Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Problem with LM675T opamp output in a circuit

Status
Not open for further replies.

hameeds01

Advanced Member level 2
Joined
Aug 23, 2005
Messages
599
Helped
75
Reputation
156
Reaction score
41
Trophy points
1,308
Location
Islamabad Pakistan
Activity points
4,540
LM675T Problem

Hey guys
I would appreciate if someone could help me,



I connected the OPAMP LM675 as show in the diagram with both i/ps having the same potential 12 volts via 4.7k the problem is that why I am getting 12 Vots at output ?? then I connected these inputs to 12 volts via 10k again the same ouput,
Is there some thing wrong with the LM675 Power opamp IC??? Its heating lil bit on no load condition, is it normal?? It has the Heat sink with it.
I don’t want to use dual voltage power supply now what should I do ??
 

AdrianN

Junior Member level 3
Joined
Jun 28, 2009
Messages
27
Helped
14
Reputation
28
Reaction score
3
Trophy points
1,283
Location
California
Activity points
1,473
Re: LM675T Problem

Take a look at its datasheet, here:

https://www.national.com/ds/LM/LM675.pdf

It needs a negative supply as well if you are to bring 0V in its input as your drawing shows. On page 2 you can see that it has a typical gain of 90 dB. This is a gain of 35000, so the output will quickly hit the rails for any tiny input voltage. You need to have a feedback as the schematic on page one shows.

For a single supply operation the input has to be biased somewhere in between rails, that is between 0 and 12V on your schematic. Don't forget the feedback though, as it will hit the power supply rail again.
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,217
Helped
2,129
Reputation
4,252
Reaction score
1,957
Trophy points
1,393
Location
Toronto area of Canada
Activity points
57,911
LM675T Problem

Its minimum supply voltage is 16V, not 12V.
Its input common mode range is 5V above its negative supply (ground in your circuit) to 5V below its positive supply voltage.

That means that its inputs do not function at ground in your circuit.
 

hameeds01

Advanced Member level 2
Joined
Aug 23, 2005
Messages
599
Helped
75
Reputation
156
Reaction score
41
Trophy points
1,308
Location
Islamabad Pakistan
Activity points
4,540
Re: LM675T Problem

Now its working fine input 0 to 5v output 0 to 12voltes, I am using a DC Brushless fan motor as a load of 500ma 12v, but there is a problem that is the LM675 gets very heat up with or without load , I am also using a reverse bias diode in parallel with motor to take care of inductive kicks,
How I can keep it running cool ???

 

hameeds01

Advanced Member level 2
Joined
Aug 23, 2005
Messages
599
Helped
75
Reputation
156
Reaction score
41
Trophy points
1,308
Location
Islamabad Pakistan
Activity points
4,540
Re: LM675T Problem

"Audioguru@ Hint: You connected positive feedback instead of negative feedback."
sorry it was just a drawing mistake, now i have corrected it ,
the problem is y it is getting too hot even without load ????
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,217
Helped
2,129
Reputation
4,252
Reaction score
1,957
Trophy points
1,393
Location
Toronto area of Canada
Activity points
57,911
LM675T Problem

It gets too hot when it oscillates at a high frequency.
It oscillates at a high frequency when it is built on a lousy breadboard that has too much stray capacitonce.

Make it on a compact pcb with short wires and supply bypass capacitors. Keep the input wires away from the output wire.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top