# Voltage Regulation for charging Supercapacitor

Discussion in 'General Electronics Discussion' started by TechTyro, Mar 2, 2018.

1. ### TechTyro

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Aug 26, 2017
Hello Forum!
I am stuck due to a simple problem that arose in my project. I want to trim the voltage that a source produces without any significant loss in the current. I have attached a couple of pictures that roughly characterizes the source I am talking about. I want the output voltage to be regulated at around 2 Volts (2V plus or minus 0.1V)for at least the first few seconds until the source voltage drops below 2 volts or till the source voltage reaches zero. I also want a significant amount of the source current at the output side as I will use that to charge a supercapacitor. In any case, the output voltage must NOT go beyond 2.4 volts as it will damage my supercapacitor. I am looking for a circuit(s) that come in-between the source and the supercapacitor that fulfills the above requirement. Suggestions please....

2. ### Alec_t

2,970
805
Jul 7, 2015
What is the mystery voltage source?
What limits the charging current?
Does the cap have a rated maximum charging current?

3. ### TechTyro

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Aug 26, 2017
The source is the regenerated voltage from a DC motor. The voltage and current depend on the speed of the motor and the maximum voltage I got from my setup was 5 volts like in the attached picture. I would like to store this energy in a supercapacitor which has a maximum rated voltage of 2.5V. I am not sure about the rated maximum current but let's assume the cap can withstand 1A. The voltage regulation seems to be the problem. It's okay even if a reasonable amount of current is available at the output (100 - 300mA) at 2 volts for 1-2 seconds, I just want to charge the cap without damaging it with high voltage.

Last edited: Mar 2, 2018
4. ### Alec_t

2,970
805
Jul 7, 2015
What picture? Use the 'Upload a file' button to post it here. A schematic of your setup would be helpful.

5. ### Alec_t

2,970
805
Jul 7, 2015
Here's a primitive regulator which might suit you :-

6. ### TechTyro

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Aug 26, 2017
Thank You! This is what I was looking for. I'll surely try it. May I know the fuction of the transistor (FZT849) and the diodes (1N914) in the cicuit? and may I also know the name of the software used for the simulation?

7. ### Alec_t

2,970
805
Jul 7, 2015
The transistor acts as a shunt to pull the voltage applied to the cap down when it tries to go above a threshold value. In this circuit the threshold is set by D1, D2 and the transistor base-emitter junction, which are all in series. Since they each drop about 0.6V to 0.7V, the threshold will be about 3 x 0.65V = 1.95V. This will vary slightly with temperature. Note that R1 will get very hot. The transistor will probably need a heatsink. Other transistors suitably rated to handle the current and power could be used.
The sim software is the free LTspice from Linear Technology. This is used by many members here, so you can get help using it. It does have a steep learning curve, but it's well worth mastering.