Low Voltage Cutoff - Page 1
Forum section: electronics / beginners

author: Grayfox
author_rank: Regular Contributor
timestamp: November 08, 2014, 12:35:23 pm
content: I got a new dashcam that runs on +12v and I will be hardwiring it into my car when I get spare time.Since it will be hardwired I will need to prevent the car battery from being fully discharged.I could easily get a device called Power Magic Pro for about $60, but I have a feeling that i can make a device just like it, that is smaller and cheaper that still does the job I am after.I just need help finding a schematic of a low voltage cut off that does not need any ICs that need programming.Circuit Voltage is 8v~15v and 2Amps or so Max.Would like to have a adjustable cut off if possible.Thank you

author: kxenos
author_rank: Frequent Contributor
timestamp: November 08, 2014, 12:42:02 pm
content: http://electronics.stackexchange.com/questions/47154/low-battery-indicator-for-2x-aa-batterieshttp://www.gorum.ca/lvdisc.htmlhttp://www.head-fi.org/t/82115/low-battery-indicator-circuits

author: Psi
author_rank: Super Contributor
timestamp: November 08, 2014, 12:43:46 pm
content: Analog comparator.Pot to set the reference threshold.Few resistors to act as a voltage divider (to scale the battery volts down to something the compactor can measure while running from battery volts itself)Mosfet to do the load switchingMaybe a 5v reg to run it all.You should keep in mind that whatever method you pick its going to be using some power to do its function. So the battery will run flat eventually.( Unless you do some funky latching relay shitgot to love latching relays)

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: November 08, 2014, 12:54:56 pm
content: Have you considered using a relay w/ coil connected to switched ACC power so the dashcam only works when the key is in the ignition and in ACC/IGN position?

author: Grayfox
author_rank: Regular Contributor
timestamp: November 08, 2014, 01:03:01 pm
content: Quote from: RobertHolcombe on November 08, 2014, 12:54:56 pmMy cars internal fusebox has a this connector which allows me to tap into to the following +12v Reverse engaged(1), +12v Headlights on(4),+12v constant battery(5) and +12v Ignition(6)Had my old dashcam connected to pin 6 but would like to utilize the dashcams Parking mode which needs a constant source of power

author: Seekonk
author_rank: Super Contributor
timestamp: November 08, 2014, 01:53:00 pm
content: A 3 pin TL431 powering a relay will do that. Look at app notes for example. Two things you must do: Power the relay from the contact that opens. A push button must be used to initially power the relay or reset it. Two, a large cap to maintain the sense voltage when you start the vehicle and the battery sags.

author: SeanB
author_rank: Super Contributor
timestamp: November 08, 2014, 03:57:58 pm
content: Most dashcams usa low power, so wiring it direct to 12V will work well enough. If you are worried about leaving it on over a week yes that will flatten the battery, but in that case you probably would be better off adding a second battery and charge circuit like used in high power audio, then powering the camera from that.

author: Grayfox
author_rank: Regular Contributor
timestamp: November 10, 2014, 06:56:36 am
content: Power use should be below 500mA.Tested in the garage and got around 200mA but this was with no GPS signal, if the GPS receiver uses any power it wont be much, but I added more since the device also can stream/export videos via WiFi.

author: darrenb
author_rank: Contributor
timestamp: November 11, 2014, 12:28:56 am
content: This is a low voltage cutout that I was working on yesterday in LTSpice.I am no expert but it seems to work in simulation. R2 and R3 divide the input voltage to 0.4 Volts when the input voltage is at 11 Volts, which is the reference voltage inside the IC. R1 is a load just for testing the circuit it is not required in use where the outputs would be the two right hand corners of the circuit.The other mosfet M2 and R5 provide reverse polarity protection. You don't have to use these parts. Pretty much any N channel mosfet would work as would any voltage comparator.People will chime in with improvements, like maybe there should be a zener for the reverse polarity mosfet gate to clamp it, but the basics are here.Actually I have a question about this circuit myself - I can't work out why the source for the reverse protection mosfet is on the high side, but the switch mosfet has the source on the low side?