Thanks for the info. I was never successfull trying to switch the weld current. Welded a lot of contacts, but no battery tabs ;-) Typical CD system discharges more volts into a step-down transformer. mike
It would need to be a very good transformer, to handle high speed, high current transients like this - I doubt I could design one off the top of my head.
Steve
When I was a Senior Scientist for Schlumberger ATE, to picky isn't it. It saved the company and customers a number of times.
Tr is rise time of a waveform - 10-90 or 20-80. Some waveforms measured from
0 to 100 would never make the two levels - unless a static condition exists.Math is based on the two levels.
Martin
I don't understand you at all... The vendor published a picture of a waveform. On it, they clearly defined a parameter Tr and quoted a number for a time associated with that graphical representation. I can't think of a more unambiguous way to define the parameter.
You have in your head a definition for a thing called Tr. That's where the ambiguity resides. And it's unrelated to the picture and number provided by the vendor. Unfortunate choice of parameter name? Seems so in your case. But it's still UNAMBIGUOUS!! Picture >= 1,000 words. Math is not required. Feel free to relabel the vendor data to suit your needs.
mike Me do loves a good pissing contest...
While the risetime typically is not the 0 -100% rise the picture is what the manufacturer indeed says this is what the waveform should look like. Not surprising since the waveform is presented for setup or checking purposes and it is much, much easier to verify with respect to the indicated points rather than finding 10% and 90% points. Unless you have a storage scope or a modern scope with memory it is a real PITA to locate intermediate points with the one-shot nature of the device. Also, the rise-time in terms of the edge of the waveform may not be the real issue but rather the the energy in the pulse which is related to the area under the curve. The 100% point as indicated does give an partial measurement of this and the voltage drop from the peak is specified.
Billh
Could an automotive relay switch them on/off quickly to acheive the wave form you posted?
So you're saying that capacitors are less likely to explode?
Thanks
You mean a starter solenoid?
Are these just safety buttons to prevent electrocution?
This CD welders (below) require 680,000 mirco F. What's the different about yours?
Energy proportional to the square of the voltage on the caps. Wonder how they get 1000A through their output connector? Wonder how they get 1000A through the small wires to their welding tweezers? My Unitek 125WS welder has half inch diameter output terminals and runs #2 wire to the head. Wonder what they use to switch the caps to the load? Would be interesting to see a graph of weld voltage and current vs time. mike
"Automotive Relay" is a pretty broad term. In general, a relay can turn on arbitrarily quickly, in terms of risetime. Turning off can be a problem if the contacts weld together. Even if they don't weld, there are mechanical and magnetic flux time constants that limit the minimum pulse width. The second pulse is problematic if the contacts are all "burnt" from the first pulse. I expect there are relays that could do the job, but not likely they'll be found in an automobile.
Again, we'd have to be more precise about the definition of explode. Take a new fully charged car batery. Shine up the connections. Slam a 1" square copper bar across the contacts. Have somebody videotape the experiment. Report back your results. It's likely that the result can be used for my definition of explosion. Do NOT try this at home.
Now pick a capacitor and voltage that gives you 100 Watt-Seconds. Short it with the copper bar. Compare the results to the battery experiment. mike
...it cost about zero to build for a start !
Steve
I found a graph for a dual pulse CD welder here.
The size of the job would presumably determine how much current is required and thus set the minimum size of the capacitors. You can also vary the amount of charge held in a capacitor by changing the voltage. The Charge is Q and the Total Charge in a Capacitor is Q=CxV. Charge is also related to Current (I) by the formula Q=IxT so IxT=CxV and then I (current)= CxV/T. This means you can increase the current capability by increasing the size of the capacitors C and/or the Voltage V or shortening the discharge time T. Note that these are ideal equations and the resistance of the leads and workpiece also factors into what happens in real life. The voltage level also has to be within safety standards. However, you can't just increase the current by shortening the time it is applied and expect it to work. The high current has to be present long enough to heat and fuse the workpieces.
The above is also related to charging the capacitors. By increasing the time to charge the capacitors you can do it with relatively small amounts of current. In this type of welding done manually that is not hard to do because repostioning for the next weld etc gives seconds to charge the capacitor so the power supply does not have to deliver high currents.
Billh
You snipped the answer from my previous post. Doubling the voltage requires 1/4 the capacitance. 18V...400V, do the math.
You have at least two major problems. You have to store the energy somewhere. The voltage and the capacitance are the easy parts. Second, you have to deliver all that current to the load. You need extremely low resistance in the switch and all the path to the weld. The simple "fix" is to just up the energy to account for the relatively huge losses. Looks good on paper, but makes the process extremely sensitive to the weld resistance which depends on force, contamination, material, luck... Tweaking it to get a good weld ain't all that hard. Getting a dozen good welds in a row is not so easy.
My Unitek 125 switches 400 odd volts with an scr into a transformer.
400V in >> 7V out into .001 Ohm. It's not your ordinary transformer. I was surprised how small it is. IIRC about 2" on a side.mike
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.