MOSFET with best drain current per dollar or per area

It is tempting to make a spot welder from the Maxwell ultracapacitors 
e.g. the 3000F ones rated for 1900 Amps each. There is something nice 
about the idea of a spot welder that does not draw large pulses of 
current from the mains, and a cordless spot welder would be a novelty.

I figure that something in the region of 10kA to 30kA output current 
would make it useful enough to justify the effort. I understand that the 
welding voltage is normally a couple of volts or so, but if the weld is 
in an awkward location then cables or tongs might drop more voltage than 
the weld itself.

If you work out the energy required for a spot weld, it is far less than 
the energy contained in one of those capacitors, let alone several in 
parallel. Therefore it is going to be necessary to switch off the 
current somehow.

Once one is doing that, it becomes tempting to regulate the current with 
a bunch of switching current regulators or buck converters, perhaps with 
multiple phases interleaved.

It seems like most cheap FET packages are limited to about 100 Amps, so 
a bunch of 100 Amp current regulators with the outputs in parallel might 
be good.

What is the cheapest FET per current? The voltages would be small, even 
a series-parallel capacitor bank might be charged to 10 Volts at most. 
Since there might be a hundred of these mosfets to reach a total of e.g. 
10kA, a small package would help. It seems to me that for any FETs that 
have a high enough current rating, the RDSon will be far better than 
required.

It only needs to withstand the current for a few hundred milliseconds, 
maybe every 5 seconds.

Also, any ideas for making cheap high current inductors? Perhaps the 
cables to the welding electrodes would have enough inductance for a 
single switching current regulator or buck converter, though if instead 
of one large buck converter, many small (100A) converters are 
interleaved, then separare insulated strands would be needed for each 
converter, and the mutual inductance might mess things up.

On Fri, 16 Sep 2016 01:38:10 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>It is tempting to make a spot welder from the Maxwell ultracapacitors 
>e.g. the 3000F ones rated for 1900 Amps each. There is something nice 
>about the idea of a spot welder that does not draw large pulses of 
>current from the mains, and a cordless spot welder would be a novelty.
>
>I figure that something in the region of 10kA to 30kA output current 
>would make it useful enough to justify the effort. I understand that the 
>welding voltage is normally a couple of volts or so, but if the weld is 
>in an awkward location then cables or tongs might drop more voltage than 
>the weld itself.
>
>If you work out the energy required for a spot weld, it is far less than 
>the energy contained in one of those capacitors, let alone several in 
>parallel. Therefore it is going to be necessary to switch off the 
>current somehow.

Why not adjust the voltage on the caps, and just dump it into the
weld? 

Switching on and off wouldn't be hard, but current regulation would be
a lot of work at 30KA. At even 300 amps per fet, that's 100 fets.

Maybe use a giant puck SCR?



-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

Good welders employ a pre-pulse a variable delay,  and a main pulse. 

Steve 

On Thu, 15 Sep 2016 11:04:37 -0700 (PDT), sroberts6328@gmail.com
wrote:

>Good welders employ a pre-pulse a variable delay,  and a main pulse. 
>
>Steve 

Chris' project should have a priority list...

(1) Make sure debris doesn't hit one in balls.

(2) ....

		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

On Thursday, September 15, 2016 at 11:38:13 AM UTC-4, Chris Jones wrote:
> It is tempting to make a spot welder from the Maxwell ultracapacitors 
> e.g. the 3000F ones rated for 1900 Amps each. There is something nice 
> about the idea of a spot welder that does not draw large pulses of 
> current from the mains, and a cordless spot welder would be a novelty.
> 
> I figure that something in the region of 10kA to 30kA output current 
> would make it useful enough to justify the effort. I understand that the 
> welding voltage is normally a couple of volts or so, but if the weld is 
> in an awkward location then cables or tongs might drop more voltage than 
> the weld itself.
> 
> If you work out the energy required for a spot weld, it is far less than 
> the energy contained in one of those capacitors, let alone several in 
> parallel. Therefore it is going to be necessary to switch off the 
> current somehow.
> 
> Once one is doing that, it becomes tempting to regulate the current with 
> a bunch of switching current regulators or buck converters, perhaps with 
> multiple phases interleaved.
> 
> It seems like most cheap FET packages are limited to about 100 Amps, so 
> a bunch of 100 Amp current regulators with the outputs in parallel might 
> be good.
> 
> What is the cheapest FET per current? The voltages would be small, even 
> a series-parallel capacitor bank might be charged to 10 Volts at most. 
> Since there might be a hundred of these mosfets to reach a total of e.g. 
> 10kA, a small package would help. It seems to me that for any FETs that 
> have a high enough current rating, the RDSon will be far better than 
> required.
> 
> It only needs to withstand the current for a few hundred milliseconds, 
> maybe every 5 seconds.
> 
> Also, any ideas for making cheap high current inductors? Perhaps the 
> cables to the welding electrodes would have enough inductance for a 
> single switching current regulator or buck converter, though if instead 
> of one large buck converter, many small (100A) converters are 
> interleaved, then separare insulated strands would be needed for each 
> converter, and the mutual inductance might mess things up.

I tried welding with two of those caps in series, just for fun.

Connected with a short length of #12(?) wire for ballast, no joy.

Regular welders do fine with 80-120A, ISTM the limitation with these caps
is getting sufficient amperage through the resistance of the workpiece.

YMMV.

Cheers,
James Arthur

Chris Jones wrote...
>
>It is tempting to make a spot welder from the Maxwell ultracapacitors 
>e.g. the 3000F ones rated for 1900 Amps each. There is something nice 
>about the idea of a spot welder that does not draw large pulses of 
>current from the mains, and a cordless spot welder would be a novelty.
>
>I figure that something in the region of 10kA to 30kA output current 
>would make it useful enough to justify the effort. I understand that the 
>welding voltage is normally a couple of volts or so, but if the weld is 
>in an awkward location then cables or tongs might drop more voltage than 
>the weld itself.
>
>If you work out the energy required for a spot weld, it is far less than 
>the energy contained in one of those capacitors, let alone several in 
>parallel. Therefore it is going to be necessary to switch off the 
>current somehow.
>
>Once one is doing that, it becomes tempting to regulate the current with 
>a bunch of switching current regulators or buck converters, perhaps with 
>multiple phases interleaved.
>
>It seems like most cheap FET packages are limited to about 100 Amps, so 
>a bunch of 100 Amp current regulators with the outputs in parallel might 
>be good.
>
>What is the cheapest FET per current? The voltages would be small, even 
>a series-parallel capacitor bank might be charged to 10 Volts at most. 
>Since there might be a hundred of these mosfets to reach a total of e.g. 
>10kA, a small package would help. It seems to me that for any FETs that 
>have a high enough current rating, the RDSon will be far better than 
>required.
>
>It only needs to withstand the current for a few hundred milliseconds, 
>maybe every 5 seconds.
>
>Also, any ideas for making cheap high current inductors? Perhaps the 
>cables to the welding electrodes would have enough inductance for a 
>single switching current regulator or buck converter, though if instead 
>of one large buck converter, many small (100A) converters are 
>interleaved, then separare insulated strands would be needed for each 
>converter, and the mutual inductance might mess things up.

 For cheap, HY1707.


-- 
 Thanks,
    - Win

On 16/09/2016 05:07, dagmargoodboat@yahoo.com wrote:
> On Thursday, September 15, 2016 at 11:38:13 AM UTC-4, Chris Jones wrote:
>> It is tempting to make a spot welder from the Maxwell ultracapacitors
....

> I tried welding with two of those caps in series, just for fun.
>
> Connected with a short length of #12(?) wire for ballast, no joy.
I think you need much thicker wire and a lot of contact pressure on the 
weld.

> Regular welders do fine with 80-120A,
That might be the current drawn by the mains input, for a small one.
AIUI, for automotive steel spot welding it is nice to have 10kA, and to 
spot-weld thick aluminium up to 40kA can be required. For example see:
https://books.google.com/books?id=i0zSBQAAQBAJ&pg=PA96

> ISTM the limitation with these caps
> is getting sufficient amperage through the resistance of the workpiece.
Agreed.

On 16/09/2016 04:11, Jim Thompson wrote:
> On Thu, 15 Sep 2016 11:04:37 -0700 (PDT), sroberts6328@gmail.com
> wrote:
>
>> Good welders employ a pre-pulse a variable delay,  and a main pulse.
>>
>> Steve
>
> Chris' project should have a priority list...
>
> (1) Make sure debris doesn't hit one in balls.
>
> (2) ....
>

Hehe the reason I have had a long obsession with getting a spot welder 
is that I was allowed to play with a big one (with foot pedal operating 
compressed air to close the electrodes) in a sheet metal workshop (also 
with round punches, guillotine, notcher, folder etc) probably before I 
was ten years old. It's not scary, it is just like a stapler for sheet 
metal. I had great fun making a dustpan out of what I found in the scrap 
bin.

On 16/09/2016 02:14, John Larkin wrote:
> On Fri, 16 Sep 2016 01:38:10 +1000, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
>
>> It is tempting to make a spot welder from the Maxwell ultracapacitors
>> e.g. the 3000F ones rated for 1900 Amps each. There is something nice
>> about the idea of a spot welder that does not draw large pulses of
>> current from the mains, and a cordless spot welder would be a novelty.
>>
>> I figure that something in the region of 10kA to 30kA output current
>> would make it useful enough to justify the effort. I understand that the
>> welding voltage is normally a couple of volts or so, but if the weld is
>> in an awkward location then cables or tongs might drop more voltage than
>> the weld itself.
>>
>> If you work out the energy required for a spot weld, it is far less than
>> the energy contained in one of those capacitors, let alone several in
>> parallel. Therefore it is going to be necessary to switch off the
>> current somehow.
>
> Why not adjust the voltage on the caps, and just dump it into the
> weld?

It would force a certain ratio of peak current vs. total energy, which 
is not nice, as they have different effects and should ideally be 
independently adjustable. Also it would be nice to be able to achieve 
arbitrary current vs. time waveforms, as good commercial spot welders can.

On Fri, 16 Sep 2016 10:54:30 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 16/09/2016 04:11, Jim Thompson wrote:
>> On Thu, 15 Sep 2016 11:04:37 -0700 (PDT), sroberts6328@gmail.com
>> wrote:
>>
>>> Good welders employ a pre-pulse a variable delay,  and a main pulse.
>>>
>>> Steve
>>
>> Chris' project should have a priority list...
>>
>> (1) Make sure debris doesn't hit one in balls.
>>
>> (2) ....
>>
>
>Hehe the reason I have had a long obsession with getting a spot welder 
>is that I was allowed to play with a big one (with foot pedal operating 
>compressed air to close the electrodes) in a sheet metal workshop (also 
>with round punches, guillotine, notcher, folder etc) probably before I 
>was ten years old. It's not scary, it is just like a stapler for sheet 
>metal. I had great fun making a dustpan out of what I found in the scrap 
>bin.

I grew up in the golden days of American education.  In Junior High
School I was required to take Wood Shop, Sheet Metal Shop and
Drafting.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |