Hi Declan!
> BTW, These diodes are a mains full wave rectifier, but have NO
> electrolytic smoothing it. That's pretty unusual on a single phase.
> That's where I'll start. I know, I know, electrolytics are useless at
> hf, but it might keep the voltage up across those diodes.
This might be on purpose!
We have some ultrasonic baths at our institute and from all of them
you at least hear an audible 50Hz (or better 100Hz) kicking - which
might be intentional.
At least I cannot see an electrolytic cap as a major factor when it
comes to production cost. On the other hand it might just be that -
another buck/quid saved on the circuitry... and for driving the
high-voltage oscillation through the piezos (I don't think you really
have "coils" under the bath, do you...) you surely don't need a
filtered mains voltage.
Did you have a look at the base and collector voltages with an
oscilloscope? You might see some ringing there, but even if not, the
faster, newer transistors (and I suppose we all agree on that) might
just lead to a steeper switching of the transformer, which will
increase the energy in the harmonics (there we are again...).
I guess it's more tricky to kill off the oscillations by artifically
slowing down the transistors without re-inventing the whole circuit,
but a semi-decent line filter should do the trick.
Now I am sitting in my office and I have one of those modern PFC power
supplies, which has a discrete line filter, instead of those metal
cans:
line
| |
( | | )
L1a( | | )L1b
( |_| )
| |
| |
o----o-| |-o----o
| | C1 | |
| | | |
|L2a( | | )L2b|
--- ( | | ) ---
C2a--- ( |_| ) --- C2b
| | | |
o----o-| |-o----o
| C3 |
| |
load
C1 0.47uF 300V~ X
C2a, C2b 2.7nF 250V~ ceramic
C1 0.22uF 300V~ X
L1a, L1b ~15 windings each on both sides of a toroid
L2a, L2b ~30 windings each on both sides of a toroid
The inductors are of the "common mode, open former" type, probably
around 0.5mH per coil....
Uwe.
--
Author: Uwe Zimmermann
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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