# Simple spot welder relay needed.

Discussion in 'The Projects Forum' started by takeitapart, Jul 9, 2010.

1. ### takeitapart Thread Starter New Member

Jun 20, 2010
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I am planning to build a CD spot welder to repair some batteries and small sheet metal projects. I have found plans that are simple to the very elaborate and complex. I have 2 "1farad" car audio capacitors, variable DC power supply and SCR. I need help with a simple switch/relay to shut off the power while triggering the scr. I would love to build the elaborate CD welders as found on this site but, they are a little much for now. I dont have PCB's or a great understanding of how to program timing circuits. However I can solder and follow a diagram. Part numbers would be great.

Apr 5, 2008
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3. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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My first version was simple like you described. I used a Variac to power a transformer from a battery charger, a bridge diode, and an automotive relay for the relay. It worked fine.

1) Almost any relay will work. I even used a toggle switch for my very first welder. The charge does not drop off very fast, so the time lost with a toggle is insignificant. You just have to remember to use it or you get a dead short after welding. I strongly suggest an appropriate fuse in the power supply circuit, just in case that happens.

2) Automotive capacitors will work, but you will be limited to whatever their voltage rating is. I found that I needed to go above 16V to get really good welds with 5 mil nickel strip. If you stick to 3 mil conductor strip, you may be OK.

What other questions do you have about the relay?

John

4. ### Dx3 Member

Jun 19, 2010
87
7
Here's an idea. A push button activates a relay that switches the charging power from the capacitors to the SCR gate.

I forgot to draw a resistor in to limit the current to the gate of the SCR. Add a resistor or the gate of the SCR will melt.

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5. ### SgtWookie Expert

Jul 17, 2007
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As Dx3 says, you'll need a resistor on the gate.
Many SCRs will trigger with 15mA or less gate current.

Just take your input voltage, subtract 0.7v from that, divide the result by 0.015, and select a resistor <= the result.

Example:
R <= (20v-0.7v)/0.015 = 1,286 Ohms. 1.2k is the closest standard value.

6. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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I$_{GT}$ varies with the device. Some larger SCR's require a minimum of 200 mA or more. It is best to consult the datasheet, if you have one, or use the datasheet from a similar device. I have found Semikron to be a useful resource in designing trigger circuits. There are extensive app notes that include studies of the way the conduction zone propagates upon triggering. I think it is best not to "under trigger," particularly when di/dt is expected to be large (e.g., in a CD welder).

Here is a useful curve relating trigger conditions to the likelihood of triggering and best practices (Source: http://www.semikron.com/products/data/cur/assets/SKKH_250_07670230.pdf)

Edit: Some people just use a series of AA batteries or even a 9V battery for the trigger. Be sure to have a common ground. Although the peak trigger current may be high, the duration is short and the batteries last quite a long time.

John

Last edited: Jul 9, 2010
7. ### takeitapart Thread Starter New Member

Jun 20, 2010
21
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Wow! thanks for all of the input. I have see the elegant post and instructions by jpanhalt/John. Maybe someday I could take that on. Now without pcb's my connections would look like a wire birds nest. Some of the suggestions are still a little over my head. I am working through an electronics book that I purchased at radio shack and last physics course was over 20 yrs ago. I am not sure what a bridge diode and a variac are. However these suggestions have given me some ideas to ponder.

8. ### retched AAC Fanatic!

Dec 5, 2009
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If it authored by Forrest M. Mims III, read the ink off of the book, and get a few more.

His books will set you straight. Also, read the E-book here.

Hopefully with age came the wisdom to not take on this high voltage stuff with out educating yourself...and it appears you have that.

To help for your next step:
Info on bridge diodes (and diodes in general):

And here is info on transformers, variacs are at the 'veri' bottom

Jul 17, 2007
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Jan 18, 2008
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11. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Yeah...I have some of Forrest's books from 30 years ago also. One never knows when they will peruse those pages again.

hasanz89 likes this.
12. ### takeitapart Thread Starter New Member

Jun 20, 2010
21
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Does the gate of the SCR unlatch when the current to the latch is removed or when the current going through the scr is complete?

13. ### takeitapart Thread Starter New Member

Jun 20, 2010
21
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Getting started in electronics is the book I have now. Also the educational section of this site is quite helpful. However those of you providing input here are the best.

14. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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For regular SCR's, the gate unlatches when the current going through the SCR stops. You can also have gate turn off SCR's (see the AAC e-book for them). There is a minimum "keep on" current in the specs. It is usually quite low.

John

15. ### Suzukiman Active Member

May 1, 2010
94
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I have seen a novel method to spot weld batteries. Just use a soldering gun and modify the tip.