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Posted by on July 26, 2007, 6:13 pm
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(this is a copy of a post made on sci.electronics.design)
---
Hi. Lately i've become interested in building a small spot welder for
DIY use. The idea is to end up with a (relatively) portable unit, with
controlable power, suitable for spot-welding anything from battery
contacts to 1.5mm sheet metal. I've ruled out buying a commercial
welder - mainly because they're rather expensive where i live, and
also because i happen to like the challenge :)
So, right now i'm contemplating two different designs, and having a
hard time deciding which one to go for:
* Capacitor discharge welder: The idea is to have a rather large
(100,000 to 300,000uF) capacitor bank which is discharged by a large
triac or similar electronic switch into the weld contacts. A cheap
microcontroller with an integrated ADC would control the charging
voltage, allowing for variable power, plus triggering the output
device.
* Mains transformer welder: The idea is to modify (or pay to have
built) a large (1 kVA to 3 kVA) transformer with only a few turs in
the secondary to get a low voltage, high current output. The power
controlling would also be handled by a microcontroller timing a relay
in the mains winding - i've had good sucess in switching off relays
very quickly (arround 5ms) by using a zenner in series with the back-
EMF catching diode.
I've considered also running a high voltage cap discharge through a
transformer to get a strong amperage pulse, as done in commercial
welders, but ruled it out - if anything, because it's overly complex
and overkill for a simple device as intended. Anyway, in both cases
i'm planning to build a simple clamp arm with copper contacts as weld
tips.
Right now i'm favoring the transformer option, if only because it ends
up being way cheaper than buying large electrolyitc caps + SCR. I have
concerns about it being used to weld small parts, as i feel that a
capacitor bank allows for much finer power control - yet, caps won't
perform as well in thicker parts... or will they?
So, my questions are... does anyone with experience with this type of
welders have any advice to share? If i go with the transformer option,
should i look into limiting the output surge current with an inductor
of some kind? How long can one expect electrolytic caps to last when
stressed with charge-discharge cycles like these? Any comments that
can help me make up my mind will be appreciated, thanks!
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Posted by Ignoramus16379 on July 26, 2007, 7:36 pm
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Miller welding machine manuals usually include diagrams of their
electrical circuits. See how they make their spot welders.
i
wrote:
> (this is a copy of a post made on sci.electronics.design)
>
>
> Hi. Lately i've become interested in building a small spot welder for
> DIY use. The idea is to end up with a (relatively) portable unit, with
> controlable power, suitable for spot-welding anything from battery
> contacts to 1.5mm sheet metal. I've ruled out buying a commercial
> welder - mainly because they're rather expensive where i live, and
> also because i happen to like the challenge :)
>
> So, right now i'm contemplating two different designs, and having a
> hard time deciding which one to go for:
>
> * Capacitor discharge welder: The idea is to have a rather large
> (100,000 to 300,000uF) capacitor bank which is discharged by a large
> triac or similar electronic switch into the weld contacts. A cheap
> microcontroller with an integrated ADC would control the charging
> voltage, allowing for variable power, plus triggering the output
> device.
>
> * Mains transformer welder: The idea is to modify (or pay to have
> built) a large (1 kVA to 3 kVA) transformer with only a few turs in
> the secondary to get a low voltage, high current output. The power
> controlling would also be handled by a microcontroller timing a relay
> in the mains winding - i've had good sucess in switching off relays
> very quickly (arround 5ms) by using a zenner in series with the back-
> EMF catching diode.
>
> I've considered also running a high voltage cap discharge through a
> transformer to get a strong amperage pulse, as done in commercial
> welders, but ruled it out - if anything, because it's overly complex
> and overkill for a simple device as intended. Anyway, in both cases
> i'm planning to build a simple clamp arm with copper contacts as weld
> tips.
>
> Right now i'm favoring the transformer option, if only because it ends
> up being way cheaper than buying large electrolyitc caps + SCR. I have
> concerns about it being used to weld small parts, as i feel that a
> capacitor bank allows for much finer power control - yet, caps won't
> perform as well in thicker parts... or will they?
>
> So, my questions are... does anyone with experience with this type of
> welders have any advice to share? If i go with the transformer option,
> should i look into limiting the output surge current with an inductor
> of some kind? How long can one expect electrolytic caps to last when
> stressed with charge-discharge cycles like these? Any comments that
> can help me make up my mind will be appreciated, thanks!
>
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Posted by on July 26, 2007, 10:40 pm
Please log in for more thread options On Jul 26, 8:36 pm, Ignoramus16379 <ignoramus16...@NOSPAM.
16379.invalid> wrote:
> Miller welding machine manuals usually include diagrams of their
> electrical circuits. See how they make their spot welders.
I have a lot of tech documents for spot welders, both from Miller
(transformer based) and Image Industries (capacitor discharge based).
The electronics on these devices are rather simple - what i'm looking
for is for wisdom from people who built similar devices for
themselves, or have experience with both types to help me decide on
the welding system.
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Posted by on July 26, 2007, 11:04 pm
Please log in for more thread options On Jul 27, 3:40 am, plisan...@gmail.com wrote:
>what i'm looking
> for is for wisdom from people who built similar devices for
> themselves, or have experience with both types to help me decide on
> the welding system.
I have played with building a couple of spot welders. The first one I
built with four big variac cores. The cores were from about a
kilowatt variac. I used the existing windings which were for 120
volts and wired two cores in series in parallel with the other two
cores in series. And then used 240 volts.
The secondary was made aluminum. It filled the hole in the variac
cores. So I had four cores with a laminated bar going thru them. I
never got to finishing it, but did use it to fuse two 3/8 diameter
bolts together. Just crossed the two bolts and clamped an electrode
on each side with a C clamp. As the bolts melted into each other I
tightened the C clamp until the bolts were pretty much in the same
plane. Took a minute or so.
Dan
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Posted by on July 27, 2007, 1:34 am
Please log in for more thread options Nice! Did you recall getting too much overheat on either the output
probes or the transformer secondary?
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>
>
> Hi. Lately i've become interested in building a small spot welder for
> DIY use. The idea is to end up with a (relatively) portable unit, with
> controlable power, suitable for spot-welding anything from battery
> contacts to 1.5mm sheet metal. I've ruled out buying a commercial
> welder - mainly because they're rather expensive where i live, and
> also because i happen to like the challenge :)
>
> So, right now i'm contemplating two different designs, and having a
> hard time deciding which one to go for:
>
> * Capacitor discharge welder: The idea is to have a rather large
> (100,000 to 300,000uF) capacitor bank which is discharged by a large
> triac or similar electronic switch into the weld contacts. A cheap
> microcontroller with an integrated ADC would control the charging
> voltage, allowing for variable power, plus triggering the output
> device.
>
> * Mains transformer welder: The idea is to modify (or pay to have
> built) a large (1 kVA to 3 kVA) transformer with only a few turs in
> the secondary to get a low voltage, high current output. The power
> controlling would also be handled by a microcontroller timing a relay
> in the mains winding - i've had good sucess in switching off relays
> very quickly (arround 5ms) by using a zenner in series with the back-
> EMF catching diode.
>
> I've considered also running a high voltage cap discharge through a
> transformer to get a strong amperage pulse, as done in commercial
> welders, but ruled it out - if anything, because it's overly complex
> and overkill for a simple device as intended. Anyway, in both cases
> i'm planning to build a simple clamp arm with copper contacts as weld
> tips.
>
> Right now i'm favoring the transformer option, if only because it ends
> up being way cheaper than buying large electrolyitc caps + SCR. I have
> concerns about it being used to weld small parts, as i feel that a
> capacitor bank allows for much finer power control - yet, caps won't
> perform as well in thicker parts... or will they?
>
> So, my questions are... does anyone with experience with this type of
> welders have any advice to share? If i go with the transformer option,
> should i look into limiting the output surge current with an inductor
> of some kind? How long can one expect electrolytic caps to last when
> stressed with charge-discharge cycles like these? Any comments that
> can help me make up my mind will be appreciated, thanks!
>