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Hysteresis problem.

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Lighty

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Hi all

I'm building a battery monitor and am having a problem with setting the to hysteresis points.

I've chosen the OPamp over say the like of the LM336 is that I Would like to have

I'm trying to achive for the output of the OPAMP to go HIGH when the battery is 13.8V and only go LOW when it is 13.2V, The output could be inverted as I'm going to drive a opto-coupler, is it can be connected to either the pos or neg rail.

Some possible problems:

1: I've noticed that the voltage across the Zener is 5.88v and not 5.6v (but is stable when the battery voltage 10-15 volt, so doubt its that?)

2: The ouput is relative to the Vss/Vdd, hence the amount of feedback varies as the battery voltage increases? Should I use a reg to optain a constant voltage for the feedback?

3: If I'm correct, the hysteresis would be relative to the voltage dividing network on the Non-inveting input? Eg: say the feed back on the hysteresis resistor is 0.1 volt, and the network is divided to 5V, 0.1 volt is less feedback compared to 0.1V over say 3V from the resistor network?

Hope that all makes sence?

Please see attached schematic.

Thanks in advance
 

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hi,
R2 and R12 will interact and affect the hysteresis setting.
So you want 13.8V and 13.2V, thats a 0.6V hysteresis level.

To reduce the interaction of R2/12 I would add a 10K resistor in series from the wipers of both the pots [R1 and R12] to the +/- inputs of the comparator.
 
Last edited:
hi,
R2 and R12 will interact and affect the hysteresis setting.
So you want 13.8V and 13.2V, thats a 0.6V hysteresis level.

To reduce the interaction of R2/12 I would add a 10K resistor in series from the wipers of both the pots [R1 and R12] to the +/- inputs of the comparator.

Thanks for your reply!

0.6V is good, perhaps adjustable (0.4V to 0.8V)

Added the 2x 10K resisters, is this correct?
 

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hi,
Look at this LTspice sim of your modified circuit.
 

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Thanks Eric, that looks 100%

What infuences would the 1K & 10K pots have?

I would imagine the 10K POT would set the threshold point? could the the 1K POT not be ommited as it does the same job?

How would I adjust the Hysteresis? replace the 680K with a POT?

Thanks
 
Thanks Eric, that looks 100%

What infuences would the 1K & 10K pots have?

I would imagine the 10K POT would set the threshold point? could the the 1K POT not be ommited as it does the same job?

How would I adjust the Hysteresis? replace the 680K with a POT?

Thanks

hi Sunny RSA.
The zener provides a fixed reference voltage, so if you connected the top of the 5v2 zener, via the 10k direct to the comparator -vin, that would remove the 10Kpot.
I would use the 10K pot in place of the 1K battery sense pot, it will give a greater range of adjustment for the comparator +vin, via the 10K.

The hysteresis pot should be 1meg, so that you can set it about 680k or so, to get the hysteresis you want,


EDIT:
Be aware when you connect the output of the comparator to the following circuit, it will most likely screw up the hysteresis.
I would use a N MOSFET on the output if you plan to drive a relay etc.
 
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hi,
Something like this.
 

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Hi Eric,

It's lovely and sunny here, but not so hot anymore, winter is on its way and its getting cold, about 20-23 deg C....... compared to normally 28+

The output of the opamp was only going to drive a opto coupler, a 4N35, and perhaps a LED, perhaps use another TL071 as a buffer?
 
Hi Eric,

It's lovely and sunny here, but not so hot anymore, winter is on its way and its getting cold, about 20-23 deg C....... compared to normally 28+

The output of the opamp was only going to drive a opto coupler, a 4N35, and perhaps a LED, perhaps use another TL071 as a buffer?

hi,
The buffer would be my choice, the other components will not interfere with the comp section.
 
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