squaring up a sinusoidal signal

On Sun, 29 Aug 2010 20:22:04 -0400, "Michael Robinson"
nospam_at_billburg.com> wrote:


"John Fields" <jfields_at_austininstruments.com> wrote in message
news:8i1l7697li0fouesu23qmpkk8fcumageug_at_4ax.com...
On Fri, 27 Aug 2010 16:18:25 -0400, "Michael Robinson"
nospam_at_billburg.com> wrote:


Before I go any further, let me describe the project. I'm designing a
low
power (no more than 200 watts) induction heater running on 12 volt
power,
to
heat steel tubing. The induction coil will be wound permanently around
the
steel tubing workpiece, and I intend to keep the tubing temperature
below
200F. So the load is in effect invariant. I'm lookng to use a series
(voltage-driven) tank. Instead of using a CR to sense the current in
the
work coil, I thought of sensing the voltage across the tank capacitor.
If
my thinking is correct, for the inverter drive to be in phase with the
tank
current, it needs to be 90 degrees out of phase with the capacitor
voltage.
And it should really switch a few degrees before that to keep the load
on
the mosfets inductive. The RC filter I described accomplishes that by
providing a phase shift (delay) of slightly less than 90 degrees. It
will
also attenuate the high voltage of the tank. And it's dirt simple.
If I use a PLL to get a phase shift of 90 degrees, then switching may
actually occur slightly late; bad for the mosfets.
I don't know enough about the 4046 to respond very cogently to your
suggestion about the XOR comparator. Just reading those datasheets is
like
picking my way through heavy underbrush. I know guys have built
kilowatt
induction heaters without PLL at all, so I'm not attached to the idea of
PLL, just willing to consider it.

---
Two questions:

1. Can you use dual supplies?

No, there's just a single power source available.

2. Can the thing be self-oscillating or does it have to be driven by
an external source?

Whatever works. I think with induction heaters usually there's a loop
that
locks the inverter in phase with the sinusoidal current in the tank (work
coil/capacitor), but with this unvarying load perhaps I could just
determine
the correct frequency empirically and set it to run at that frequency,
open
loop.
Does that answer your question?

---
Yes.

Just a thought:

Version 4
SHEET 1 1476 916
WIRE -224 -336 -496 -336
WIRE 32 -336 -224 -336
WIRE 208 -336 32 -336
WIRE 32 -288 32 -336
WIRE 208 -240 208 -336
WIRE -112 -192 -304 -192
WIRE 32 -160 32 -208
WIRE 160 -160 32 -160
WIRE -224 -144 -224 -336
WIRE -304 -128 -304 -192
WIRE -256 -128 -304 -128
WIRE 32 -128 32 -160
WIRE -112 -112 -112 -192
WIRE -112 -112 -192 -112
WIRE -256 -96 -352 -96
WIRE -224 -48 -224 -80
WIRE 32 -16 32 -64
WIRE -112 96 -112 -112
WIRE -112 96 -304 96
WIRE 32 96 32 48
WIRE -496 128 -496 -336
WIRE -224 128 -496 128
WIRE -224 160 -224 128
WIRE -304 176 -304 96
WIRE -256 176 -304 176
WIRE -112 192 -112 96
WIRE -112 192 -192 192
WIRE 32 192 32 160
WIRE 32 192 -112 192
WIRE 208 192 208 -144
WIRE 352 192 208 192
WIRE -352 208 -352 -96
WIRE -256 208 -352 208
WIRE -352 224 -352 208
WIRE 32 224 32 192
WIRE 352 240 352 192
WIRE -352 320 -352 304
WIRE 352 320 352 304
WIRE 352 320 -352 320
WIRE 32 368 32 288
WIRE 352 384 352 320
WIRE 32 448 32 432
WIRE 208 528 208 192
WIRE 32 544 32 512
WIRE 160 544 32 544
WIRE 352 544 352 464
WIRE -496 576 -496 128
WIRE 32 592 32 544
WIRE -496 720 -496 656
WIRE -224 720 -224 224
WIRE -224 720 -496 720
WIRE 32 720 32 672
WIRE 32 720 -224 720
WIRE 208 720 208 624
WIRE 208 720 32 720
WIRE 352 720 352 624
WIRE 352 720 208 720
WIRE -496 816 -496 720
FLAG -496 816 0
FLAG -224 -48 0
SYMBOL voltage -496 560 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL res -368 208 R0
WINDOW 0 -38 37 Left 0
WINDOW 3 -62 80 Left 0
SYMATTR InstName R3
SYMATTR Value 1e6
SYMBOL nmos 160 -240 R0
SYMATTR InstName M1
SYMATTR Value Si7868DP
SYMBOL pmos 160 624 M180
SYMATTR InstName M2
SYMATTR Value FDS4465
SYMBOL res 16 -304 R0
SYMATTR InstName R6
SYMATTR Value 510
SYMBOL res 48 688 R180
WINDOW 0 36 76 Left 0
WINDOW 3 36 40 Left 0
SYMATTR InstName R8
SYMATTR Value 510
SYMBOL diode 16 -128 R0
WINDOW 3 36 32 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D3
SYMBOL diode 16 448 R0
WINDOW 3 41 30 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D1
SYMBOL diode 16 96 R0
WINDOW 3 37 34 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D2
SYMBOL diode 16 224 R0
WINDOW 3 41 32 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D4
SYMBOL diode 16 -16 R0
WINDOW 3 41 31 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D5
SYMBOL diode 16 368 R0
WINDOW 3 43 32 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D6
SYMBOL ind2 336 368 R0
WINDOW 0 61 31 Left 0
WINDOW 3 39 65 Left 0
SYMATTR InstName L1
SYMATTR Value 1.59e-5
SYMATTR Type ind
SYMBOL cap 336 240 R0
WINDOW 0 -39 5 Left 0
WINDOW 3 -84 59 Left 0
SYMATTR InstName C2
SYMATTR Value 1.59e-7
SYMBOL res 336 528 R0
SYMATTR InstName R1
SYMATTR Value .2
SYMBOL Opamps\\LT1498 -224 -176 R0
SYMATTR InstName U2
SYMBOL Opamps\\LT1498 -224 128 R0
SYMATTR InstName U1
TEXT -480 760 Left 0 !;ac oct 256 20 1000000
TEXT -480 792 Left 0 !.tran .001 startup

I just picked a couple of MOSFETs out of LTC's menu based on Vds(on)
and Rds(on), and an LT1498 because it can source/sink about 30 mA.

There are two in a package, and I'm using them as unity-gain buffers,
so I connected them in parallel instead of sparing one.

---
JF

"John Fields" <jfields_at_austininstruments.com> wrote in message
news:mr6n76h7ps46lil100mbe01vue0ehu2c79_at_4ax.com...
On Sun, 29 Aug 2010 20:22:04 -0400, "Michael Robinson"
nospam_at_billburg.com> wrote:


"John Fields" <jfields_at_austininstruments.com> wrote in message
news:8i1l7697li0fouesu23qmpkk8fcumageug_at_4ax.com...
On Fri, 27 Aug 2010 16:18:25 -0400, "Michael Robinson"
nospam_at_billburg.com> wrote:


Before I go any further, let me describe the project. I'm designing a
low
power (no more than 200 watts) induction heater running on 12 volt
power,
to
heat steel tubing. The induction coil will be wound permanently around
the
steel tubing workpiece, and I intend to keep the tubing temperature
below
200F. So the load is in effect invariant. I'm lookng to use a series
(voltage-driven) tank. Instead of using a CR to sense the current in
the
work coil, I thought of sensing the voltage across the tank capacitor.
If
my thinking is correct, for the inverter drive to be in phase with the
tank
current, it needs to be 90 degrees out of phase with the capacitor
voltage.
And it should really switch a few degrees before that to keep the load
on
the mosfets inductive. The RC filter I described accomplishes that by
providing a phase shift (delay) of slightly less than 90 degrees. It
will
also attenuate the high voltage of the tank. And it's dirt simple.
If I use a PLL to get a phase shift of 90 degrees, then switching may
actually occur slightly late; bad for the mosfets.
I don't know enough about the 4046 to respond very cogently to your
suggestion about the XOR comparator. Just reading those datasheets is
like
picking my way through heavy underbrush. I know guys have built
kilowatt
induction heaters without PLL at all, so I'm not attached to the idea of
PLL, just willing to consider it.

---
Two questions:

1. Can you use dual supplies?

No, there's just a single power source available.

2. Can the thing be self-oscillating or does it have to be driven by
an external source?

Whatever works. I think with induction heaters usually there's a loop
that
locks the inverter in phase with the sinusoidal current in the tank (work
coil/capacitor), but with this unvarying load perhaps I could just
determine
the correct frequency empirically and set it to run at that frequency,
open
loop.
Does that answer your question?

---
Yes.

Just a thought:

Version 4
SHEET 1 1476 916
WIRE -224 -336 -496 -336
WIRE 32 -336 -224 -336
WIRE 208 -336 32 -336
WIRE 32 -288 32 -336
WIRE 208 -240 208 -336
WIRE -112 -192 -304 -192
WIRE 32 -160 32 -208
WIRE 160 -160 32 -160
WIRE -224 -144 -224 -336
WIRE -304 -128 -304 -192
WIRE -256 -128 -304 -128
WIRE 32 -128 32 -160
WIRE -112 -112 -112 -192
WIRE -112 -112 -192 -112
WIRE -256 -96 -352 -96
WIRE -224 -48 -224 -80
WIRE 32 -16 32 -64
WIRE -112 96 -112 -112
WIRE -112 96 -304 96
WIRE 32 96 32 48
WIRE -496 128 -496 -336
WIRE -224 128 -496 128
WIRE -224 160 -224 128
WIRE -304 176 -304 96
WIRE -256 176 -304 176
WIRE -112 192 -112 96
WIRE -112 192 -192 192
WIRE 32 192 32 160
WIRE 32 192 -112 192
WIRE 208 192 208 -144
WIRE 352 192 208 192
WIRE -352 208 -352 -96
WIRE -256 208 -352 208
WIRE -352 224 -352 208
WIRE 32 224 32 192
WIRE 352 240 352 192
WIRE -352 320 -352 304
WIRE 352 320 352 304
WIRE 352 320 -352 320
WIRE 32 368 32 288
WIRE 352 384 352 320
WIRE 32 448 32 432
WIRE 208 528 208 192
WIRE 32 544 32 512
WIRE 160 544 32 544
WIRE 352 544 352 464
WIRE -496 576 -496 128
WIRE 32 592 32 544
WIRE -496 720 -496 656
WIRE -224 720 -224 224
WIRE -224 720 -496 720
WIRE 32 720 32 672
WIRE 32 720 -224 720
WIRE 208 720 208 624
WIRE 208 720 32 720
WIRE 352 720 352 624
WIRE 352 720 208 720
WIRE -496 816 -496 720
FLAG -496 816 0
FLAG -224 -48 0
SYMBOL voltage -496 560 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL res -368 208 R0
WINDOW 0 -38 37 Left 0
WINDOW 3 -62 80 Left 0
SYMATTR InstName R3
SYMATTR Value 1e6
SYMBOL nmos 160 -240 R0
SYMATTR InstName M1
SYMATTR Value Si7868DP
SYMBOL pmos 160 624 M180
SYMATTR InstName M2
SYMATTR Value FDS4465
SYMBOL res 16 -304 R0
SYMATTR InstName R6
SYMATTR Value 510
SYMBOL res 48 688 R180
WINDOW 0 36 76 Left 0
WINDOW 3 36 40 Left 0
SYMATTR InstName R8
SYMATTR Value 510
SYMBOL diode 16 -128 R0
WINDOW 3 36 32 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D3
SYMBOL diode 16 448 R0
WINDOW 3 41 30 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D1
SYMBOL diode 16 96 R0
WINDOW 3 37 34 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D2
SYMBOL diode 16 224 R0
WINDOW 3 41 32 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D4
SYMBOL diode 16 -16 R0
WINDOW 3 41 31 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D5
SYMBOL diode 16 368 R0
WINDOW 3 43 32 Left 0
SYMATTR Value 1N4148
SYMATTR InstName D6
SYMBOL ind2 336 368 R0
WINDOW 0 61 31 Left 0
WINDOW 3 39 65 Left 0
SYMATTR InstName L1
SYMATTR Value 1.59e-5
SYMATTR Type ind
SYMBOL cap 336 240 R0
WINDOW 0 -39 5 Left 0
WINDOW 3 -84 59 Left 0
SYMATTR InstName C2
SYMATTR Value 1.59e-7
SYMBOL res 336 528 R0
SYMATTR InstName R1
SYMATTR Value .2
SYMBOL Opamps\\LT1498 -224 -176 R0
SYMATTR InstName U2
SYMBOL Opamps\\LT1498 -224 128 R0
SYMATTR InstName U1
TEXT -480 760 Left 0 !;ac oct 256 20 1000000
TEXT -480 792 Left 0 !.tran .001 startup

I just picked a couple of MOSFETs out of LTC's menu based on Vds(on)
and Rds(on), and an LT1498 because it can source/sink about 30 mA.

There are two in a package, and I'm using them as unity-gain buffers,
so I connected them in parallel instead of sparing one.

---
JF

I copied the circuit into LTSpice and took a look. I can see that it will
need something to get the tank started oscillating, which is probably why my
effort to sim it came to nothing. I think to get that circuit to start I
will need to insert a supply above or below the rails and give a kick to one
of the mosfets to get the tank going.
Gives me a good excuse to start learning Spice.

I copied the circuit into LTSpice and took a look. I can see that it will
need something to get the tank started oscillating, which is probably why my
effort to sim it came to nothing. I think to get that circuit to start I
will need to insert a supply above or below the rails and give a kick to one
of the mosfets to get the tank going.
Gives me a good excuse to start learning Spice.

On Mon, 30 Aug 2010 12:53:48 -0400, "Michael Robinson"
nospam_at_billburg.com> wrote:


I copied the circuit into LTSpice and took a look. I can see that it will
need something to get the tank started oscillating, which is probably why
my
effort to sim it came to nothing. I think to get that circuit to start I
will need to insert a supply above or below the rails and give a kick to
one
of the mosfets to get the tank going.
Gives me a good excuse to start learning Spice.

---
Just a hunch...

If you're not familiar with LTspice, then maybe it's working but you
haven't picked what to display.

If you can see the schematic, but no waveforms, right-click on an
empty spot in the schematicf pane, and then click "RUN" in thye box
that appears. Then, when everything settles down, left-click the
schematic pane and then mouse around until you see some pencil-like
thingies appear when you get close to wires.

Those are voltage probes, and if you left-click when they're pointing
at a wire, then what's on that wire will appear on the waveform
display pane.

If you see a clamp-on looking thing with an arrow going through it
appear when you're over a component, that's a current probe, and left
clicking will display the current in that component.

Good luck :-)

---
JF
Ok, got it. And thanks for posting this elegant little circuit, John!

"John Fields" <jfields_at_austininstruments.com> wrote in message
news:ev6o76lifeb01nsoj5vlgvcb8qr7f9fj0e_at_4ax.com...
On Mon, 30 Aug 2010 12:53:48 -0400, "Michael Robinson"
nospam_at_billburg.com> wrote:


I copied the circuit into LTSpice and took a look. I can see that it will
need something to get the tank started oscillating, which is probably why
my
effort to sim it came to nothing. I think to get that circuit to start I
will need to insert a supply above or below the rails and give a kick to
one
of the mosfets to get the tank going.
Gives me a good excuse to start learning Spice.

---
Just a hunch...

If you're not familiar with LTspice, then maybe it's working but you
haven't picked what to display.

If you can see the schematic, but no waveforms, right-click on an
empty spot in the schematicf pane, and then click "RUN" in thye box
that appears. Then, when everything settles down, left-click the
schematic pane and then mouse around until you see some pencil-like
thingies appear when you get close to wires.

Those are voltage probes, and if you left-click when they're pointing
at a wire, then what's on that wire will appear on the waveform
display pane.

If you see a clamp-on looking thing with an arrow going through it
appear when you're over a component, that's a current probe, and left
clicking will display the current in that component.

Good luck :-)

---
JF
Ok, got it. And thanks for posting this elegant little circuit, John!