Generating 0-10VDC With Op-Amp and DAC - Is There A Better Way To Protect Op-Amp and Limit Current?

Thread Starter

Mahonroy

Joined Oct 21, 2014
315
Hello, I am currently generating a 0-10VDC with an op-amp and a DAC (digital-to-analog-converter). The op-amp has a limit of 350mA output max. So in an effort to protect the op-amp (say an accidental short), I have that 33 ohm resistor in place.

This solution works good when the output has small amounts of current being drawn from it. The problem is, the more current that is pulled from the output, the larger the voltage drop is over that 33 ohm resistor. E.g. instead of getting a 0-10VDC signal, I now get a 0-9VDC signal.

I'm wondering if there is a better way to handle this? For instance allow me to get into the higher currents and not have the output get pulled down... but still offer protection for the op-amp.

Here is my circuit currently:
0_to_10_question1.jpg
 

OBW0549

Joined Mar 2, 2015
3,509
I'm wondering if there is a better way to handle this?
Yes. It's easy: just disconnect R27 from the op amp's output and connect it to the other side of R26. R26 will therefore be "inside" the feedback loop and effectively disappear. I've done this many times as a way of dealing with output protection resistors.

By the way, the LMC7101 has an Absolute Maximum Rating on output current of ± 35 mA. How do you figure on getting 350 mA out of it?
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
315
Yes. It's easy: just disconnect R27 from the op amp's output and connect it to the other side of R26. R26 will therefore be "inside" the feedback loop and effectively disappear. I've done this many times as a way of dealing with output protection resistors.

By the way, the LMC7101 has an Absolute Maximum Rating on output current of ± 35 mA. How do you figure on getting 350 mA out of it?
Thanks for the response, and sorry for the late reply!
I am not understanding how this change will offer current draw protection for the op-amp. If the output is short circuited, won't that cause an excessive current draw from the op-amp now that the 33 ohm resistor is in the feedback loop?

Here is a link to the LMC7101, it shows 300mA per channel:
https://www.digikey.com/product-detail/en/microchip-technology/LMC7101BYM5-TR/576-2575-1-ND/1821735
 

OBW0549

Joined Mar 2, 2015
3,509
I am not understanding how this change will offer current draw protection for the op-amp. If the output is short circuited, won't that cause an excessive current draw from the op-amp now that the 33 ohm resistor is in the feedback loop?
Yes, if you leave R26 at 33 ohms; but since it is inside the feedback loop you can increase it to a safer value, say 390 ohms, without affecting output accuracy. This will limit the output current to about 31 mA with the output shorted to ground.

Here is a link to the LMC7101, it shows 300mA per channel:
That 300 mA figure is the typical supply current with the output shorted, NOT an operating limit representing the op amp's output capabilities. It is a warning to the designer that if the output is shorted, excessive power dissipation will result.

LMC7101.png
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
315
Yes, if you leave R26 at 33 ohms; but since it is inside the feedback loop you can increase it to a safer value, say 390 ohms, without affecting output accuracy. This will limit the output current to about 31 mA with the output shorted to ground.


That 300 mA figure is the typical supply current with the output shorted, NOT an operating limit representing the op amp's output capabilities. It is a warning to the designer that if the output is shorted, excessive power dissipation will result.

View attachment 214434
Thanks again for the response. I simulated the circuit with putting the 33 ohms in the feedback loop, and simulated an almost short circuit by putting a 1 ohm load. It shows that 10 amps is flowing out of the op-amp in this case. It doesn't look to be limited by the feedback.
1597084713291.png

What are you driving with the opamp that might draw 350mA?

Bob
Light dimming signals. There can be hundreds of lights in parallel all pulling a small amount which adds up. Also long wire runs add resistance and pull additional current as well. Plus I want some headroom. I could decrease the maximum current rating if I can figure out how to limit current better.
 

OBW0549

Joined Mar 2, 2015
3,509
Thanks again for the response. I simulated the circuit with putting the 33 ohms in the feedback loop, and simulated an almost short circuit by putting a 1 ohm load. It shows that 10 amps is flowing out of the op-amp in this case. It doesn't look to be limited by the feedback.
Good grief.

That circuit is NOT what I described in post #2. Go back and read it again-- CAREFULLY.

Furthermore, your simulation showing 10 amps out of the op amp is not just wrong, it defies all common sense. Use a real simulator like LTSpice, not a newbie toy that gives absurd results.
 
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