Monday, 29 December 2014

Micro Tesla RF Coil

Directly inspired from a Hackaday article I saw I've made a Micro Tesla coil circuit.  To be honest it isn't a Tesla coil really it's an RF tuned circuit.  Here is a link to the original article:

Micro Tesla coil makes a perfect stocking filler

Here is the circuit diagram:

It's very similar to the flyback circuit I posted about before but on a much smaller and lower power scale.  The transistor basically turns on and off as fast as it can and energy is passed into the primary winding of the RF transformer.  The magnetic field in the primary winding is coupled into the secondary winding and this causes a very large voltage to be induced.  This voltage can be used to cause compact fluorescent light bulbs to glow - it's quite visually impressive.  The amount of RF field this circuit gives off will be high so don't use it near any sensitive equipment.  It may well affect LCD monitors causing them to flicker.

I did try and perform a simulation of the circuit but it didn't work as well as I would like so I'm not going to discuss it.  Getting simulators to model magnetic coupling well is difficult!

The transformer is actually a fancy pair of windings on a straight ferrite rod.  The ferrite rods are available from old FM radio receivers and switch-mode power supplies.  The original article shows how to find them and wind them.

Because it's me and I couldn't find any strip board, I made a small PCB.  Here is the layout:

MicroTesla PCB Top layer with Components

MicroTesla PCB Bottom Layer

Here are some renders of the populated PCB...just showing off the 3D capabilities of the EagleUP ULP!

Isographic image of populated PCB

Top view of populated PCB
The hardest part of realising this circuit is winding the transformer.  Wrapping 270 turns or more around ferrite core can be very tedious and hard to achieve.  To make it easier for me I designed a 3D printed stand!  Here is a render of the print from Sketchup:

MicroTesla Stand
Winding the coil is difficult and tedious...there is no real short cut or quick method unless you have a coil winder.  I didn't....What I did was cover the ferrite rod in insulating tape and then apply double sided tape on top of the insulating tape.  I then started winding...and winding....and winding until most of the ferrite rod was covered. I got the ferrite from an old FM radio and the really thin wire from an old relay coil that I had lying about - Raid the junk pile!

The components required are:

1x 22k 1/4W resistor
1x BC548C NPN transistor
1x 5mm LED (any colour)
3x 5mm Screw terminal connectors (optional
1x SPST or DPST slide switch 

For the RF transformer

1x old relay (for the really thin enamel coated wire)
1x Ferrite rod (old FM radio or similar) 
Double sided tape

Here are some photos of the construction of the PCB:

Cleaned single side PCB ready for ironing on the design

layout reasonable successfully transferred!

Into the etch tank!

The etched and drilled PCB

Once I had the ferrite and secondary winding complete I connected the end of the secondary to a pin so that it could easily be inserted into the screw terminals - the relay wire is very thin and breaks easily, be careful.

Here is a short video of the circuit in operation.

The circuit works...I didn't really get it to light a compact fluorescent light bulb but it works fine on the LED with a coil of ten turns.  It caused a CFL lightbulb to glow in one corner.  It isn't a circuit with practical uses apart from low power inductive charging and is similar to those used for mobile telephones.  It is difficult to transfer energy wirelessly.  The BC548 transistor gets incredible hot to the touch!  I had fun building this circuit and playing with it but I doubt I'll make another!

Cheers - Langster!


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