Voltage sensing DC 12/volt   - Page 1
Forum section: electronics / beginners

author: m42665
author_rank: Newbie
timestamp: July 31, 2018, 03:35:03 am
content: Let me put it this way we have a 12/volt battery on a table that has a light on but in time the battery is dying with voltage going lower and lower the light brakes at 9/volt and lower . It is this I want to stop , put something on the line that would sense the lower voltage and shuts off and only let 12v or high come through . I am a newbie and I have to get this done so please somebody help.2ND POST --Thank you so much for your help ! I need you to remember that I am new at this I didn't know what a comparator was and had to look it up and a lm385 too but it looks like they are the same thing , now this doesn't mean that am good now. I still need help putting this together , some part numbers or names of what is needed to do the job.Well it looks like what I need is the comparator I saw a video on it and want to try it . Looking at a picture I posted I believe I can do but if you can hold my hand and help with maybe a picture or how to wire it would mead a lot to me !As far as my example of the battery going dead was so it could be understood what I wanted to do .The battery never go's dead but the wire I have to use and must use changes voltage from 6 to 12volts so I want to stop anything less then 12volts coming out on a wire I'm going to tie to it that is going to a relay that could turn on at the wrong time . Please get back thanks in advance Mitch .3RD POST---Thank you so must for your help !!!!! I put up a 2nd post where I say it's not so much the battery I care about , it's the wire I must use that has 6 and 12volt at different times , I want to tie on to that wire but I only want it when it has 12volt not 6volt . So I must stop 6volt from coming out of that wire that I must use . Please get back with something like you sent me and how to wire it up .I Thank everybody so much for your ideas and remember that I am new at this so make it easy so I can understand .

author: JS
author_rank: Frequent Contributor
timestamp: July 31, 2018, 04:56:49 am
content: A comparator, check a known voltage from a reference, like lm385 or other voltage regulator with a division of your rail, that comparison will tell you if the battery voltage is higher or lower.As your battery will recover some voltage after the load is disconnected you need to deal with that. Hysteresis might help but likely not enough, as recovery voltage is quite high, depending on bat chemistry. I'd probably use a way to dissconnect everything from the battery till a charger is detected, and only then reconnect the comparator and light. Something like a mosfet powering the comparator and light, keept on by the comparator till it hits a threshold and then a separate circuit which powers the mosfet while detects a charger.That way once you hit the level ypu want the light off it turns off and as soon as a charger gets plugged it goes back on, till voltage drops again.JS

author: m42665
author_rank: Newbie
timestamp: August 01, 2018, 05:36:58 am
content: Thank you so much for your help ! I need you to remember that I am new at this I didn't know what a comparator was and had to look it up and a lm385 too but it looks like they are the same thing , now this doesn't mean that am good now. I still need help putting this together , some part numbers or names of what is needed to do the job.Well it looks like what I need is the comparator I saw a video on it and want to try it . Looking at a picture I posted I believe I can do but if you can hold my hand and help with maybe a picture or how to wire it would mead a lot to me !As far as my example of the battery going dead was so it could be understood what I wanted to do .The battery never go's dead but the wire I have to use and must use changes voltage from 6 to 12volts so I want to stop anything less then 12volts coming out on a wire I'm going to tie to it that is going to a relay that could turn on at the wrong time . Please get back thanks in advance Mitch .

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2018, 06:37:54 am
content: You can buy them off ebay, if you don't feel like making your own.https://www.ebay.co.uk/itm/Battery-Against-Excessive-Discharge-Controller-12V-Low-Under-voltage-Protection-/252928364204Another possibility is the TL431. How much current does the light draw? If it's more than a couple of hundred mA, use this circuit to switch a relay or use a P-MOSFET for Tr1.

author: m42665
author_rank: Newbie
timestamp: August 01, 2018, 03:39:56 pm
content: Thank you so must for your help !!!!! I put up a 2nd post where I say it's not so much the battery I care about , it's the wire I must use that has 6 and 12volt at different times , I want to tie on to that wire but I only want it when it has 12volt not 6volt . So I must stop 6volt from coming out of that wire that I must use . Please get back with something like you sent me and how to wire it up .I Thank everybody so much for your ideas and remember that I am new at this so make it easy so I can understand .

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2018, 08:25:13 am
content: Quote from: m42665 on August 01, 2018, 03:39:56 pmI'm not sure what you want. The circuit I posted can cut-off at any voltage between 2.5V and 36V, which can be altered by changing the values of R1 to R3.The cut-off voltage can be calculated, using the following formula:V = VREF*(1+R1/R2)Where VREF= 2.495V but can normally be rounded to 2.5V to make calculations easier.R3 controls the hysteresis. When the output is off, it's in parallel with R2 (assuming the load on the circuit is a very low resistance compared to R1 to R3) and when the output is on, it's in parallel with R1. This means the turn on voltage of the circuit is higher than the turn off voltage, which is necessary to stop the circuit from oscillating. Without the hysteresis, when the circuit turns off, the load would be disconnected from the supply, causing the voltage to rise and it to turn on again, resulting in an endless loop.This means there are two separate calculations for VONand VOFF.VON= VREF*(1+(R1|R3)/R2)VOFF= VREF*(1+R1/(R2|R3))The | character means the equivalent of two resistors in parallel, so R1|R3 = (R1*R3)/(R1+R3) and R2|R3 = (R2*R3)/(R2+R3).With the values in the circuit above: VON= 10.6V and VOFF= 10.2V rounded to 1 decimal place.To make calculations easier, don't worry about the separate on and off voltages, just use the first formula.By the way, the LM358 is totally different to a comparator IC, such as the LM393. It is an operational amplifier. It can be used as a comparator but it's seldom the best choice of that function. It's more suitable for amplifying