Cant understand The function of OPAMP?

[message4guru]

Hi all,

I found this circuit to be delivering 5 Ampere current. Resistance R6 and R8 form the voltage adjust function. I cant understand the operation of opamp. How to analyses and understand the operation of OPamp?

 

[mtwieg]

It implements a current limit, with R3 acting as the sense resistor. The current limit will be roughly equal to Io=1.25*R2/(R3*R5). When Io is below the limit, the opamp output is saturated high (though its output voltage is clamped roughly to Vo through the diode on pin 8). When Io goes above the limit, it acts as an integrating error amplifier to keep the output current constant.

 

[E-design]

Here is my take on this:

When in normal constant voltage mode, a sample of the control voltage is combined with the output voltage coming from the collector of the bypass transistor. This is applied to the inverting input of the op-amp. When not in current limit, this voltage will be lower than the non-inverting input pin. This will cause the output of the op-amp to be more positive and to reverse bias the LED so it has no control over the LM350's adjustment pin.

When more current is drawn the output voltage of the transistor will have to increase accordingly. This will result in the voltage at the inverting pin to go more positive at some stage than the non-inverting pin, thus forcing the output of the op-amp in the negative direction. This will bias the LED on and pull down the LM350's control pin to fold back until no over-current condition exists. The points at which the current limit acts are determined by the ratio of R2 and R5, which can be varied with R2.

mtwieg - I think we are basically saying the same thing. I only saw your post after posting.

 

[FvM]

The schematic (copied from good old LM101A data sheet) doesn't tell how OP V+ is connected. Using the circuit with +35V input seems rather marginal.

 

[message4guru]

Thanks to all..

I am a college going student. Please give me some books which give clear perspective of what is happening in the OPAMP circuit and if i chance certain parameter is changed what can happen?

 

[E-design]

You may find this document from TI useful:

https://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

 

[bigdogguru]

In its current configuration the LM301A is essentially a comparator, with the set point applied to the inverting input and the sense applied to the non-inverting input. Its primary function appears to be to turn on and off the current limiting indicator LED (D3). Also it should be noted the LM301A has no internal compensation and thus requires external compensation afforded by C4 and C5.

There are a multitude of textbooks available which cover Op Amps, two of my favorites are:

Operational Amplifiers: Applications, Troubleshooting, and Design

Linear Integrated Circuits

While both are out-of-print, they can be purchased "used" at a mere fraction of today's textbook prices. Bell's text includes an entire chapter on Op Amp Compensation which is rarely covered in today's texts.


BigDog