Ignoramus965 wrote:

My objective is to make a square wave inverter for high amperage DC
for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

It can do up to 10 volts, supposedly, and any frequency.

Can I simply use it to drive either MOSFETs or IGBT's that make up the
H bridge?

Not directly.

You'll need some level translation.

If you don't know what that means then you shouldn't be attempting to make a
welder.

Unless you fancy welding *yourself* to something ? Terminally maybe ?

Btw - electronics hobbyists are advised to try out *low current* stuff
before progressing to the 'arcs and sparks' scenario !

Graham

Pooh Bear wrote:
Ignoramus965 wrote:


My objective is to make a square wave inverter for high amperage DC
for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

It can do up to 10 volts, supposedly, and any frequency.

Can I simply use it to drive either MOSFETs or IGBT's that make up the
H bridge?


Not directly.

You'll need some level translation.

If you don't know what that means then you shouldn't be attempting to make a
welder.

Unless you fancy welding *yourself* to something ? Terminally maybe ?

Btw - electronics hobbyists are advised to try out *low current* stuff
before progressing to the 'arcs and sparks' scenario !

Graham


There's quite a bit of gate capacitance in larger MOSFETS that has to be
quickly filled or emptied. Drivers are by no means trivial.

In fact, the design of the driver stage is usually much more difficult
than the output stage.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

Many thanks to you, Don Lancaster, for all your teachin's over the years.
Bet your sweet bippy I've bookmarked your site!

Bob Swinney
"Don Lancaster" wrote in message
...
Pooh Bear wrote:
Ignoramus965 wrote:


My objective is to make a square wave inverter for high amperage DC
for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

It can do up to 10 volts, supposedly, and any frequency.

Can I simply use it to drive either MOSFETs or IGBT's that make up the
H bridge?


Not directly.

You'll need some level translation.

If you don't know what that means then you shouldn't be attempting to
make a
welder.

Unless you fancy welding *yourself* to something ? Terminally maybe ?

Btw - electronics hobbyists are advised to try out *low current* stuff
before progressing to the 'arcs and sparks' scenario !

Graham


There's quite a bit of gate capacitance in larger MOSFETS that has to be
quickly filled or emptied. Drivers are by no means trivial.

In fact, the design of the driver stage is usually much more difficult
than the output stage.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

Ignoramus965 wrote:
My objective is to make a square wave inverter for high amperage DC
for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

Can I simply use it to drive either MOSFETs or IGBT's that make up the
H bridge?

i

No. You need a different signal for the high side drivers referenced
to the midpoint of the bridge. Wasn't this question answered already
in an earlier thread? Also, large power stages require a lot of
current to drive them. In basic EE classes, they teach you the gate
driver problem (how can a circuit with no dissipative elements still
dissipate power). You need to understand this problem before designing
gate drivers. I believe Don Lancaster has a nice article on this
matter somewhere on his web site.

Why don't you reference a working design and ask questions based on
that? This will give a reality check and stop you from wasting
people's time and generating all kinds of noise?

I have spent some time checking the following reference design out. I
am fairly sure it will work:

http://www3.telus.net/public/a5a26316/TIG_Welder.html

Sure, you can argue all day about using a signal generator or using
MOSFET's instead of IGBT's, but why don't you reference some questions
to this design. Also, you might be able to get some advice at
www.diywelder.com. There are other designs there as well.

Good luck.

Ignoramus965 wrote:

My objective is to make a square wave inverter for high amperage DC
for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

It can do up to 10 volts, supposedly, and any frequency.

Can I simply use it to drive either MOSFETs or IGBT's that make up the
H bridge?

thanks

i

I would not use the signal generator because it isn't really optimal for the
job, and if you get for example a drain to gate short in a blown MOSFET,
you will quite likely destroy your Wavetek. Get the National Semiconductor
datasheet for a 555 timer and build yourself an oscillator. This is
seriously just as good for what you are doing, and when it blows up you
will be less upset (or at least I would). If you need to drive MOSFET
gates fast, I recommend TC4421 or TC4422 chips connected after the 555,
they are supposed to put out 9 Amps, which your Wavetek can't do. The 555
can run off a 5V regulator. Keep the wires from the TC4421 to the MOSFET
shorter than 1 inch for low inductance, and put ceramic and electrolytic
decoupling caps right next to each 4421.
Chris

Chris Jones wrote:
Ignoramus965 wrote:


My objective is to make a square wave inverter for high amperage DC
for TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

It can do up to 10 volts, supposedly, and any frequency.

Can I simply use it to drive either MOSFETs or IGBT's that make up the
H bridge?

thanks

i


I would not use the signal generator because it isn't really optimal for the
job, and if you get for example a drain to gate short in a blown MOSFET,
you will quite likely destroy your Wavetek. Get the National Semiconductor
datasheet for a 555 timer and build yourself an oscillator. This is
seriously just as good for what you are doing, and when it blows up you
will be less upset (or at least I would). If you need to drive MOSFET
gates fast, I recommend TC4421 or TC4422 chips connected after the 555,
they are supposed to put out 9 Amps, which your Wavetek can't do. The 555
can run off a 5V regulator. Keep the wires from the TC4421 to the MOSFET
shorter than 1 inch for low inductance, and put ceramic and electrolytic
decoupling caps right next to each 4421.
Chris


Triple 5's work great but you might want to look into an astable
multivibrator with a tuning capacitor as you can build one up to handle
voltages you can't imagine. Then if you hookup a pocket O'scope ($160
allelectronics.com), or build a simple freq counter($30 parts find free
scats online). Then you can monitor your output frequency. And as an
alternative to mosfets try a power triac(2 Z diodes chained together) or
a variac might work but cost prohibitive if they smoke up, I usually
just use them as clean-low power power supplies. What Chris recommended
will work I just wanted to give you a different perspective and some
other options.

X

On Fri, 09 Sep 2005 03:08:52 +0000, Ignoramus965 wrote:

My objective is to make a square wave inverter for high amperage DC for
TIG welding, from a 300A CC DC welding power supply.

I would actually be content with 200A current. AC would be used for
aluminum only.

I own a Wavetek 171 signal generator:

http://www.labx.com/v2/spiderdealer2...m?LVid=2779965

It can do up to 10 volts, supposedly, and any frequency.

Can I simply use it to drive either MOSFETs or IGBT's that make up the H
bridge?

I would disrecommend an H-bridge - the workpiece has to be grounded
to ground, or you run a risk of serious injury.

I'd be very surprised if you could design and build a circuit that would
turn your DC welder into an AC one, for less money than you can just go
buy an AC welder.

Good Luck!
Rich