Comparator circuit + OP Amp 741 help

H

Hussain

New Member

• Jan 15, 2009

• #1

Hi guys ...

This is my first post , and I seek some help ....

I'm building a comparator circuit , and it's working perfect . The signal of the of the output goes to trigger a coil of relay. Unfortunately , the signal is not enough to energize the coil !!!!

I have been advised to install another circuit as OP amplifier with BC 337 connected with a resistor 100Ω . I tried it last night , but it still won't work .>>>


What shall I do bro ...

Waiting u ........

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 15, 2009

• #2

Check my PIC tutorials under 'Hardware extras'



It shows how to switch a relay.

Also check here



Which even uses 741's, and has exactly what you want.

 

H

Hussain

New Member

• Jan 15, 2009

• #3

Thanx a lot bro ...

I'll try it tonight and I'll let u know with the result

 

H

Hussain

New Member

• Jan 16, 2009

• #4

I tried the circuit , but it did not work as i want ....

The comparator circuit works fine with the set point of potentiometer , however, if i connect the OP / AMP 337 , the coil is energized as soon as switch on the circuit .....and it can not be controlled with the pot.

notice that the LED still showing that it varies proportionally with the pot.


How can I control the energization of the coil with my pot.?????

See the attached picture , it shows the connection of my circuit ....

Please help me ASAP

 

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aljamri

Member

• Jan 16, 2009

• #5

If you are doing this circuit for demo, connect an LED (Lower load). and if it is made to drive the Relay coil, no need for another 741, a proper transistor will do the job. In my case i'm using 2n2222a.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 16, 2009

• #6

As you're using a split supply, the emitter of the transistor needs to go to the negative supply, check the link I posted which has exactly what you're trying to do.

It's also absolutely essential to have a diode across the relay.

 

H

Hussain

New Member

• Jan 16, 2009

• #7

aljamri , thanks for ur commens .... yest I'm using the circuit for to drive a relay coil .



Mr. Nigel . . .

I connect the circuit as shown in the link ....except the diode that I'm going to install tomorrow ....

You have mentioned that the emettier has to be connected in the negative supply !! I connect it to the GND !! Isn't that enough ???

Is negative -12 Vdc?? or the GND can be considered as Lower potential source??

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 16, 2009

• #8

Like I said check the second link I posted - for exactly what you want.

And no, you can't use ground, you need to use -12V.

 

aljamri

Member

• Jan 17, 2009

• #9

Nigel Goodwin said:

Also check here

Click to expand...

In this link, the AUTO FAN circuit is exactly simmilar to the circuit you are looking for.

 

audioguru

Well-Known Member

Most Helpful Member

• Jan 17, 2009

• #10

The 741 opamp is shown as an inverter. So its output will always be negative which will always turn off the transistor.

I think the transistor is shorted because it powered the relay coil without the protection diode.

 

H

Hussain

New Member

• Jan 22, 2009

• #11

Mr. Nigel ... Thanx alot ... it works now !!

Mr. Audiofuru .... You are 100 %right , when I troubleshooted the circuit , I found that the transistor was shorted .....!!


Now .... is working fine .....