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Im new to electronics and have been trying to devise a cutoff circuit for a 2s 7.4v Lipo Battery. I have found a few places that have the schematics but am unsure as to that parts are needed for a specific layout. see the link bellow.

Adding voltage cutoff to a circuit?

I have read that a good way is to use the op-amp schematic referenced in the first post. now it shows 2 ICs and im unsure as to if each will be the same TLV431 also is there a way to incorporate a speaker that would sound when the cutoff voltage has been achieved (6.2V)?

parts list that I have come up with is:

TLV431 x 1 (IC1), unknown x 1 (IC2), STP80PF55 x 1(P-channel mosfet), 15k x 1 (R1), 10k-100k? x 1 (R3) How does one figure out what resistor is needed?, R2= R3 x [Vmin/1025 -1]This on im confused of

Any Help will be greatly appreciated!

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user27686
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1 Answers1

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"1025" in your formula above is an error due to the shocking diagram quality (drawn by me :-( ).

It should be 1.25 as the TLV431 regulator output = 1.25V.

So to achieve 6.2V cutoff you want
R2 = R3 x (6.2/1.25 -1 )
= R3 x (4.96-1)
= 3.96 x R3 ~+ 4 x R3.

If R3 = 10k then R2 = 39K is OK.

IC2 is any opamp that will work on the available power supply voltage and which has enough output swing to drive the MOSFET off and on.

The 2nd circuit using a transistor instead of an opamp will work as well and saves you needing to use an opamp. A resistor from MOSFET gate to ground would be auseful precaution to ensure it is off when not driven on - anything in the 10k-100k range would work OK.
The transistor can be almost any small PNP. I'd use a BC327 if through hole or a BC807 if SMD BUT almost any PNP transistor will work.
Using R1 = 39k and R2 = 10k should work OK.

Note that this circuit draws a small current via R1+R2 when the battery is cut off.
Iquiescent ~+ Vbat/(R1+R2) = 6.2 / (49k) =~ 125 uA.
This will often not be a problem.

Russell McMahon
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  • So Russell if i can clarify on schematic 1 IC1 is an TLV431 also correct? – user27686 Aug 21 '13 at 02:33
  • Ok Russell it took me a minute or two to understand the 1st diagram. I also now understand the 2nd Diagram also thanks for your explanations! – user27686 Aug 21 '13 at 02:49