Electronics > Beginners
Protecting 2.7V supercapacitors in series, powered from USB 5V
JDP:
Hi all,
I'm very much a beginner and have no engineering or electrical background. So please excuse any apparent incompetency (but do of course let me know :)).
I'm currently working on a simple safe-shutdown implementation for a raspberry pi zero using two 10F 2.7V supercaps in series to provide reserve power while the pi shuts down.
Since I'm charging the caps in series from a 5V 1.5A USB power supply, I've gathered that i really should implement some level of protection. To do this I use this guide by Andreas Spies: http://www.sensorsiot.org/simple-and-cheap-way-to-protect-your-super-caps-video-139/
The relevant circuit diagram:
In the diagram two TL431X are used to shunt the caps in case of overvoltage (>2.7V), based on a reference voltage.
My main question is if it is correct that he uses two voltage dividers to generate separate reference voltages (R1-R2 and R4-R5) in stead of using just one voltage divider between 5V and ground?
And a side question. Wouldn't it be advisable to limit the current from the USB power supply with a current limiting resistor, or can one assume that the USB supply itself will limit the current?
I of course plan to add a diode to get rid of any backcurrent woes.
Oh, and in case you're curious. I plan on using a HT7044A-1 voltage detector with a pullup resistor to the pi's 3V rail connectet to its Vout go generate an active low signal for the pi to react to.
cheers
-JDP
David Hess:
--- Quote from: JDP on December 19, 2019, 08:10:52 am ---My main question is if it is correct that he uses two voltage dividers to generate separate reference voltages (R1-R2 and R4-R5) in stead of using just one voltage divider between 5V and ground?
--- End quote ---
I think the circuit as shown is overkill but TL431s are cheap. I would have just used a pair of resistors to make a voltage divider which slowly balances the capacitors. A single operational amplifier could have been used also to drive their common point at higher current with lower quiescent current.
--- Quote ---And a side question. Wouldn't it be advisable to limit the current from the USB power supply with a current limiting resistor, or can one assume that the USB supply itself will limit the current?
--- End quote ---
Yes, as shown the circuit violates USB specifications for capacitance and current draw. Some sort of current limiting should be used to isolate the capacitors from the USB supply.
schmitt trigger:
Your balancing circuit should be fine.
You will have to calculate R5 and R6 to ensure that under worst case conditions, no more that 100 mA are shunted by U1 and U2.
atmfjstc:
An interesting project. Will you be using some regulator downstream to boost the caps' voltage to 5V?
Zero999:
The TLE2426 would be a better solution: pin 2 "in" to +V, pin 2 (out) to both capacitors and pin 3 (common) to 0V.
http://www.ti.com/lit/ds/symlink/tle2426.pdf
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