# Using Op-amp as comparator with low supply voltage (1.1v)

As part of a school physics project my group was given 2 TEC1-12706 and was told to find some way to generate electricity via Seeback effect and recycled materials. Due to low efficiency of the TEGs, we are only able to generate around 1.5~1.1V when connecting both TEGs in series during testing. Our idea is to connect the TEG to a pot and use it as an egg timer. Our circuit is as shown:

HA17393

At time = 0, the TEG will charge the capacitor.

Few seconds later, due to drop in temperature of hot water, Vse < Vcap, stopping charging of capacitor.

Few minutes later, the Vse < Vnon-inverting input (adjusted using the variable resistor), and op-amp's output will turn on, allowing the capacitor to turn on an LED, or some other device to indicate that the egg is cooked.

However, I am facing an issue with my circuit. Often, when the TEG voltage drops below the capacitor voltage, the capacitor begins to discharge quickly. This happens even if no output is connected to the op-amp. I suspect this is due to the supply voltage of the op-amp being less than stated in the datasheet (Vs = 2-36V). Is this indeed the case or have I made another design error in my circuit?

• "the capacitor begins to discharge quickly"- How quickly? What's the time constant? How does it compare to what you would expect from the resistor loading, the Schottky leakage and the quiescent current of the op-amp? Why use and op-amp as a comparator, why not get a low voltage comparator? – John D Mar 19 '18 at 15:40
• @JohnD that device IS a comparator. I added the spec-link. – Trevor_G Mar 19 '18 at 15:53
• @Trevor_G Thanks, I didn't check, I just read the problem statement that called it an op-amp. – John D Mar 19 '18 at 15:58
• @Trevor Yeah it's a bad way, unfortunately the school project requires us to use electricity generated from waste heat via a TEG. A 555 timer would not work due to low voltage. – Wet Feet Mar 19 '18 at 16:13
• @WetFeet, or connect the LED+R from Vcc to the comparator output and swap the inputs. – The Photon Mar 19 '18 at 17:40