Circuit feedback with mutual inductance on a toroid

Question

I have been reviewing a circuit for the past few days and I am hoping to get some feedback to verify my thoughts as to how the circuit is working. This is a driving circuit for an ultrasonic cleaner and I am trying to figure out how to improve manufacturing consistency.

Here is the circuit:

There is a digital control circuit that simply turns the driving circuit on and off. I included a note for where it connects but I did not include the digital circuitry. I've also circled what I consider the most important component in the system (and also the one which I suspect is causing the most problems). This system is powered by a 120VAC 60Hz US outlet.

Here is what I have figured out so far:

1. When the user powers on the ultrasonic cleaning, the relay applies power to the driving circuit. The RLC oscillator must start up fairly quickly.

2. The RLC circuit made up of L5-A, C15 and R16 begins oscillating. This oscillating circuit then drives the transformer L7. From what I can tell, Q5 and Q6 are BU406 transistors with heatsinks.

3. L5-C is a feedback loop on the inductor and is used to auto-tune the oscillator circuit to the ideal driving point of the piezo transducer (42kHz ideal operating point). This is a hand wrapped inductor and I've included a picture below. As you can see, the white and yellow wrappings (L5-A and L5-B) are pretty good but the L5-C wrapping in red insulated wire is pretty loose.

I am trying to figure out why some of the units we get back from the manufacturer work well and why others have very weak cleaning power. I think the problem lies in the tuning feedback.

In order to drive the piezo element the circuit needs to drive the transducer at the frequency with the highest current draw. From what I understand L5-C is supposed to change the impedance of the L5-B and L5-A inductors to force the RLC circuits to oscillate at the right frequency. My concern is that manufacturing variations in the inductor wrapping put the tuning circuit out of the frequency range for which the auto tuning will work.

Does anyone here have any thoughts or advice on my analysis? Is there anything I am missing? Also, if anyone has any ideas for improving or tuning the circuit I'd be interested in hearing them. Let me know if I am missing any information and I'll update my question to include the missing information.

Thanks!

Answer 1

Toroids have one serious problem. Due to eddy currents there are peaks and dips in the magnetic flux around the core. The way this is avoided is to make sure each winding wraps around the entire perimeter of the toroid.

The red winding I see in the photo is bunched up in one spot. That spot could be a dead zone or a hot zone in terms of magnetic flux density. The chances of a more predictable output and/or transfer of energy is much greater if the red winding is spread out around the entire toroid.

Having wound toroids of my own, I have run into this very problem myself. You can test it by using a single turn that you can move around the toroid, with the turn connected to an oscilloscope. As you move the sense wire around the toroid you will see peaks and dips in signal strength.

Always have each winding spread out around the toroid as much as possible.