Zener creating problem - the comparator supply voltage

[sush]

zener izmin

Hi,
I am trying to comapre 2 signal for which i have used Comaprator which has to work on 36 volts. The circuit is working fine giving me an output either 0volts or 29 volts. But this 29 volts is required to be changed to 5 volts for interfacing this signal with Microcontroller. I tried the shown diagram but due to zener leakage it gives 4.3 volts. If i replace it somtimes become 4.7volts (i.e. different zeners are giving different outputs) I think they are of low qualitity type. But can there be a better & accurate method for changing this signal from 29 volts to 5 volts. I cannot change the comparator supply voltage.

Thanks for reading this.

 

[BABESH]

izmin 1n4733

Hi ,
I think there should be a series resistor (Rs) between Zener Kathode and V input to zener.

Actuall your o/p is dropeed to 5.9, instead of that use 4k7 and 3k9 you will get 13V
now connect a series resistor as per equ (Vin(13)-Vz(5))/(IZMin.)

Refer for any zener there will be parameter called IZmin, which is the min current required to maintain Vz,

In general it will be around 1mAmp in worst case for a 5V Zener.

So choose Rs in the range of 7K to 8K, it should work.

Thanks
Babesh.

 

[pranam77]

Re: Zener creating problem

Hai sush
Use this level converter to change the voltage levels between two circuits. Connect the open pin of the 47K resistor to the Op-amp output. Note that the ground shoud be common to both the microcontroller and the OP-amp. Other way is using a Opto isolator like MCT2E whose led can be used on 29 volt side whereas the collector and emitter many be used to provide logic to the micro. Hope this post helps you. Donot forget the helped me button. Good luck.

 

[sush]

Zener creating problem

Thanks both of you. I clicked "helped me" for both of you.pranam77 i'm very weak in analog circuits can you please tell me the role of each resistor in your circuit & how you calculated value of each resistor. I want to learn so please help me. Thanks once again.

 

[biff44]

Re: Zener creating problem

Just raise the 1K resistor until you get the voltage closer to 5 volts.

BTW, your microcontroller probably views anyting greater than 2.4 V as a digital "1".

 

[IanP]

Re: Zener creating problem

Microcontrollers will read input as a "1" when the voltage is roughly ≥2V (subject to dc characteristics of a particular IC), so if your Zener diode produces 4.3V it should be just fine.
BTW. you even don't need 1kΩ resistor in parallel with Zener diode, the diode will happily sink some mAs of current, however, if you want to keep it (1kΩ) there it's your choice.
And this circuit with BJT working as buffer is not a brilliant idea, so I wouldn't worry much about it ..

Rgds,
IanP

 

[pranam77]

Zener creating problem

Hai Sush
Thanks for pressing the "Helped me" button. In fact the two resistors 47K and 2.2K are wired as voltage divider network. As the 29 volts is connected to the 47K, and the result grounded by 2.2K, the base will get a 1.29 volts which makes the transistor conduct and thus the micro get the 5volts voltage from the formula given below

R-Gnd x Voltage
----------------------
R-Gnd +R vcc

Good luck

 

[IanP]

Re: Zener creating problem

Pranam, it looks like you have no idea how your own circuit works ..
If the Base is biased with 1.29V the Emitter voltage will be lower by Ube (≈0.7V) ..
The Emitter voltage is 5V only when the Base voltage is 5V + Ube ≈ 5.7V ..

I think one resistor and one Zener diode can do the job, so why complicate things that can be very simple ..

Rgds,
IanP

 

[sush]

Re: Zener creating problem

Thanks Ianp for replying. If transisitor is not a good idea what will be the perfect method. & For a similar situation but instead of comparator here I'm using a dual opamp to calculate DC current flowing. I'm converting the current into 0-5volt output, but again the zener dosen't allows the voltage to go beyound 4.3volts. I want to keep the zener in case of some fault which makes the output to go upto 30 volts, then it would damage the, ADC or full controller. Here what can i do. I have attached the circuit diagram.

 

[sush]

Re: Zener creating problem

As u say Ianp reagrding the bias voltage please see the diagram, I'm in doubt (sorry for my bad knowledge of analog electronics, but I want to improve) . usually i have seen this type of diagram for activating a 12v relay from microcontroller output. This means i should activate or deactivate a relay with 0/0.7 volts . Is 0/5v is not proper. The circuit i have attached is a test result from a simulator.

 

[BABESH]

Re: Zener creating problem

Hi sush....
From this transistor relay ckt, relay will ne ON always as coil is connected between 11V pos and neg terminal..

From zener prob.. u r into relay ...

 

[pranam77]

Re: Zener creating problem

If the Base is biased with 1.29V the Emitter voltage will be lower by Ube (≈0.7V) ..The Emitter voltage is 5V only when the Base voltage is 5V + Ube ≈ 5.7V ..I think one resistor and one Zener diode can do the job, so why complicate things that can be very simple ..

Rgds,
IanP

Yes IanP. You are correct. I was actually thinking about the reverse way (Emitter connected to ground) and thus had calculations.
Sush..If you are using my circuit calculate the required output from the emitter as per the resistor didviders connected to the base. The formula iis given. Good luck.

 

[IanP]

Re: Zener creating problem

With regard to the Zener diode, make sure that it is not rated for 4.3V, as it looks like that’s the case ..
If you have more of them, grab 1k resistor and connect them one by one (with 1k resistor in series) to, say, 12Vdc and measure voltage across them ..
You will find out that the voltage will vary ..
Choose one that gives you reading of more-the-less 5.1V ..


As far as the circuit with the LM358 is concerned, it is not rail-to-rail opamp, so the output won’t work close to 0V and close to Vcc ..
Consider lowering the supply voltage to this opamp to 5V, then you won’t need any Zener to protect the microcontroller pin(s) ..
To make it working use 100k/18k (not 100k/100k) resistor pairs (this will reduce the input voltage by the factor of 6) and then amplify the signal in the second stage by a reasonable factor ..

All that can be achieved only when you use rail-to-rail opamp and there are version of the LM358 which can do this, for example, the LMV358 ..

And lastly, any BJT (transistor) needs a base resistor ..
You can easily drive something like the BC547 from a microcontroller (0-5V output range) when you connect 1-10k resistor between the microcontroller pin and its Base ..

Rgds,
IanP

 

[sush]

Re: Zener creating problem

Thanks again Ianp for replying. My zeners are 1N4733 for which i checked on net & which is rated of 5.1Volts. but still its giving results which i'm unable to understand. As you said I tried please see the attachment.

 

[pranam77]

Zener creating problem

Hai sush
when you connect the zeners for 12volts, to get 5.1 volt, you are dropping just 7 volts. Try a lower value resistor to get the specified output. You may try 100 ohms and below. I guess 100E may work well. Good luck

 

[sush]

Zener creating problem

How do you calculated this 100 ohm value?

 

[IanP]

Re: Zener creating problem

As with all Zeners, their characteristic is not square and they are not perfect voltage regulators ..
As current increases the voltage increases too ..

The 1N4733 is a 1W diode, so you can allow up to 200mA and according to its data sheet it should rich ≈5V @ 40-50mA ..
See fig.1/page4:
**broken link removed**

So, [12V-5V]:40mA =>175Ω

Rgds,
IanP