Why don't ultrasonic cleaners use simpler circuit? - Page 1
Forum section: electronics / beginners

author: TNb
author_rank: Regular Contributor
timestamp: June 07, 2017, 09:48:25 am
content: I've read some stuff on ultrasonic cleaners recently and discovered that they use quite strange (for me) circuit.Here is an example -http://jumperone.com/2012/02/ultrasonic-cleaner-re/(the pdf of circuit -http://jumperone.com/files/2012/02/ultrasonic-cleaner-re/CD7810_Ultrasonic_Cleaner.pdf)It seems like it is sort of class B amplifier, but the frequency is set by some kind of self-oscillating circuit based on 3 coils on a toroidal core. I think that it would be a lot simpler to just take some IC like 555 timer and configure it to produce 42 kHz signal to amplifier, that way we don't need that beefy toroid and possibly we can even omit LC filter on the output. Why manufacturers decide to use toroid circuit? Seems to be more expensive and harder to assemble.

author: coppice
author_rank: Super Contributor
timestamp: June 07, 2017, 10:01:03 am
content: Ultrasonic transducers for driving liquids are high Q. They need to be driven bang on their resonant frequency. Therefore they are usually driven by something that self adjusts it's drive frequency to match the exact resonance of a particular sample of transducer.

author: TNb
author_rank: Regular Contributor
timestamp: June 07, 2017, 10:09:53 am
content: So if I replace it with 555 timer I either need to adjust frequency or I don't hit resonance and output less power? bummer...

author: dmills
author_rank: Super Contributor
timestamp: June 07, 2017, 10:16:35 am
content: That toroid supplying the feedback is not particularly beefy bearing in mind that it is just supplying the base drive.The real advantage of the circuit as given is that it is self tuning to the series resonance of the transducer, because the feedback is taken from the transducer current, so nothing to adjust.The transducers are narrow band, so you would have to tune the thing if it was not self adjusting.L4 which is fairly large you would need anyway (It resonates with the transducer + C19 capacitance) as otherwise you will have a huge amount of reactive power flowing due to the fixed transducer capacitance, which will cause excessive heating in the power switching transistors, it also doubles as a voltage step up by forming an L network with the transducer, I would guess you are probably getting 500V or so across the piezoceramic.A 555 would also require a low voltage rail referenced to something sane, which is more power supply stuff to deal with, this thing runs directly off the rectified mains. There is sometimes something to be said for clever discrete circuits.C6,15,20 probably want to be rated for more then 250V however, unless that is an AC rating, and D10 is clearly redundant....

author: coppice
author_rank: Super Contributor
timestamp: June 07, 2017, 10:17:15 am
content: Even if you tweak up the frequency of your 555 to match the transducer, the resonance may shift with temperature. Are you sure the circuit you are looking at is class B? Class C is a popular approach.

author: TNb
author_rank: Regular Contributor
timestamp: June 07, 2017, 10:29:36 am
content: hm... yea, it makes more sense now, especially with temperature fluctuations, thanks!I guess it is time to learn more about self-adjusting circuits

author: dmills
author_rank: Super Contributor
timestamp: June 07, 2017, 10:47:03 am
content: My bet is class C once the thing is up and running, the C/B resistors just serving to provide some bias during startup to provide sufficient small signal gain to get the thing going.Technically that probably makes conduction angle just greater then 180 degrees, so AB, for small signal but probably class C once up and running.Regards, Dan.

author: wraper
author_rank: Unknown
timestamp: June 07, 2017, 10:50:42 am
content: Actually this is the most basic circuit. Decent ultrasonic cleaners have significantly more complicated circuit. 555 would only increase component count (high current PSU for it and output transistor driver, most likely you'll still use transformer), and you need circuit to work at transducer/tank resonant frequency in the first place.

author: TNb
author_rank: Regular Contributor
timestamp: June 07, 2017, 11:51:39 am
content: Quote from: wraper on June 07, 2017, 10:50:42 amMy train of thought was that toroidal cores are more expensive and you need to manually wind them, which seems to be more effort when producing it in quantities of thousands. I thought getting a box of 555 times is dirt cheap and easier to assemble.Now I see that extra component would probably add up to a price of toroids and labor. Though I suppose the main reason is of course hitting the resonance frequency, so it makes perfect sense now.

author: dmills
author_rank: Super Contributor
timestamp: June 07, 2017, 01:08:28 pm
content: Toroid winding machines are standard fare in the magnetic components industry, no need to hand wind.Even in very modest quantities (100's) getting a specified toroidal core wound with a specified number of turns is easy, and getting it wound to a specified inductance and tolerance is only slightly more expensive.One of the things that always surprises beginners is how quickly paying for a run of a custom part becomes cost effective in a production situation, custom magnetics are an expensive pain when building one, when doing a run of even 100 units it is cost effective and by the time you take a thousand units you are paying pretty much parts cost and machine time plus profit.Regards, Dan.

author: ConKbot
author_rank: Super Contributor
timestamp: June 07, 2017, 01:09:52 pm
content: "You need to manually wind them" nope!http://www.coilwindingmachines.eu/toroid_winding_machines/toroid_winder_wh900_sample_winding.html.1200 turns put down quite quickly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2017, 03:41:41 pm
content: Not complicated at all -- looks to be very much the same as a typical CCFL driver circuit (which is also resonant, but for different reasons). It's simply a class D self-excited, current driven half-bridge oscillator circuit.An interesting feature of that arrangement is the BJTs are driven at constant hFE, determined by the feedback transformer ratio. One amp of load current, into the primary of a 1:N transformer, results in 1/N amps through the secondary, which goes into the B-E circuit, thus driving the transistor at a forced hFE = N.In practice, the transformer is not ideal, and leakage and winding inductance reduce the current, particularly after spending some time in one state or the other -- the inductance steals base current, increasing forced hFE over time. Eventually, forced hFE > actual linear hFE and the transistor begins to turn off. The stored inductive current acts to turn the transistor off suddenly, and turn on the opposing transistor, resulting in a sharp switching waveform.FWIW -- to do any better, say with a traditional digital SMPS circuit -- you need a full PLL + error amp + VCO + driver circuit. This effectively emulates the above oscillator with a control loop. The added complexity is really only justified for circuits with a wide operating range (an inductive cooktop might benefit from this approach), or high power (where the tighter control is combined with protective circuitry to make a safe, reliable and efficient system, justified by economy of scale, and safety).Tim

author: james_s
author_rank: Super Contributor
timestamp: June 07, 2017, 05:07:42 pm
content: If you want to build one yourself, you don't *have* to use a toroidal core, one of the various split core types ought to work too. As others have said it really does have to be self adjusting to keep the thing in resonance, there are many factors that will affect the precise resonant frequency.

author: CopperCone
author_rank: Super Contributor
timestamp: June 08, 2017, 06:56:32 am
content: I wonder how safe it is to put the 1Macross the X2 Capacitor.I'm guessing zener diodes might be more survivable then a linear regulator. everything that I would have transformer coupled is capacitor coupled...

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2017, 10:34:07 am
content: Quote from: coppice on June 07, 2017, 10:17:15 amThe mechanical load in the tank significantly changes the resonate frequency and this happens when the fluid level is changed.The transducer can be driven outside of its resonate frequency and more complex designs sweep the frequency around the resonate frequency but it is less efficient.

author: GreyWoolfe
author_rank: Unknown
timestamp: June 08, 2017, 11:40:07 am
content: Stupid question here, but don't you want a variable frequency? I've seen it said that fixed frequencies could cause components to be vibrated off and a variable frequency mitigates that. I look to be enlightened.

author: ed_reardon
author_rank: Regular Contributor
timestamp: June 08, 2017, 11:53:49 am
content: With regard to custom parts my ex-father-in-law owns a short run inductor and transformer outfit, I was quite staggered how inexpensive (comparibly) some components were sold for, clearly more expensive than 'off the shelf' items but much less than I would expected an order of say 100 custom transformers would have cost.Anyway a very useful thread for me, I'm very interested in this subject so I'll leach away!

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2017, 08:02:51 pm
content: Quote from: GreyWoolfe on June 08, 2017, 11:40:07 amIt is a nice feature for better performance but not absolutely necessary.