opamp design / short circuit protection

Discussion in 'The Projects Forum' started by themindflayer, Oct 30, 2010.

Oct 29, 2010
44
1
Hi all,
I want to understand how to design an opamp circuit given what i want.
e.g I want to implement short circuit protection, I already have a current sensing implemented using a current transformer, I know that X amount of current is expected (anything above the uC turns the system off but it is too slow, to account for inductive loads etc), I decide that 2X current is my limit for short circuit i.e any current above 2X is to be considered a short circuit. Hysteresis is to be +/- 0.25 times short circuit current (once short circuit has been detected the op-amp is not to deactivate until current falls down to 1.5X.)

So given all this how do I approach op-amp design? I am familiar with the 2 golden rules (Art of Electronics - Paul Horowitz) and I can solve circuits to some extent .... however when it comes to right down intuitive designing I am at a loss ... Normally I look at circuits, solve them and then tweak to produce desired results ... I dont suppose this is how things are meant to be i.e learn circuits and equations and then solve where necessary .... I was wondering how does one intuitively develop circuits around op-amps .... any handy literature or how one starts solving the above problem would be a blast ...

thanks

2. Wendy Moderator

Mar 24, 2008
20,772
2,540
One of the reasons the 741 was so beloved by universities (in spite of local critics) is the output is fairly tough, you could usually short it with no problems.

Many newer op amps also have this feature, datasheets are your friend.

The big thing to remember about op amps in general is they are self correcting. If the output can reach the goals the op amp can compensate for almost anything. The really big hitters with op amps in general are input response (how close the inputs can get to ± edges), the output ± response, slew rate, and frequency response.

Last edited: Oct 30, 2010
3. eblc1388 Senior Member

Nov 28, 2008
1,542
102
Short circuit protection of an opamp is not the same as short circuit protection(for another circuit) using opamp.

4. Wendy Moderator

Mar 24, 2008
20,772
2,540
Amen to that! Transistors don't care, they'll fry their little hearts out if requested.

Oct 29, 2010
44
1
thanks for the reply ... Bill i believe you misinterpreted .... i want to intuitively design using op-amps ... a design example using the said conditions (short circuit protection with hysteresis) would be much appreciated ....

6. eblc1388 Senior Member

Nov 28, 2008
1,542
102
If you want fast action on short circuit current detection, then you should not be using an opamp.

You would use a comparator instead, parts like LM393 or LM311 etc...

If you want to learn how to design with them, the easiest way is to check out your design using simulator. Some colleges provide their students with access to SPICE simulator but one can always get a free one like LTSpice to use at home.

Input your desire current profile as the sensed current to the comparator and watch how the comparator responses. You can also tune the hysteresis characteristic too using the simulator.