Wireless Power Transfer Circuit

Thread Starter

Roessleralec

Joined Jul 16, 2019
28
Good Afternoon,

Recently I have made a wireless power transfer circuit for a project of mine. The goal is to illuminate an LED roughly an inch away. To do this I used a 555 timer to create a square wave that drove an NPN transistor with a 28.5 uh coil. Testing on the breadboard it worked fine but as soon as I made a PCB for it, the distance it worked decreased dramatically. Now, it is spotty and I am lucky if it works up to half an inch. Initially, I thought that with the PCB traces capacitance was added that affected the resonance but after measuring with a multimeter it hadn't changed. Below I have attached the schematic and PCB files. If anyone has any advice I greatly appreciate it.

PS: One thing that has eluded me is why the signal I measure from the output of the timer is roughly 60khz. This doesn't match what the values are. Also, it is worthy to note that when driven with a function generator, the circuit performs well in this range. (50-70khz)
 

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Papabravo

Joined Feb 24, 2006
16,773
Good Afternoon,

Recently I have made a wireless power transfer circuit for a project of mine. The goal is to illuminate an LED roughly an inch away. To do this I used a 555 timer to create a square wave that drove an NPN transistor with a 28.5 uh coil. Testing on the breadboard it worked fine but as soon as I made a PCB for it, the distance it worked decreased dramatically. Now, it is spotty and I am lucky if it works up to half an inch. Initially, I thought that with the PCB traces capacitance was added that affected the resonance but after measuring with a multimeter it hadn't changed. Below I have attached the schematic and PCB files. If anyone has any advice I greatly appreciate it.

PS: One thing that has eluded me is why the signal I measure from the output of the timer is roughly 60khz. This doesn't match what the values are. Also, it is worthy to note that when driven with a function generator, the circuit performs well in this range. (50-70khz)
Using the values from the schematic, what frequency would you expect?
 

BobTPH

Joined Jun 5, 2013
3,652
Actually, it is probably lower capacitance on the PCB that detuned it. Breadboards have lots of stray capacitance.

Bob
 

Thread Starter

Roessleralec

Joined Jul 16, 2019
28
Checking the capacitance on the breadboard reads around 290pf and on pcb is 224pf. Adding capacitance to increase it to 290 seems to work slightly but not noticeably enough to compare to before.
Thanks
 

Papabravo

Joined Feb 24, 2006
16,773
Checking the capacitance on the breadboard reads around 290pf and on pcb is 224pf. Adding capacitance to increase it to 290 seems to work slightly but not noticeably enough to compare to before.
Thanks
Measuring low values of capacitance like that is a tricky proposition. You need LAB quality instruments to get reliable readings. Capacitor tolerance is another thing to account for.
 

Thread Starter

Roessleralec

Joined Jul 16, 2019
28
With that said, do you think the issue is of an incorrect value of capacitance that doesn't put the circuit into complete resonance?
 

Papabravo

Joined Feb 24, 2006
16,773
With that said, do you think the issue is of an incorrect value of capacitance that doesn't put the circuit into complete resonance?
I don't think resonance has anything to do with this problem or this circuit. The two resistors and the capacitor determine the time it takes to charge and discharge the capacitor from 1/3*Vcc to 2/3*Vcc. That is not resonance that is exponential attack and decay. The equation is simple, straightforward, and requires nothing more than algebra for a solution. You can verify the waveforms with a scope if you have one. The frequency is low enough that RF effects are limited.

One more thing to keep in mind is that the datasheet mentions limits on capacitor and resistor values. When you violate these restrictions you are essentially venturing into "no man's land". 200-300 pf seems like it might be on the small side.

One more thing: It looks like the schematic has two 680 Ω resistors and a 6.8 nF capacitor. If that is the case why are we talking about 200-300 picofarads?
 

Thread Starter

Roessleralec

Joined Jul 16, 2019
28
What I meant by resonance was in the second stage of the circuit. This includes the coils and capacitors that operate in resonance. Sorry if I wasn't clear.
 

Papabravo

Joined Feb 24, 2006
16,773
What I meant by resonance was in the second stage of the circuit. This includes the coils and capacitors that operate in resonance. Sorry if I wasn't clear.
Yeah - I have no idea what you may or may not expect with this experiment. I do believe that disappointment is highly likely since I know of no reliable theory to predict results.
 
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