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RE: Ultrasonic heaters! (again)

Posted by Triode_Kingdom (A) on October 28, 2014 at 11:03:40

In Reply to: RE: Ultrasonic heaters! (again) posted by jorisdallaire on October 28, 2014 at 09:16:07:

"do you mean that the output is a rectified sinewave at mains frequency that is "chopped" by some square-wave generator afterwards? So adding a cap at the bridge output basically gives a DC supply that is then chopped?"

Yes, that's how these modules are configured. With no filter cap, the output from the module is essentially 120Hz chopped at the HF rate.

"Wouldn't an LC stage after the rectifier limit the raise in DC voltage?"

There are two issues with that. First, the choke would have to meet critical inductance at 120 Hz without incurring excessive loss. A value of 0.1 to 0.2H is probably about right for this. A more serious issue is the initial voltage overshoot when power is first applied to the LC. Depending on the DCR of the choke, that might be 50% or more of the static voltage. Personally, I wouldn't want to connect such a circuit to a filament. If regulation is your goal, a small SS device might be a better choice.

Remember that everything ahead of the HF output transformer is attached to the mains. These components must be well isolated from the chassis. Meeting that requirement will be more difficult as the circuit becomes physically larger.

"...you still need to drop voltage with power resistors?"

If the transformer is toroidal, and you don't mind rewinding both the primary and secondary, the exact ratio can probably be created for the filament. Otherwise, if you only unwind turns from the secondary, resistors will probably be required to "trim" the voltage.

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