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• Multiple Capacitors : Series or Parallel ?

  Frédéric Druppel • 07/17/2017 at 18:26 • 4 comments

  As I will be using 2 capacitors, I have the choice to put them in series or parallel, with benefits on each choice. The capacitors I have are 2.5V 310F Ultracapacitors from Maxwell.

  First of all some formulas :

  Where C = capacity (Farads), Q = charge (Coulombs), U = Voltage(Volts), W = Work/Energy (Joules).
  

  If I put the 2 capacitors in parallel, the overall voltage will stay 2.5V, but I'll have 620F in total. Which means I'll have a charge of 1550 Coulombs (Q=C*U), and a stored energy of 1937.5 Joules. As it is "only" 2.5V, it can have some trouble pumping the required current through the circuit (as I=U/R), but it's easier to charge as I won't have to "balance" the two capacitors as in the "series" option.

  If I put the 2 capacitors in series, the overall voltage will become 5V, and the total capacity will become 155F. At the end that means I'll have 755 Coulombs of charge, but still 1937.5 Joules of stored energy. Because it's 5V, it could pump more current through the circuit (as I is still equal to U/R), but the overall charge won't be as high as the "parallel" option.

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  TL;DR ? Here's a quick table :

  Option :	Parallel	Series
  Voltage :	2.5 Volts	5 Volts
  Capacity :	620 Farads	155 Farads
  Stored charge :	1525 Coulombs	755 Coulombs
  Stored energy :	1937.5 Joules	1937.5 Joules

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  So yeah, I don't really know which one I'll be using yet, so I'll do some testing ^^

• OLED Splash Screen & bitmaps

  Frédéric Druppel • 07/17/2017 at 14:59 • 0 comments

  As the Pulse Tab Welder will have an OLED display, I thought it was a good idea to add a splash screen during boot up. Here's the one I'll be using (drawn pixel by pixel in Adobe Photoshop :p ) :

  Okay, It looks a bit pixelated, but that's because the original image size is only 96 by 64, in order to integrate it directly on the OLED display.

  I'll probably draw other images and logos to "simplify" the use of the device (like icons and pulse waveforms for some menus, but I have to be careful not to use too much program space).

• Welding modes

  Frédéric Druppel • 07/17/2017 at 14:52 • 0 comments

  There are different welding modes I'd like to have in my welder (like continuous, pulsed, etc.). Here are the ones I will probably add :

  • CDC : Continuous Direct Contact
  • TCDC : Time Controlled Direct Contact
  • CPC : Continuous Pulsed Contact
  • TCPC : Time Controlled Pulsed Contact
  • ACPC : Amount Controlled Pulsed Contact
  • FCPC : Frequency Controlled Contact

  (Note : The above acronyms are not industry standards, in fact, I think I made them up)

  I will not explain each mode here, as I don't know if I'm going to integrate them all as some may not be as useful as others. (If you still want an explanation about a mode, leave a comment !!)

  I still have to make some research on how commercial tab/spot welder work in different modes, as well as their "industry-normalized" names.

  The modes will be selected with a rotary encoder and a GUI on an OLED display I'm currently kind of working on ("kind of" because I haven't received the rotary encoder I'll be using)

• Welding principle

  Frédéric Druppel • 07/16/2017 at 21:46 • 0 comments

  The welding principle on a short, high current discharge to weld a zinc strip on a battery cell terminal. It has to be short enough in order not to overheat the cell, but "hot" enough to make a soldered point. The pulse length will be controlled with an Arduino, more on that in an other log.

  I choose to use supercapacitors (in my case, Maxwell Ultracapacitors), primarily because I had them laying around for a while, and also because they can provide really high discharge currents.

  As stated just above, welding requires high currents, and in order to pulse current up to 250A (in this welder anyway), you need a fast switching system that can support high currents. I opted for a MOS-FET, more precisely an IRFS3004-7PPbF. They can support currents up to 400 Amps (silicon limited) !! I found them relatively cheaply on Arrow.com.

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  That's it for now ^^

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