Simple supercapacitor protection circuit - Page 1
Forum section: electronics / beginners

author: bn
author_rank: Newbie
timestamp: January 08, 2020, 07:00:37 pm
content: Hi,recently I've been testing a simple supercapacitor protection circuit based on TLV431, such as added. The goal was to trigger discharging resistors when voltage exceeds around 2.9 V.It has worked fairly well in LTSpice, but real circuit misbehaves a little (all passives values and transistors were the same as in simulation). The problem is that response when voltage reaches 2.9 V TLV431 seems to not pull PNP's base hard enough to ground and as a result both transistors are barely conducting and very little current is sunk from source. Even when voltage is raised well above the threshold, cathode-anode voltage of TLV doesn't drop to expected ~1 V.On the other hand, I've also done a quick test with a lab supply. One channel was acting as a supercapacitor while the other was directly fed to ref pin of TLV431 (after removing voltage divider connections). This time when reference voltage was increased barely above 1.24 V circuit immediately started to sunk much more current while compared to tests with voltage divider. It seems like voltage divides is the problem but I can't figure out what's wrong.Any suggestions?Thanks in advance

author: Mr Evil
author_rank: Regular Contributor
timestamp: January 08, 2020, 10:07:53 pm
content: The circuit looks ok, so I suspect a wiring error or a broken component. Start with the minimum number of components that will do something. For this circuit, that would be U1, R1, R2 and R12. Test it by applying a voltage with your lab power supply, and see if it pulls down on R12 as expected. If that works, then add Q2, R3 and R6. Lastly, try it with Q1, R5 and R4.

author: GerryR
author_rank: Frequent Contributor
timestamp: January 08, 2020, 11:34:04 pm
content: I may be all wet here, but from the data sheet:"Slow or inaccurate responses can also occur when the reference pin is not provided enough overdrive voltage.This is the amount of voltage that is higher than the internal virtual reference. The internal virtual referencevoltage will be within the range of 1.24 V ±(0.5%, 1.0%, or 1.5%) depending on which version is being used.The more overdrive voltage provided, the faster the TLV431 will respond. This can be seen in Figure 27 andFigure 28 where it displays the output responses to various input voltages.For applications where TLV431 is being used as a comparator, it is best to set the trip point to greater than thepositive expected error (that is, +1.0% for the A version). For fast response, setting the trip point to > 10% of theinternal Vref should suffice.For minimal voltage drop or difference from Vin to the ref pin, TI recommends using an input resistor < 10 kΩ toprovide Iref."If your V1 is 2.9V, then the ref input is just setting at ~ 1.234 V,which is ~ equal to the internal reference, therefore sluggish behavior.Try changing the divider to Vref and see what happens.

author: bn
author_rank: Newbie
timestamp: January 10, 2020, 12:42:39 pm
content: @Mr EvilI tested the first stage with removed connection to Q2's base. At the threshold level, VKAwas successfully pulled down so I assume this part is fine. Next I added R3,Q2 and R6. Again it seems fine since voltage measured at R6 ,after crossing threshold, was pulled up to around V1 - 0.5 V. As it turns out final part might be the problem. With R5,R4,Q1 connected VKAis no longer ~1V, and Q1's VBEis ~0.7 V.@GerryRI am aware of that, but unfortunately even if reference input TLV is overdriven it's not working as expected.Thank You both for suggestions.