# Monitor Super Capacitor Voltage

Discussion in 'The Projects Forum' started by bweiss, Apr 11, 2014.

1. ### bweiss Thread Starter New Member

Mar 20, 2014
2
0
Hello All,

I am trying to implement a design for a Super Capacitor charger that will monitor the voltage on the super capacitors while they are charging using an Analog to Digital converter from an 8051 microcontroller. The attached image shows what I am basically trying to do. I want to replace ?1 and ?2 with whatever will work, can be resistors, capacitors, transistors, op amps, etc.

Thanks for any help!!!

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2. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,558
2,142
Make sure your 8051 will run on 3.3V. Older ones require 5V.

?1 - This can be a simple 3-terminal regulator like an LM217 if you can dissipate 5 watts. For better efficiency, a small switcher circuit like a National Semi (not TI) Simple Switcher can do this and run cool.

?2 - Why does this need to deliver 0.5 A? If this is just driving the input to the A/D, it can be two resistors and a bypass cap.

ak

3. ### bweiss Thread Starter New Member

Mar 20, 2014
2
0

That voltage regulator is exactly what I was looking for I think. I am going to try it out and see if it works. As for the ADC input side I was thinking a bit to hard about it I think. A simple resistor network should work fine as long as I can limit the current. I was thinking of using an op amp to bump the voltage down to 2.4 volts max as my A/D converter is setup to only read a max of 2.4 volts. In this case would a 1kohm resistor on the input and a 470 ohm resistor for Rf be sufficient to bump 5.2 volts down to 2.4 volts?

Vo = -(Rf/R1)Vi

Would that 1kohm resistor also limit the current enough to prevent it from ruining the opamp/8051?

Thanks.

Last edited: Apr 11, 2014
4. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,558
2,142
You describe an inverting amp, so your output will try to be *minus* 2.4V. If you need an opamp to assure a nice low source impedance for the A/D input, use two resistors to divide your input voltage down by 44% (1.00K and 1.27K) and configure the opamp as a voltage follower. The opamps internal current limiting should protect itself and whatever it is driving, although I would think the load current into an A/D would be microamps.

ak