[R-390] 3DW7: A 3TF7 Tubester

[David Wise]

If it isn't the ultimate, it's certainly
as far as I can take it.  For me, it's a
tour de force.

As soon as I considered the "tubester" form factor,
I knew that DC regulation was out.  With a grounded
supply and load, half-wave rectification is necessary,
doubling the required reservoir capacitance.  You also
need a ground.  While it would be possible to contact
the shield's bayonet base, I found this distasteful.
The DC choices are (1) linear, and (2) high-frequency
switching.  In either case, the reservoir cap eats up
75% of the available space, leaving not enough for the
brains and RF filter (switching) or the heat sink (linear).

If the input to your pass device is other than DC,
you must measure true-RMS.  Those "clipped sinewave"
designs won't work without it.  Another idea is a
saturable reactor.  It's simple but way too big.

My first try used a forward phase-controlled triac,
with a light bulb and photocell as RMS sensor.  I
synchronized the control to the sine wave, using
an exponential ramp circuit of my own invention.
(At least, I haven't seen it anywhere else.)
This was marginally usable, but the light bulb kept
drifting.  Eventually I gave up on it and found an
RMS converter IC.  This worked great, but my
suspicions about RF noise were confirmed.
I changed to reverse phase control with controlled
fall time.  This doesn't put any detectable noise
into the receiver, but the parts count is high.
Even so, I was able to squeeze! it into the available
circuit board space to confirm it could be done.
That was months ago, when I first considered
announcing the 3DW7.  At that point it would
have been an analog design.

Dissatisfied with the density, I took the digital
leap and breadboarded up a microcontroller.
After months of "interesting" evenings debugging,
I got the program working really well.  It uses
power mosfets switched at zero-cross to stay quiet,
adjusts to voltage changes in one half-cycle without
overshoot, and (like its analog predecessor) powers
itself when not conducting, making it a two-terminal
device.  It factors its own power usage into the
computed load current.  It senses overloads and
short-circuits, so quickly that no fusing is required.
I don't have the facilities to test it, but I think
it will regulate ugly-shaped, frequency-varying
waveforms like what you get from a generator or inverter.

It needs neither ground nor shield.  The main heat
source is the current sense resistor.  It has recessed
Up and Down buttons on top for calibrating between
270mA and 320mA, and stores the setting in eeprom.
I'm figuring on a transparent plastic envelope.  No I
will not blow glass :-)  And sorry, it doesn't glow,
takes too much power.

This design could be adapted to a variety of
voltages and currents.

Regards,
Dave Wise

> -----Original Message-----
> From: Drew Papanek [mailto:drewmaster813@hotmail.com]
> Sent: Tuesday, December 24, 2002 12:02 PM
> To: r-390@mailman.qth.net
> Subject: [R-390] 3DW7: A 3TF7 Tubester
>=20
>=20
> Hello David,
>=20
> Runs cool and regulates great?  Sounds like the ultimate ballastube=20
> replacement!  I am envisioning a switching regulator or phase=20
> control type=20
> of arrangement possibly with controlled rise and fall times=20
> to minimize=20
> noise generation.
>=20
> All of this sophistication in a 3TF7-sized package-now that's an=20
> accomplishment!
>=20
> Drew
>=20
> On Mon, 23 Dec 2002 14:40:22 -0800 "David Wise"=20
> <David_Wise@Phoenix.com>=20
> wrote:
>=20
> >Subject: [R-390] 3DW7: A 3TF7 Tubester
> >
> >For the past six months I have been working on
> >a solid-state ballast the size of a 3TF7, and I
> >think I've done it.  It's a two-terminal device;
> >plug it in and go, no modifications whatsoever.
> >It runs cool and regulates great.  <snip>
>=20
>=20
>=20
> _________________________________________________________________
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