# continuously charging capacitor circuit question

Discussion in 'General Electronics Chat' started by jiyluv, Feb 2, 2011.

1. ### jiyluv Thread Starter New Member

Aug 22, 2010
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Hello guys-

I just had a question about my charging circuit.

I used pspice simulator to design a simple full bridge rectified circuit that charges a capacitor, as shown in the attached pic, thinking that I will see it charge up incrementally to a very high voltage, since it will be just charging the capacitor. However, as you can see from the simulation readouts, the value doesn't rise above a certain voltage value. (I am hoping to see it rise to 15+ V, but it just stays at around 1.2 max, after 500ms)

Seeing that there is an initial rise in voltage, it means up to a certain value, the capacitor charges, but after that it saturates to a value and cannot go above it.

What do I have to change in this circuit to make the charge accumulate continuously without limit?

Some things I were thinking of that might be the reason and additional questions that came to mind were-
1. If the problem is that I am tying the charge circuit to ground, then how would i be able to simulate this circuit without supplying a ground?
2. is it that the capacitor I am using for this model is not a polarized one? If so, I couldn't find the polarized model- would it make a difference?

Thanks for your help in advance- you guys have been very helpful in the forums, and I appreciate it. Please be patient with me if my mistakes and comments sound elementary to you all.

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Sep 7, 2009
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3. ### Adjuster Well-Known Member

Dec 26, 2010
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Your circuit is a simple bridge rectifier. Its maximum output will be equal to the peak input voltage (2V) less two diode drops.

The 1.2V you obtained implies a diode drop of only 0.4V, which is actually very low for silicon diodes. Presumably this is explained by the fact that the capacitor is unloaded.

If you want to obtain a larger voltage using just diodes and capacitors, you might try using a voltage multiplier circuit.

How much voltage do you require, and at what current?

4. ### jiyluv Thread Starter New Member

Aug 22, 2010
13
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Hey guys, thanks for the feedback.

I am right now trying to achieve around 12V of generated voltage, at about 50-100mA current (but the current part is not certain- right now I am just trying to get the circuit to show constant charging).

Attached is a page of the paper published by a group at MIT involving an energy harvesting circuit. I can see that there exists separate circuitry that serves as a charge reservoir that exists until it reaches a certain voltage(using mosfet + zener).

If you see the waveform readout, it actually does pull up the energy until it reaches over 12-14 V.

Do you guys think the reason why their system actually can store and accumulate charge is because of the reservoir-like circuitry in addition to the initial rectifier circuit? Or any other suggestions?

thanks for the help!

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5. ### marshallf3 Well-Known Member

Jul 26, 2010
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First thing I don't like is that C1 appears to be far too small considering the bridge rectifier indicating that AC of unknown voltage/current isn't specified. I also see +5V specified into the TX-66 and while it's creative I believe it could be improved on as to that

It seems to me that you could simplify the heck out of this circuit, especially if you want to read more than a tag every 15 minutes.

Is this the receiver or transmitter part?

6. ### jiyluv Thread Starter New Member

Aug 22, 2010
13
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Yeah, for me right now its not even for reading a tag- I just want it to accumulate voltage for more than the amplitude of the source voltage. The circuit diagram attached as a pdf is the receiver- but the transmitter only consists of the piezo network, it seems.

Last edited: Feb 3, 2011
7. ### Ron H AAC Fanatic!

Apr 14, 2005
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The reason the voltage continues to rise in your PDF is that they are using a piezo voltage source, which puts put high voltage spikes. See Parasitic Power Harvesting in Shoes, which is a much more detailed report on the same project.

8. ### jiyluv Thread Starter New Member

Aug 22, 2010
13
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ah, thanks for the clarification!
oh so this piezo network provides high spikes in voltage to begin with that will enable such high accumulation of charge be possible..?

so it seems that the capacitor will only be able to charge up to the maximum peak to peak amplitude of the source voltage- is this right?

9. ### Ron H AAC Fanatic!

Apr 14, 2005
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Yes.

That is a fundamental property of capacitors.

10. ### jiyluv Thread Starter New Member

Aug 22, 2010
13
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thanks for the reply!