I watched some tutorials how people use 2 car batteries in series, with a few mosfet amp's to make a spot welder, for 18650 batteries, and whatever else. I want to build one for myself, except use my power supply.
I have a nice switched, filtered, regulated power supply I had for years and used for stepper motors and all types of projects. It's awesome, it's heavy steel case, industrial, looks indestructible and I got a steal deal from ebay on it for something like lunch money.
I'd like to avoid destroying that if possible, but I can't ignore the amount of current it generates to be tempted to use it for spot welding. I also have a decent 48v power supply with high amps (I have to check) pulled from a POE telephone switch rack.
Spot welding is pretty much a short circuit. I saw a few tutorials where people use 2 car batteries and used arduino, with a foot-pedde to trigger mosfet's for a few short pulses. I would like to use my mighty power supplies instead.
Is there circuitry I can built to protect the power supply? Should I just create a bank of capacitors to charge and draw from that power (though that idea is kind of expensive since terminal capacitors aren't cheap).
As a rule, you shouldn't short-out a power supply... Do you have an idea of how much current you need?
48V across 1 Ohm is 48 Amps, so a 1 Ohm resistor would limit the current to a safe level for a 50A power supply.
A capacitor (or a bank of capacitors) on the load-side of the resistor could supply more short-duration current, but I'm not sure if "average" capacitors are rated for multiple short-circuit discharges.
Something like this ultra-capacitor should work just fine. Maybe a bit pricey, but hey, with 1.9KA@50V you can weld with a capacitor!!
Probably not a good idea to short it with your good screwdriver..
Harbor freight sells 90 amp MIG welder. The manufacturer's manual, claims "63 ~ 68 amps; High setting: 79 ~ 90 amps". Youtube video's I have watched, people have gauged the output at 20.03 - 19.87 amps.
Quoting the manufacturer of that welder, from Harbor Freight:
"
Name 90 Amp-AC, 120 Volt, Flux Cored Welder"
SKU 61849
Brand Chicago Electric Welding
Amperage (amps) 20
Duty cycle 20% @ 90 AC amps
Maximum amperage output (amps) 120
Maximum Diameter 0.035 in.
Minimum amperage output (amps) 60
Minimum Diameter 0.030 in.
Open Circuit Voltage 27
"
I can probably buy a step down transformer and all expensive parts, when all said and done - I would end up building a welder from scratch. I was trying to avoid buying expensive heavy batteries to lug around, and was interested in using these power supplies I already have.
Figure out how many joules it takes to make the weld, and go from there.
And the resistance of the steel (and electrodes) involved - hint it will be in the milli-ohm zone or lower, steel is
a metal and metals conduct electricity very very well (OK, steel isn't copper, but still...)
Water cooling isn't needed for occasional use, you can just wait for the electrodes to cool down again!
There's something to be said for living in the UK. We have 240vac mains , and I have a 10-200 amp TIG/stick welder which runs off an ordinary 13A socket. And 100% duty cycle at 160 amps. I can pick it up with one finger. Just.
It works fine!
Be perfect for your spot welding job, as the TIG gun has an on/off switch - just bypass with a relay and timer.....
Alanhurst: Thanks for the information. I could probably save myself the time, and just BUY a spot welder, then pay for a 220 electric line for service - if I really wanted to, just for spot welding.
For hobby and learning purposes, I just want to try to build a little spot welder and figure out what to do to protect a power source from short circuit. I will not be welding aircraft joints for combat fighters with this technology.
So, I suppose if I draw from capacitors, and have a diode across the positive and negative to protect power from going wrong direction. Then I guess I would only need something to limit the amount of current being drawn from the power supply, so if I accidentally draw everything and keep going - the power supply isn't 100% taxed. Other than that, I don't know what else could happen that would damage a power supply.
The point is that a welder is DESIGNED to take short circuits - your psu isn't, and I'd be worried about damaging it.
Can't you buy invertor welders for 110vac? - depends if you've a use for a welder, I suppose.
If you put a 1/2 ohm 1200 watt resistor in series with your psu, it would limit the current to 50amps.
Pretty beefy component. I wouldn't worry too much about reverse voltage etc - your big psu would stand that.
For a capacitor solution as I said I'd figure the joules....how many grams do you need to heat up to eg 1000C? typical metals have a heat capacity about 1/7 that of water.
So to heat say 1 gram needs 1000/7 or about 140 calories or 600 joules . Charged to 24vdc you'd need
energy = 1/2 C V^2.....
600 = 1/2 C *576 so C = about 2 farads , with a VERY low esr.....
Yes, I can buy a welder, a spot welder, co2 laser to melt metal, and / or pay a guy to bring their welder to me and weld stuff for me.
I never thought about calculating how much power I need, but the mathematics are interesting. I figured as long as I could control the pulse of electric in ms or ns, then a pwm signal can control how much of whatever energy was available would be delivered.
allanhurst:
The point is that a welder is DESIGNED to take short circuits - your psu isn't, and I'd be worried about damaging it.
Can't you buy invertor welders for 110vac? - depends if you've a use for a welder, I suppose.
If you put a 1/2 ohm 1200 watt resistor in series with your psu, it would limit the current to 50amps.
Which is absolutely useless as you need an order of magnitude more current - you have to dump
kilowatts into milliohms, the maths is simple I = sqrt (P/R)
DocStein99:
For hobby and learning purposes, I just want to try to build a little spot welder and figure out what to do to protect a power source from short circuit. I will not be welding aircraft joints for combat fighters with this technology.
There are projects on the web to build these from microwave oven transformers.
If you can find one of course as most are inverter these days.
I got confused when it comes to inductor coils, and became even more confused when I tried to read about them, what they do. I can probably spend a week or two trying to study the information in order to get the basic theory. It started explaining about magnetic energy and how it passes through some variable energy time where current is converted to voltage.
If I have a 12v power supply, and only want 30 ma to drive a LED, I use a 400 ohm resistor. So if my power supply is 600 watts, and I used .036 in that circuit, it should still be able to deliver 599.64 more watts. I know if I add a capacitor on the circuit, it will charge from the power supply and continue to deliver power after the power supply is shut off until the power in the capacitor is drained.
So I thought a choke coil acted the same way as a capacitor to store power, when I connected one in line to a d/c power supply then to a motor. After I cut the power supply, this choke coil somehow stored enough power to bring the motor slowly down to a stop, instead of immediately. I don't know anything about how the science of what's going on and how the energy is manipulated inside of the coil choke. I know it's possible to calculate something by the gauge of the wire, length and / or the number of turns around the ferroid core - but I'm lost on how/what that is used for.
We aren't talking about inductors, we are talking about transformers. In a transformer you can
pretty much ignore the inductance and just assume it magically makes the sum of amp-turns of
all the windings zero. Thus 1A in the 1000 turn primary must be balanced by -1000A in a one-turn
secondary. If you ignore the direction of the winding you can just say Ip x tp = Is x ts, for current
and turns of primary(p) and secondary(s).
So 10A in a 240V primary with 100 turns means 1000A in a 1 turn secondary (at 2.4V)
A decent transformer has enough inductance that the above approximation is a good one
at the frequency of operation.
MarkT:
Easy to tell the difference, an oven with a transformer weighs twice as much (or more!)
I have one , recently replaced the fans, a year later i am still trying to find someone to help lift it back into place.
Weighs a ton i think.
While it still works i am loathe to build a spot welder from it.
On by to do list though, purely for battery tags.
DocStein99:
Alanhurst: Thanks for the information. I could probably save myself the time, and just BUY a spot welder, then pay for a 220 electric line for service - if I really wanted to, just for spot welding.