[SOLVED] Why do we use zener diode as Voltage reference in Comparartors

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speedEC

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Dear All,

I have to monitor the voltage of the 6v battery. If the battery reaches 6.9v (maximum), I have to show the Green LED as Battery Full to the user. Also, if the voltage goes down 5.5v then RED LED should be shown as Battery Low. I found one schematic using LM741 and UA741. What I want to know is how to set 6.9v as reference voltage? and why do we use Zener as reference voltage?

advance thanks.
pmk
 

speedEC said:
..............
What I want to know is how to set 6.9v as reference voltage? and why do we use Zener as reference voltage?
Click to expand...

The voltage across Zener diodes remains nearly constant in spite of very large current variations.
See the V-I characteristic curve.
Thus, the diode can be seen as a voltage source with a very small differential resistance (0.1 ohm and less).
 
Thanks for your reply. Which zener range should I select to set 6.9v as reference voltage?

---------- Post added at 14:43 ---------- Previous post was at 14:25 ----------





Pl see this voltage monitor circuit using LM741 IC. They told to select one-half of the battery voltage. i.e for 6v battery they told us to select 6/2 = 3v. i.e. 3.3v Zener. I have connected 3.3v zener to Pin 2 (Vref) as shown in the Picture.

What I want to know is how the circuits works and How the voltage compared? Can you elaborate pl. The circuit works fine. But, I want to understand the functions. I don't want to follow the circuit blindly.

thanks
Pmk
 
Last edited:

speedEC said:
What I want to know is how the circuits works and How the voltage compared?
Click to expand...
The amplifier is being used simply as a comparator.

If you have read about operational amplifiers you should know that when the non-inverting input's voltage is higher than that of the inverting input, the output of the amplifier goes high, and vice versa.

In the circuit shown, the reference (zener) voltage is on the inverting input. If the voltage (from the potentiometer) on the non-inverting input is higher than the zener voltage, the amplifier's output will be high and the l.e.d. will not be lit.

If, on the other hand, the voltage from the potentiometer is lower than that of the zener, the amplifier's output will be low, and the l.e.d will be lit.
 

Thanks Syncopator. I can now understand. But, without load connected to the Battery, the UA741 circuits works fine for Low and High Voltage monitoring. But, If I connect load to the battery, say lamp (Car's Head Light), I receive unpredictable result. Why? help needed. May be, The lamp sucks entire current from the battery and it won't allow sufficient current to flow to zener diode. So, the zener starts malfunctioned that I believe. Is this correct? Someone told me we have to wait for certain time to settle Lead-Acid battery bcoz chemical reaction take sometime to settle. Is this correct? If so, How can I monitor the voltage live?
 

speedEC said:
May be, The lamp sucks entire current from the battery and it won't allow sufficient current to flow to zener diode .... Is this correct?
Click to expand...
Measure the battery voltage with a meter, both without the lamp connected and with the lamp connected. The readings should be the same. If the reading when the lamp is connected is lower, the battery is almost discharged or has a high internal resistance and is possibly beyond repair (but it might be possible, with some patience, to revive it).

Someone told me we have to wait for certain time to settle Lead-Acid battery bcoz chemical reaction take sometime to settle. Is this correct?
Click to expand...
No, it's a load of rubbish.
 
I have measured the Battery Voltage:

When Lamp NOT Connected to the Battery : 6.39v

When Lamp Connected to the Battery : Suddenly Voltage down to 5.92v and starts decreasing slowly with respect to time.

Is that Battery problem?

thanks
Pmk
 

battery open cell voltage will be higher than when you load it....it's ok if it tends to droop due to depletion....it depends on your current draw....
 

Then why UA741 gives unpredictable results? I have checked with the LM358 also. It works better, but still It shows Battery Low/High wrongly when load (lamp) is connected. For example, Green Lights ON bcoz the battery full (say, I set 6.2v). If lamp connected, the voltage goes down to 5.92v as I already said, the Green LED goes off correctly. After disconnecting the Load (Lamp), the battery voltage get back to around 6.2 and above. So Green LED lights-up again. How can I correct this? Pl note that Without Load connected to Battery Everything (High/Low voltage monitoring) works fine.

---------- Post added at 18:19 ---------- Previous post was at 18:16 ----------

Also, I checked with other battery. Same result. So, Battery is not the problem. Shouldn't we use very high load? Is this Lamp beyond the limit of 6v, 4.2Ah Battery? Bcoz, I used load as a lamp which is used in Car Head Light. Any suggestion pl?
 

speedEC said:
I have measured the Battery Voltage:

When Lamp NOT Connected to the Battery : 6.39v

When Lamp Connected to the Battery : Suddenly Voltage down to 5.92v and starts decreasing slowly with respect to time.

Is that Battery problem?

thanks
Pmk
Click to expand...
Yes. The battery's voltage shouldn't drop like that. If you have access to another battery, try it just to satisfy yourself.

It may be possible to rejuvenate the battery. To do so you would need to fully charge it at a low current, say 0.5A. Then discharge it, say with a car headlamp bulb. Then recharge it slowly again. Then discharge it. And repeat the cycle half a dozen times. Sometimes there will be little improvement, sometimes a large improvement. Worth a try anyway.
 

speed...

once the load is disconnected its natural that the battery voltage jumps or bounces to a litle higher voltage......as i have seen with li-ion batteries....

i used a discrete hysterysis circuit to avoid the false triggering.......you can find many discrete/ic(simple op-amp based too) solutions online.....go through the link below just as a reference....

Need help w/ hysteretic comparator - /etc... Analog Forum - /etc... Analog - TI E2E Community

not sure if am addresing your issue correctly.....
 
speedEC said:
Hi Syncopator, I also checked with other battery (New 6v , 4.2Ah). But the same result.

thanks
pmk
Click to expand...

Ok, I think I was overlooking something. Each cell in a lead-acid battery has a nominal voltage of 2. When the battery is fully charged, each cell voltage will be rather higher, may be approaching 2.6/2.8. Once disconnected from the charger this rapidly settles to about 2.2V. Then, once you start drawing current from the battery, the cell voltage goes down to 2V and stays very close to that figure during a discharge cycle.
 

Hi Syncopator,

Sorry for the delay.

As mentioned by ea.arun, I have to use hysterysis circuit in order to show correct result by the UA741 IC. I'll try that one. I need one more clarification regarding setting the reference voltage on UA741 or LM358 ICs. Really I am very much confused on setting reference voltage. As you said, When Battery voltage goes beyond/below Vref, The LED will ON/OFF. OK. fine. I understand. But, How can I set the exact voltage reference. I have to set 6.9v as fully charged and 5.5v as Low Battery Indication. Can you please help me?

thanks
pmk
 

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