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wireless power circuit needs improvement

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mik3ca

Member
This is the circuit I have so far for wireless power: circuit.png
I tested it on a breadboard and so far I can get an LED to light up. This image is actually two circuits. The power provider (which takes 3/4 of the image) and the receiver (LED circuit) which takes up the top-left portion of the image.

When testing, the battery was measured at 7VDC, and what shocked me (thank god not physically) was the voltage coming out of the NPN emitter. I couldn't measure it right with the voltmeter, even at the 1000V setting. Sometimes the meter reads -1 and sometimes it gives a high number. Luckily, no parts were fried in the process.

The voltage at the power pins of the rectifier was measured at 3.6V when I have the insulating material of the inductors ending in UHB touching each other. They are 470uH axial.

I think a couple of ways I can increase the voltage (since I'm looking for about 8V output instead of 3.6V) is to increase the wireless power provider supply voltage and/or change the transistor or maybe increase the oscillator frequency?

What should I do?
 

Beau Schwabe

Active Member
What about a simple ZVS driver? The Primary coil will have the most power at it's resonant frequency.
I'll see if I can pull up a schematic for the video I made below.... the ZVS was modified so that a Center tapped coil was not necessary... instead, two resonant Capacitors were used
 

AnalogKid

Well-Known Member
Most Helpful Member
What are the diodes in bridge B1? They need to be high frequency switching types, not power rectifiers like a 1N4004.

ak
 

Les Jones

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I would have thought that the oscillator and both tuned circuits should be on about the same frequency. From a quick calculation the oscillator is about 100 Khz the tuned circuit in the transmitter (470 uH and 100 pF) is about 730 Khz. The tuned circuit in the receiver (470 uH and 1 nF) is about 230Khz. Maby someone else will check my calculations.

Les
 

mik3ca

Member
What are the diodes in bridge B1? They need to be high frequency switching types, not power rectifiers like a 1N4004.

ak
Mine is a DIP IC bridge rectifier DF04N. is there a DIP IC rectifier that has HF switching diodes or will I need to make my own bridge? and what about voltage? would high-speed diodes handle 1kV?

I would have thought that the oscillator and both tuned circuits should be on about the same frequency.
See I'm a bit fuzzy as to exactly what frequencies I should use for maximum power transfer.
 

mik3ca

Member
Now I have an issue. I hooked everything up except I changed the inductors labeled 470UHB to actual coils I think that are 350uH. They were advertised as 125khz RFID coils when I bought them. When I ran tests, there were problems with the transmitter. I measured the resistance of the coils and they read 4 ohms. I ran a diode test on the transistor (2n2222A) as per directions from https://www.techwalla.com/articles/how-to-test-transistors-in-a-circuit and saw no issues there. I tested voltages. at NPN base, the voltage is about 2, NPN collector voltage is about 0.1 and about the same with NPN emitter, yet I connected everything to the circuit board securely. Now when I ran an LTspice simulation, with the inductor disconnected at the collector, I get 18V output at the emitter, however in real tests, output shows nothing when tested with the voltmeter.

I'm expecting about 800+ volts at NPN emitter. everything worked on a breadboard but not on PCB.

Is there a certain way I need to make my design on PCB? I used 60mil traces with 20mil spacing and a ground plane.
 

alec_t

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Most Helpful Member
2V on the base and 0.1V on the collector and emitter? If those measured voltages are DC you have a dead transistor.
 

AnalogKid

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Are you sure the 555 is oscillating? If it isn't, the transistor will burn up.

ak
 

mik3ca

Member
I'm going to double-check the oscillator, and is the transistor still going to burn up with a malfunctioning oscillator even though I'm only giving about 90uA to the NPN base? and all my voltage measurements are in DC. I'm going to keep investigating because the only thing I found so far was the 470uH between collector and VCC turned into an open circuit.
 

mik3ca

Member
Now I'm not 100% sure if the oscillator is working, but I ran some tests on the oscillator output pin. I tested with an LED hooked both ways (one test where LED cathode is to output pin and anode to +ve through 1K resistor and one test where LED anode is to output pin and cathode to ground through 1K resistor) and in both tests, the LED appeared to stay on. The oscillator itself i believe runs around 200Khz. I use a 3.3K and 2.2K resistor with 1nF capacitor in the 555 timer circuit at time of testing.

The only other way I could test is by using a multimeter. I don't own an oscilloscope. Before thinking about replacing the transistor, is there anything else I can do?
 

dr pepper

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Most Helpful Member
If the osc runs at 200kc you'll probably be able to pick it up on a Am broadcast radio, the 3rd harmonic is 600kc near the bottom of the broadcast band, just hold the circuit near an am receiver and sweep the tuning if the osc is running you'll hear a dead spot on the dial.
It sounds like the main issue you have is tuning, the tx and rx coil tuning is critical, ie they need to have the same resonance, try fiddling with the capacitance across the rx coil, there will be a sweet spot where the o/p is greatest.
This is something I made at xmas, the truck receivers power wirelessly, the tx oscillator is just a joule thief, and the rx similar to yours.
 
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mik3ca

Member
Ok this is the funny part. when I made the whole thing with breadboards, things worked and I can pick up the power from either inductor. I tested with LTspice and the voltage from the collector inductor is slightly lower than in the emitter inductor but the values were still in the high hundred volt range even though my battery was 9V. I just got severe problems when I put everything onto PCB's. The only thing majorly different is that the inductors used for the power transfer are pre-wound coils used for RFID. I'm gonna see what happens when I go back to the inductors I originally used. Another thing to note, the original inductors (470uH) measured 10 ohms with an ohmmeter and the pre-wound coils measured 4.7 ohms. If switching the coils to inductors is the answer, then I'm gonna have to figure out how to make my own large-area coil so that I don't have to be ultra precise in touching insulation of the inductors together when it comes to powering my projects.
 

dr pepper

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Most Helpful Member
One of the principles of inductance is that the coil doesnt 'want' current through it to change, and if you interrupt current through it then it will reverse the voltage and step it up to whatever is required to maintain said current, even if that is a very high voltage.
Lager area coils will work better, pcb choke will not be as good, if anything they are meant to emit a low magnetic field as it would interfere with other devices on a board.
 

mik3ca

Member
They are meant to emit a low magnetic field as it would interfere with other devices on a board.
so I guess my design is a complete flop for having the inductors too close to everything else? If so, what minimum spacing should be recommended between the magnetic-field-causing inductors and the rest of the circuitry? (sorry, I'm trying to conserve on circuit board material as it tends to be expensive)

circuit.png
 

dr pepper

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A propriety choke most likely wont be designed to emit a magnetic field, if anything emision would be seen as parasitic so it makes sense designers/users would want this to be a minimum , it really depends on the inductor and of course theres bound to be some emission, just most likely not as good as a coil meant to do that.
Looking at your layout I'd say from the size your coils indeed will not have that great a coupling, if you look at my project, the coils are air cored and wound over 3 fingers, the physical size gives better coupling over a distance (10mm), but its still naff most of the energy is wasted.
Your boards are not necessarily scrap, you could just unsolder the chokes and place a coil around the outside of the board, your circuit will most likely work perfectly electrically, the chokes on the board are very unlikely to intefere with the likes of a 555.
If you look at circuits designed to charge phones and the like you'll see the coils are much bigger than a pcb choke, in the region of 4".
 
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mik3ca

Member
So I'm free to use a larger value of inductance and the thing can work only if I have a super huge coil away from the board but still connected? and do I need special wire to make such inductor? Theres a store near me that sells standard hookup wire and also wire with special red coating that is normally used inside motors
 

mik3ca

Member
Oh... I came across a website that also uses a 555 timer for the oscillator but he is running it at a lower voltage and decided to connect 555 output directly to NPN base. Here's the link:
http://www.theorycircuit.com/wireless-power-light-array-using-ic555/

Is connecting the oscillator output directly to NPN base going to overheat the transistor? The only resistance I think I have stopping the path to ground then is the 10 ohms and 470uH inductor, or do I calculate inductor reactance as an additional resistor value based on 555 timer frequency and add that to 10 ohms?
 

dr pepper

Well-Known Member
Most Helpful Member
That is my opinion yes.
Larger coils tend to go further.
You can connect the tx coil direct to the transistor, however the coil and frequency are critical, if the inductance is too low the transistor will blow up, if too high very little energy will be transferred.
 

mik3ca

Member
...if the inductance is too low the transistor will blow up, if too high very little energy will be transferred...
Then based on my observations, 470uH is too high since the transistor isn't broken after measurements. So I guess I sum the reactance of both inductors and add my 10 ohm resistor in series and make sure the current based on total resistance doesn't exceed NPN collector current defined in the transistor datasheet?
 
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