Re: low-cost 1800-amp heating source

On Jul 30, 7:09 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Mon, 30 Jul 2007 16:46:03 -0700, the renowned MooseFET



<kensm...@xxxxxxxxx> wrote:
On Jul 30, 9:20 am, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Mon, 30 Jul 2007 07:04:35 -0700, John Larkin

<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Sun, 29 Jul 2007 17:58:01 -0500, Spehro Pefhany
<speffS...@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sun, 29 Jul 2007 13:47:28 -0700, the renowned John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sun, 29 Jul 2007 07:56:57 -0700, Winfield Hill <h...@xxxxxxxxxxx>
wrote:

Some evaporation sources need lots of current, but
have modest voltage requirements, for example
needing 400 amps, but dropping less than a volt.

I imagine one good way to get a high ac current
would be to pass a big fat wire through the center
hole of a toroidal 50 / 60Hz power transformer.

For example, AnTek makes nice low-cost toroidal
transformers,http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

These transformers have plenty of room inside their
center hole (1.5") to pass even several turns of big
wire, etc.

I purchased a 1kVA part, p/n AN-10265, for $100.
Using a one-turn secondary to measure the turns
ratio, I found 225 primary turns. This means we'll
get up to 530mV with a one secondary turn, 1.06V
with two turns, or 1.6V with three turns, and we
can draw up to 1875, 938 or 625A respectively.

The question then becomes, what's the best way
to control the total output current, to obtain a
precise oven temperature.

One of those Omega 1/4 DIN (or whatever) temp controllers should work.
Some of them do full-cycle zero-crossing burst control, sort of the AC
power equivalent of delta-sigma. SCR phase control is very nasty into
transformer loads.

John

Shouldn't be with a shorted secondary.. it should look pretty
resistive.

If the scr/triac conduction isn't perfectly symmetric, you can wind up
with net DC into the primary, which can get weird. And sometimes an
scr won't stay triggered when driving an inductive load, ie the
transformer leakage inductance. There are hazards, that's all.

John

Yup. A single SCR on a the DC side of bridge rectifier with the AC
side in series with the primary would assure symmetry.

This is assuming there is not mains ripple getting into the triggering
circuit.

Yeah, and that the diodes are pretty much the same.

At the voltages involved, we would have to use some strange diodes to
make enough mismatch to matter.


It only ensures that it is as symmetrical as the triggering
is. A triac running from that same trigger signal would be about as
good.

Triacs, especially those of the sensitive-gate persuasion, don't like
to commutate when there is much dv/dt.

Even an SCR would likely need a snubber in this circuit.

Alternistors are better, SCRs
are the best thyristor.

We could put a large capacitor in series to ensure no DC component.
It leads to some fun waveforms and interesting shopping trips at these
current levels, however.

I don't think it's an issue with a reasonable design. although
toroidal xfmrs are supposed to be worse in that regard than E-I lam
types.

Neither really do I. At worst we will have a small fraction of the
peak current as an average.


Heck, if you want to get fancy, why not detect any miniscule DC
component and actively null it?

I think that is a somewhat silly idea so lets do it :)

We know that the current stops when the SCR is off. We can use a
current transformer and restore the zero when the current stops. I
assume we are tripping the SCR with a tranformer so we can have the
whole circuit isolated from the mains.

The current sense transformer would drive a circuit like this:

-----/\/\---------
( !
( !
( !
! --/\/\---+
-GND ! !
! !
---/\/\-+-!-\ ! C1
( ! >----+---!!--+---!+\ To DC servo
( GND-!+/ ! ! >--+---
( / -!-/ !
---GND SW1 O ! !
! ------
GND

The first opamp lets us use a cheap design for the current transformer
and get a big enough signal to drive C1.

C1 is snapped to ground by SW1 when the SCR is off to reset the zero.

.

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