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Help with Low Voltage Cutoff design

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Flash

New Member
Hello,
My name is Brian and i am new to this site.
I have a decent electronics background in the way that I can solder great and can read a schematic and use a multi meter.

I am into RC trucks and in my current project I need a low voltage cutoff circuit.

I am using 11.1v lithium battery (3 cell 1500 mAh) so they can be damaged when voltage drops below 9v or 3v per cell.

So I would like to make a circuit that I can hook in between the battery terminal and the truck connector so that when the voltage drops to 9v the truck turns off all together.
to cut the power to everything connected to the battery.

If possible maybe have it beep a few times before it cuts out completely.

Normal ESC have a lipo cutoff and mine does have this.
it is set to 9v.
But in my RC truck I also have 24 LED's for headlights and such, as well as a modified train smoke maker.
these are hooked dirrectly to the battery.

I have found a few circuits to make a buzzer go off, or LED's turn on.
I do like the buzzer idea, but would like to have it kill power as well.
the LED idea is not good for me bacause I have so many LED's already I dont think I would even notice another LED if it turned on. lol

please let me know if there is a simple circuit I can make.
then i can buy the components and build it myself if I have a schematic.

thanks for the help and for reading.
 

on1aag

New Member
Hi Brian,

Here's a circuit that will switch the power off when the voltage
drops below 9 volt. It's as simple as I could make it but there's
no buzzer. It uses cheap and readely available parts so you can
obtain them from a local supplier. The only somewhat difficult
part is the P-channel mosfet, select the one with the lowest
Rdson available.
This circuit is easy to build so you can test it and see how it
behaves. Later you can substitute the TL431 with a ICL7665S
and include a buzzer.

on1aag.
 

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Flash

New Member
wow that looks great and pretty easy for me to understand.

what do you mean by this?

"Later you can substitute the TL431 with a ICL7665S
and include a buzzer."

how would I go about doing that I dont understand?

How did you draw that schematic? did you use a program?
It looks like in the pic of the graph that you where able to test it is that so? haha
I want a program like that!

Again thanks for your help
 

on1aag

New Member
Hi Brian,

If you take a closer look at the datasheet of the ICL7665S
you will find examples on how to design similar circuits.

ICL7665S*CMOS Micropower Over/Under Voltage Detector

The simulator, Switchercad, can be downloaded here:

Linear Technology - Linear Home Page

But you might want to search the internet to find some
extra simulation models.

LTspice : LTspice/SwitcherCAD III

Start with the simple circuit and test it.
Then you'll know if a hysteresis is necessary or not.
Perhaps you can use it separately for the leds and the
smoke generator.
With a slight modification you can add a hysteresis.

on1aag.
 

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Flash

New Member
ok so I notice you put in one more Resistor R5
is that the hysteresis?
I am new to that term ?
I notice also that the voltage drops to 0v then turns back on at around a little over 9v.
is that so that it will turn back on once It hits that voltage and that would mean a new battery or fresh charged battery?

I was actualy thinking of using a seperate battery just for the smoker and LED's so that would be perfect.
just to shut them off.
The truck has the ESC cutoff.
then i would get longer run times as well.
with dual battery's

thanks for the links I am going to check them out now.

Brian
 

audioguru

Well-Known Member
Most Helpful Member
The LM339 is a quad comparator in a 14 pins package. The schematic has the pin numbers correct but the input (+ -) labels are backwards. Use an LM393 dual comparator in an 8 pins package instead.

These are very low current comparators and the output current is a minimum of only 6mA so use a low current piezo beeper without an LED.
 

Flash

New Member
so will this 1st schematic drawn for me work correct in what I need to do for my voltage cutoff?



if so is there a way to include this schematic in the mix so I can have a buzzer as well as a cutoff?

Low Voltage Alarm

also when you say the + and - are reversed where are you saying that is?


Also I would like it to switch off power and have it stay off.
untill I replace the dead battery with a new one.


thanks
 
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Katrine

New Member
I know this is an old post, but!

Hey, I know this is an old post, but I just so happen to have run into the same problem as flash, with slight modification.

I just want to run lights (no vehicle - so not a terribly high power draw) off a LiPo battery, so I only need to shut down those lights as the LiPo reaches 9v

However, I can't really figure out where I hook up my LED's from the diagram.. I realize I might have to add some voltage control for those, but where should I connect them? :)

Any help would be greatly appretiated!
 

audioguru

Well-Known Member
Most Helpful Member
Leds operate from current, not voltage. So you must use a current-limiting resistopr or circuit in series with them.
You probably have a 3-cells Li-po battery that is 9V when discharged and is 12.6V when fully charged.
Then you will see the LEDs dimming if a resistor is used.
 

Katrine

New Member
Alright, I guess I got hung up on the fact that they have a max V in the datapages I was looking at. My plan was to have a fixed volt output, say 8v, thats enough to fit roughly 2(or more) of any colors in series with resistors, atleast of the ones I looked at (2-4v spanning red, white, blue and green). Should that not keep the diodes shining bright untill cutoff, because I calculated the resistors based on 8v - which is available till the very end?

But my basic problem still is I can't see from the diagram he drew where I connect my LED's(with resistors) :) I'm likely missing the obvious, but if someone could either draw on the diagram or point out where (so I get the cut-off effect), I'd be very happy :D

And thanks for the answer :)
 

Katrine

New Member
I caught as much. But that says nothing(to me) of where I connect the LED's. Not sure how to explain it clearer. Where do I connect the + and - of my LED? (assume the resistor is there).
 

audioguru

Well-Known Member
Most Helpful Member
Your LEDs and current-limiting resistor are the 10 ohms load resistor in that circuit. Your battery voltage will drop to about 9V when the LEDs should be disconnected by the Mosfet.
 

exilus

New Member
I got 3 questions:

1- With the right R1/R2 ratio, can I use this schema for a 1S battery (cutoff at 3v) ?
(the load is +/- 0.45A)

2- Do i need to change R4 value (for a 1S battery) ?

3- If I can't find a FDS4685, what can i use to replace it ?



Thanx
 

audioguru

Well-Known Member
Most Helpful Member
I got 3 questions:

1- With the right R1/R2 ratio, can I use this schema for a 1S battery (cutoff at 3v) ?
(the load is +/- 0.45A)

2- Do i need to change R4 value (for a 1S battery) ?

3- If I can't find a FDS4685, what can i use to replace it ?
You need to use parts that work at only 3V:
1) Replace the TL431 with a TLVH431.
2) Adjust R1 and R2 for the correct cutoff voltage.
3) Replace the Mosfet with one that works when its gate is only 3V or less.
 

exilus

New Member
Thanx for the answer,

How you sure the TL431 can't work at 3v ?

I'm not good in electronic, could you give me a few details on how to chose the Mosfet?
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
The output voltage of a TL431 does not go low enough to turn on a Mosfet when its supply is only 3.0V.
I don't know of any very low voltage Mosfets and don't know where to find one. Maybe you can do a search on Digikey's website.

Maxim-IC and other manufacturers make low voltage disconnect ICs.
 
hi, may i ask how the values of the resistors were computed to make the cutoff to 9v? i was trying to modify the circuit to have a 9v source and cut off at about 7v but i dont know how to properly solve for the new values of the resistances. thanks in advance!!!
 
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