Trying to wirelessly power an LED.

[vk6kro]

Hi,
I'm not sure if this is enough to reproduce my results, but it may save you some work:
Turn off the phase plot and get both scales linear.

Version 4
SHEET 1 880 680
WIRE 464 96 336 96
WIRE -80 128 -128 128
WIRE 32 128 0 128
WIRE 192 128 112 128
WIRE 336 144 336 96
WIRE 336 144 304 144
WIRE 368 144 336 144
WIRE 464 144 464 96
WIRE 112 208 112 192
WIRE 112 208 32 208
WIRE 304 224 304 208
WIRE 336 224 304 224
WIRE 368 224 336 224
WIRE -128 240 -128 208
WIRE 32 240 -128 240
WIRE 192 240 192 208
WIRE 192 240 32 240
WIRE 32 272 32 240
WIRE 192 272 192 240
WIRE 336 272 336 224
WIRE 336 272 192 272
WIRE 464 272 464 224
WIRE 464 272 336 272
FLAG 32 272 0
SYMBOL voltage -128 112 R0
WINDOW 123 24 114 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 5 0
SYMATTR InstName V1
SYMATTR Value SINE(0 5 3000000)
SYMBOL ind2 16 112 R0
SYMATTR InstName L1
SYMATTR Value .0017
SYMATTR Type ind
SYMBOL cap 96 128 R0
SYMATTR InstName C1
SYMATTR Value 3e-12
SYMBOL res 176 112 R0
SYMATTR InstName R1
SYMATTR Value 1000
SYMBOL res 16 112 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 50
SYMBOL ind2 352 128 R0
SYMATTR InstName L2
SYMATTR Value 6.96e-6
SYMATTR Type ind
SYMBOL cap 288 144 R0
SYMATTR InstName C2
SYMATTR Value 422e-12
SYMBOL res 448 128 R0
SYMATTR InstName R3
SYMATTR Value 1000
TEXT -136 96 Left 0 !;tran 0.000001
TEXT -176 48 Left 0 !.ac lin 50000 1e4 5e6
TEXT -88 -24 Left 0 !K1 L1 L2 0.1

 

[Mike_In_Plano]

I worked on this problem about 12 years ago for a trans-dermal power supply (medical implant). On the primary side, the problem is one of getting sufficient flux linkage in the area of the pick up coil - N, I, and geometry.

For even a few 10's of mW at an inch away, the current in the primary side gets very high - on the order of an amp. To reduce losses on the primary side, the current in the primary recirculated through a parallel resonant tank circuit. Silver mica caps proved to have low loss for this application.

The secondary side was not tuned to resonate with the primary. Instead, it's parallel capacitor served to work with it's leakage inductance to obtain maximum power transfer. Since the coupling between primary and secondary coils would change with the application, the optimum value for the secondary capacitor would vary. Our pragmatic fix for this was to optimize the value for the poorest reasonable coupling - thus you get an improvement in transmission range at a cost in maximum power transfer.

I've attached a couple of drawings to communicate the basic idea and to give some idea as to why the power transfer varies with different orientations of the secondary.

Have fun,

Mike

 

[md5fungi]

vk6kro, I spiced the circuit using LTSpice and also Multisim. The results were satisfactory. I still, however, cannot measure the power correctly.

We've done all sorts of things to measure the power. We're calculating Real Power by using Vmax values and using the equation P = 1/2(Re(VI*)). Besides this, we've tried apparent power and RMS calculations.

We simplified the calculation by building a voltage doubler/rectifier on the output and measuring that as a DC value. No matter what we do, we measure more power on our receiver coil. We also made sure we are picking up no stray signals.

We really need a solution to this problem; we're not sure what's happening, we've tried multiple oscilloscopes/probes and approaches to the problem.

 

[vk6kro]

If you can be sure your resistor is 1000 ohms, you would measure the peak to peak value of the RF signal (that is the total amplitude of the waveform) divide by 2.828 to get RMS and then using V*V / R, calculate the power.

However I very much doubt that your resistor would be 1000 ohms at 3 MHz. I have an antenna analyser which can measure such things and I have seen resistors give very peculiar values at radio frequencies.

The best I have seen are surface mount resistors. At such low power, just a single 1000 ohm resistor would probably be OK. You have two 1 K resistors and I suggest you check or change them both.

Some metal film resistors seem OK. Others are really bad. I can't think of a way of testing them unless you have access to an antenna analyser.

Have you tried having two oscilloscopes, one on the transmitter and one on the receiver?
If you had an oscilloscope on the transmitter and then moved it to the receiver, this could remove a load from the transmitter which might then transmit more power than when you were doing the measurement.

A simple RF probe for an oscilloscope is to use a half wave voltage doubler
http://dl.dropbox.com/u/4222062/RF%20probe.PNG

This gives a reading close to the peak to peak value of the input signal. The capacitors I have shown as 22 pF should be as small (in pF) as possible to still get a good reading.

 

[md5fungi]

If you can be sure your resistor is 1000 ohms, you would measure the peak to peak value of the RF signal (that is the total amplitude of the waveform) divide by 2.828 to get RMS and then using V*V / R, calculate the power.

However I very much doubt that your resistor would be 1000 ohms at 3 MHz. I have an antenna analyser which can measure such things and I have seen resistors give very peculiar values at radio frequencies.

The best I have seen are surface mount resistors. At such low power, just a single 1000 ohm resistor would probably be OK. You have two 1 K resistors and I suggest you check or change them both.

Some metal film resistors seem OK. Others are really bad. I can't think of a way of testing them unless you have access to an antenna analyser.

Have you tried having two oscilloscopes, one on the transmitter and one on the receiver?
If you had an oscilloscope on the transmitter and then moved it to the receiver, this could remove a load from the transmitter which might then transmit more power than when you were doing the measurement.

A simple RF probe for an oscilloscope is to use a half wave voltage doubler
<circuit>This gives a reading close to the peak to peak value of the input signal. The capacitors I have shown as 22 pF should be as small (in pF) as possible to still get a good reading.

Wow, these are both very good points.

It's a good point that the resistors may behave differently at high frequencies, and possibly have a significant self-inductance.

I think we have access to antenna analyzers that will help us make some impedance measurements at HF.

It's also a good point that our oscilloscope may be loading our circuit, and that our circuit may deliver more power when it's not hooked up. I think we tried using two oscilloscopes simultaneously before, but then again, I'm not sure we were measuring power correctly.

Thanks for the help again, vk6kro, this is probably the most helpful response I've gotten so far.