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Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1

Low battery cut off circuit

Looking at this circuit here is it the 4v7 Zener and the trimmer that sets what the voltage that causes the Lm741 to go high?

It's the one that shows the relay coil, For that transistor It's not listing what it is so I guess anything is ok? What about a BC547?

Given I'm using a micro relay I doubt I need a diode on the coil?

»homemadecircuitsandschematics.bl···age.html
--
It's NOT Ni-kon It's NE-KON!




LG is NOT Lifes Good It's Lucky Goldstar!



shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
Any relay would need a diode unless it is solid state.

The BC547 will do; just about any NPN transistor would work.


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to Subaru
I don't like that circuit. Yes, it's dropping the load, but it's adding it's own load to an already low battery.


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
Reviews:
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2 recommendations

reply to Subaru
OT - but I'm amazed how long people have held on to the 741, even where it's not meant to be used.

I prefer the Philbrick




Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1
reply to SmokChsr
said by SmokChsr:

I don't like that circuit. Yes, it's dropping the load, but it's adding it's own load to an already low battery.

But what would be a better option then? The load of the 741 can't be as much as the load that's connected.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 recommendation

reply to SmokChsr
said by SmokChsr:

I don't like that circuit. Yes, it's dropping the load, but it's adding it's own load to an already low battery.

I agree. Modern voltage monitor ICs draw only microamps and provide more functionality.

Subaru See Profile you could try this out in the future. You could use a pot at R2 to adjust the on/off voltages. The MOSFET switch needs to handle max LED current but oversize a bit.


Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1
said by lutful:

said by SmokChsr:

I don't like that circuit. Yes, it's dropping the load, but it's adding it's own load to an already low battery.

I agree. Modern voltage monitor ICs draw only microamps and provide more functionality.

Subaru See Profile you could try this out in the future. You could use a pot at R2 to adjust the on/off voltages. The MOSFET switch needs to handle max LED current but oversize a bit.

Yeah looking at this data sheet it's more suited for what I need it for I wished I known about it when I placed my order at mouser..

So now R2 how would I really know when it turns on and off at a lower voltage? Would it just be trial and error?
--
It's NOT Ni-kon It's NE-KON!




LG is NOT Lifes Good It's Lucky Goldstar!


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
said by Subaru:

So now R2 how would I really know when it turns on and off at a lower voltage? Would it just be trial and error?

You can calculate them based on the desired off/on voltages for the battery.

R4 = 100K is fine.
R3 = R4 x (Voff - 1.15) / 1.15
R2 = R4 x (Von - Voff) / 1.15


SmokChsr
Who let the magic smoke out?
Premium
join:2006-03-17
Saint Augustine, FL
reply to Subaru
said by Subaru:

But what would be a better option then? The load of the 741 can't be as much as the load that's connected.

It wasn't the 741 as much as the relay and associated electronics. On any battery under 10AH in size it would not take long for the circuit to completely discharge and ruin the battery.

Lutful, posted a much better option.


Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1
I see, I looked at the data sheets of both and the one Lutful posted would be better off as far as how much power is needed for operation.

I will be working with 350 mA but I can move all the way up to 1A if needed.


shdesigns
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join:2000-12-01
Stone Mountain, GA
Using a MOSFET is more efficient. If you have either a N-channel of P-channel MOSFET with a low enough Rds-ON, it will work well.

A MOSFET, zener reference, and something like an LMC6262 would draw less than 0.1ma.


Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1
reply to lutful
In your circuit diagram R3 what's the closest I can get to 940k?

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 edit
said by Subaru:

In your circuit diagram R3 what's the closest I can get to 940k?

You can use a pot or multiple fixed resistors. Just plug in the numbers for R3/R4 and see how close you come to the desired cut-off voltage.

*** please stick with these equations:
R4 = 100K is fine.
R3 = R4 x (Voff - 1.15) / 1.15
R2 = R4 x (Von - Voff) / 1.15

So for 12.0V cut-off
R3=100K*(12.0-1.15)/1.15=943K


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:4
reply to lutful
The resistor values don't seem to correspond to the upper and lower voltages shown. Of course, I didn't have coffee, yet.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
Yes, R4 comes to 978K for 12.4V cut-off. Lower value gives him a bit more run time.

b10010011
Whats a Posting tag?

join:2004-09-07
Bellingham, WA
Reviews:
·Comcast Formerl..
reply to SparkChaser
said by SparkChaser:

OT - but I'm amazed how long people have held on to the 741, even where it's not meant to be used.

True an LM339 would have been my choice in this circuit.
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Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1
reply to Subaru
Finally going to get around to ordering the parts from mouser.. since I think an order from Digikey got lost in the mail I'm just going to order some capacitors for another project and get other stuff for this project in the cart as well.

After that the only thing holding me up is going to find a decent price for white insulated wire so it wont stick out so much.. I did a demo test and for only 350 mA it's pretty darn bright and it's right were I need it.
--
It's NOT Ni-kon It's NE-KON!




LG is NOT Lifes Good It's Lucky Goldstar!



Subaru
1-3-2-4
Premium
join:2001-05-31
Greenwich, CT
kudos:1
reply to Subaru
So in my order I ordered the following

MAX8212CPA+
MIC502YN
NTCALUG02A103G (NTC 10K 2% Low Therm Gradient) hah this NTC is rated for 100 mW this time around.