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Marko

Sat Feb 04 2012, 06:19PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi thomas,

Do you still get a simply shorted power supply? It's very hard to guess what else could be wrong this way.

What capacitance, voltage and other markings are on your capacitors? They should be MKP/FKP.

Also make sure you don't have a faulty feedback diode or something! If mosfets and diodes are OK and the circuit is properly wired I can't think of a reason why it wouldn't work, even if the caps aren't the best.

Make sure to measure the mosfets and diodes before installing them in order to avoid installing dead parts!

Apart from that, you should at least replace the steel bolts you screwed the copper pipe onto your board with brass, or best just solder the pipe directly if possible. Having steel in HF current path is intolerable as it introduces lots of resistance.

Marko

Legit_bacon

Sat Feb 04 2012, 07:07PM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Why are your MOSFETS not bolted onto your heatsinks?
They'll blow very quickly not being properly attached to there heatsinks.

THOMAS18

Sun Feb 05 2012, 07:41PM

Registered Member #4267 Joined: Fri Dec 16 2011, 09:17AM
Location:
Posts: 16

I had just put the board together and took the picture before screwing them down and soldering to the board. Its just as the mosfets osscillate I assumed they work working, iv added my circuit above as a Jpeg but cannot upload the eagle files. It must be something small I'm overlooking but I really can't figure it out. Thanks for the help. Marko could you list the parts you used? As I'm fairly sure I ordered all exact components, incase my first attempt was due to tollerances of components. Kind regards

THOMAS18

Wed Feb 08 2012, 11:30AM

Registered Member #4267 Joined: Fri Dec 16 2011, 09:17AM
Location:
Posts: 16

Iv, now soldered straight to board as suggested still no joy.
My 12v 4a supply drops from 12v dc to around 6v dc when board is connected and i enable switch.
The mosfets Oscillate but no power to a LED, with 15 wraps of wire around.
Or an exactly the same size 15cm diamater copper pipe atatched to an 24v bulb with a 6.8nf capacitor.
Iv drawn a simple version of my receiver to check its correct, basically just a bulb, capacitor and copper attatched?

THOMAS18

Wed Feb 08 2012, 11:31AM

Registered Member #4267 Joined: Fri Dec 16 2011, 09:17AM
Location:
Posts: 16

Recicver rough pic

Conundrum

Wed Feb 08 2012, 07:52PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Interesting re. steel versus brass.

Also plating silver onto the copper then spraying it with varnish would substantially decrease resistance at RF due to the skin effect. (see Radcom magazine)

Putting the capacitors inside the coil is not likely to cause a problem, they don't contain ferrous metal.

That coil looks like it would work in the high tens of MHz, IIRC the frequency used is 13.56 MHz (ISM band) but this does mean that the resonance of the receiver has to be spot on.

A good way to test these is to use a 'scope and connect its input to the baseband output of a TV tuner with a ramp generator on the sweep input and the tuner's 33V rail so that it can be used as a primitive spectrum analyser.

This allows even a cheap 'scope to show a large chunk of the RF spectrum on its small display.

See

Obviously what you will pick up is the 2nd or 3rd harmonic of the baseband 13.56 MHz but you can easily calibrate your analyser using a 4 pin can oscillator.

Pickup coil can be a simple short wire antenna...

-A

THOMAS18

Mon Mar 05 2012, 12:15PM

Registered Member #4267 Joined: Fri Dec 16 2011, 09:17AM
Location:
Posts: 16

I really am at a loss with this circuit at the minute. Iv tried different coild, and mosfets. I even tried to simply replicate marko s design, yet i still couldnt get a functioning circuit.
Does any one have a working board they could send at a price?
This could really help my troubleshooting.

Kind regards

Nathan

THOMAS18

Tue Mar 27 2012, 02:06PM

Registered Member #4267 Joined: Fri Dec 16 2011, 09:17AM
Location:
Posts: 16

Iv finally done this now!!
Could anyone help me with some cool suggested receiver circuits.

KInd Regards

Conundrum

Wed Mar 28 2012, 06:52AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi, btw double post..
Suggested receiver I'd like to try, use my and other's "reverse biased LED" tuner circuit.
the plan here is that you have an array of blue LEDs which are encapsulated in resin to keep out the light, in inverse parallel.
The centre point of each LED/capacitor string is connected via a blocking inductor to a variable DC source, this then allows tuning of the resonant point with varying distance etc.

Not sure if this approach has even been attempted but logic suggests it should work.
I would add some protection on the LEDs though as they don't like excess forward or reverse voltage.
5V max it seems would be a sensible limit.

-A

THOMAS18

Wed Mar 28 2012, 12:49PM

Registered Member #4267 Joined: Fri Dec 16 2011, 09:17AM
Location:
Posts: 16

Is this similar to a varactor diode?
Would you have time to sketch up a simple diagram on paint or something similar?
As I don't quite get the circuit ?
Kind regards

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