MOSFET with best drain current per dollar or per area

On 9/15/2016 12:07 PM, 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
>> 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
>
Short answer, don't even try to switch the weld current.
My experiments with using caps and a sacrificial
mechanical switch resulted
in dismal repeatability.

I spent a lot of time fiddling with a modified microwave transformer
to weld battery tabs.  I switched the primary with a SCR.
Ignoring the several critical electrical issues, the problem is
getting enough energy into the weld REPEATABLY.
If your source resistance is low and the weld time is long,
even a tiny change in weld resistance will make a major change
in the weld energy.  With practice, I was able to make a good weld
about 90% of the time.  Do the math.  I rarely constructed a complete
battery pack with all good welds.

I switched to a commercial welder.  It varies the voltage on a capacitor
between 0 and 600V to set weld energy.  Switching via a single
SCR.  The high ratio transformer produces up to 7000 A into one milliohm.

http://i.imgur.com/ZeZerGx.png

Open circuit voltage is relatively high, so changes
in weld resistance are proportionally reduced.
Raising the weld resistance has relatively little effect on the
ENERGY delivered.
The short weld time
gives less opportunity for the heat to escape through the weldment.

Weld repeatability is WAY BETTER.

I expect there's lots of magic in the design of that transformer.

There's also a lot of magic in the electrode management system
if you expect repeatability.
This is what I ended up with:

http://i.imgur.com/er1BqSb.jpg

http://i.imgur.com/yd8c0rf.jpg

On Fri, 16 Sep 2016 11:03:42 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>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.

One could PWM into the load, and keep the efficiency up.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On 9/15/2016 8:47 PM, Chris Jones wrote:
> On 16/09/2016 05:07, dagmargoodboat@yahoo.com wrote:
...
>> 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
...

I have a small MIG (90A max) that runs on a 120v 20A ckt.  My 225A stick 
welder runs on a 50A 240v ckt.

My homemade spot welder produces about 2kA (calculated), which is enough 
to weld 2 pieces of 1/8" mild steel together.  I don't see why 10-20kA 
would be needed.  Except to keep the time very short & then I don't see 
the need to do that.  Except for battery tabs.

Bob

On 9/15/2016 11:38 AM, 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'm missing something.  Why do you need to switch the current?  They 
don't have a switch on the secondary side of a transformer based welder, 
it is on the primary side.  If you are concerned about safety, just 
unplug the cables when you aren't using it.

The switch on a welder is in the welder's hand.

-- 

Rick C

On 9/15/2016 11:12 PM, rickman wrote:
> On 9/15/2016 11:38 AM, 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'm missing something.  Why do you need to switch the current?  They
> don't have a switch on the secondary side of a transformer based welder,
> it is on the primary side.  If you are concerned about safety, just
> unplug the cables when you aren't using it.
>
> The switch on a welder is in the welder's hand.

Sorry, I missed that it was a spot welder... duh!

-- 

Rick C

On 9/15/2016 8:47 PM, Chris Jones wrote:
> 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.

Yes, the spot welder I've seen clamped the work like a vise grip.  I 
think the issue is to get a good contact between the welder and the 
metal so most of the resistance is between the two materials so that is 
the part that gets hottest.


>> 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.

Isn't it an issue of work piece resistance vs tool resistance?  What is 
the internal resistance of the caps.  Is that the limitation?

-- 

Rick C

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.
>
   Be damn careful about eye-squared-tee; a parameter that everyone 
including their pet goldfish dropped from datasheets back in the late '70s.

On Thu, 15 Sep 2016 09:14:57 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

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

Such welders do exist, and they do work, but they are considered less
optimal than a current regulated one.

The problem is that you cannot control the resistance of the
workpiece. With a constant amount of energy delivered to each weld,
due to the variable resistance, the weld will sometimes be a low
current over a long time and other times a high current over a short
time. This leads to inconsistent results.
-- 
RoRo

On 16/09/2016 17:02, Robert Roland wrote:
> On Thu, 15 Sep 2016 09:14:57 -0700, John Larkin
> <jjlarkin@highlandtechnology.com> wrote:
>
>> Why not adjust the voltage on the caps, and just dump it into the
>> weld?
>
> Such welders do exist, and they do work, but they are considered less
> optimal than a current regulated one.
>
> The problem is that you cannot control the resistance of the
> workpiece. With a constant amount of energy delivered to each weld,
> due to the variable resistance, the weld will sometimes be a low
> current over a long time and other times a high current over a short
> time. This leads to inconsistent results.

Isn't that what you require, constant energy?

-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

On Fri, 16 Sep 2016 10:47:27 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>a lot of contact pressure on the 
>weld.

You want enough force to get good electrical contact, but you don't
want too much force, as you will then squish out the molten metal,
leaving the weld thin and weak.
-- 
RoRo