10 volts and it increases, it turns on when the input voltage reaches 13.8 volts or beyond. If the voltage decreases while it's running, it does not turn off until it passes below 13 volts. The process repeats again.