Inductor value

Hi all,

I am building the attached circuit but am having problems sourcing the inductor choke rated for 10A. Just wondering is this a typo or is a 10A rated one necessary.

Is this Arduino related.
Why not ask on the website of the author.


I have tried what you have suggested but no response, have also done numerous internet searches but know answer.


These kind of designs are unfortunate because they are sort of outdated. The reason for the big choke is to protect the two transistors from over current peaks when they switch where the overlap current would go very high and most likely burn out the transistors if it were not for that choke.

Does it have to be rated 10 amps?
Well the winding primary is 7 and 7 turns and the secondary is 240 turns so it is a step up which means that the current in the primary is about 240/7 times higher than the output current. 210/7 is a nice round 30x, so with only 333ma on the output the input will be around 10 amps.

However, there appear to be capacitors on the output, and that means that the secondary current could actually go higher that if there was some resistive load.
So the limiting factors are just the transformer resistances and the MOSFET resistances and maybe a little wire resistance. That's not the best idea at all.
In fact, if you can stand an efficiency loss you can insert some resistance to keep the current lower, although it will take longer to charge the caps.
Of course the resistor(s) used have to be able to handle the power too, so you'll have to figure that into the sizing (10watts, 20watts, etc.).

Alternately you could look for a more modern design that uses a switching regulator IC and some PWM current limiting.

As it stands now though, if the choke current rating is too low it will most likely saturate and that would kill the transistors. It is hard to specify the actual max current though because it depends highly on resistances that are not shown in the schematic.

You can see the choke on the picture. The ferrite bar standing up in the middle of that circuit board.
Not too hard to wind yourself.
The author talks about the wire of a MOT (microwave oven transformer).
The fillament winding could be thick enough for that.

2309-H-RC Bourns Inc. | Inductors, Coils, Chokes | DigiKey ?

A terribly old-fashioned design? - don't think so - I bet there are millions built every day driving cold cathode backlights for tv's, laptops etc.

It'a a resonant royer topology running probably at a few tens of kHz - I've used it myself. They're very efficient.

TOKO and probably many others make special transformers for the job.

The inductor value looks plausible



I guess a two-resistor voltage divider is a "terribly old-fashioned design", but we haven't really found a better one.

Thank's all for your advise and explanation i will do more research to try and understand the circuit better. As for the wire from an MOT transformer the author was referring to the secondary on the flyback core for that. However all of your thoughts and advise is appreciated.

Yes, I read about the secondary wire, but a MOT transformer ALSO has a THICK fillament wire.
THAT could be used to wind the inductor on a ferrite rod.

A modern version of that circuit would use fewer components, most can be replaced by a small
microcontroller, if the MOSFETs are switched for logic-level ones.

As I understand it this circuit configuration isn't very efficient as the transformer secondary's
resistance acts as a current limiting resistor until the capacitor is near to charged. The transformer
has a fixed voltage ratio which means it can't be efficient until the capacitor voltage rises.

A flyback converter would avoid this problem and only need a single switching device, albeit a
high voltage one. A single inductor is simpler than a transformer too. Having said that its
hard to get high efficiency in any wide-ratio step-up converter.

The resonant Royer isn't as inefficient as you think - in charging the capacitor it acts a charge transfer pump, moving the charge on the resonating C to the load C.

That's why they're used

and of course the transformer can be any ratio you like - much better than trying to get a boost convertor with a single inductor to go from say 12 to 500 volts....



Yes i am aware that the primary winding of a MOT transformer has a thick wire as i have used one to build a simple portable spot welder. As the circuit calls for an inductor value between 47-250uH am i right in thinking that the lower this value is the cooler the FET'S will run hence the charge time will increase

The lower the value the less effective it will be at reducing shoot-through currents - higher values will
limit the rate of current spike build-up better.

Yes i am aware that the primary winding of a MOT transformer has a thick wire as i have used one to build a simple portable spot welder.

I hope you have used the fillament for the spotwelding, not the primary or secondary.

I hope you have used the fillament for the spotwelding, not the primary or secondary.

doesn't the usual MOT to spotwelder conversion involve a NEW hand-wound low-turns secondary winding? ~1000W at low voltage makes for a lot of current...