Robot spot weld control missing SCR... what is it ?

1999 Medar Medweld 3003 AC Welding control ... previous owner removed the SCR (seen in the open blue box at the bottom of the page, lower left of the box here -http://www.medar.com/documentation/ml00281r1-0.pdf )

Here's a photo of the actual Medar 3003 box
http://www.practicalmachinist.com/medar.jpg

What function does this serve in this situation ? I'm guessing AC/DC conversion and timing...but you tell me. And is replacment likely to require it be exact duplicate of what was in there originally ? Expensive ?

(fwiw, I've called Medar and can only get recorded messages)

The "SCR" is a high frequency power supply so the mass of the transformer on the welder can be reduced. Maybe around 1000 HZ instead of the 60 HZ power coming in.

SCR also stands for "Silicon Controlled Rectifier". It is a power switching device. Not sure if that is what is meant in this cast tho...

One use of an SCR is in a light dimmer circuit. AC electricity is a sine wave with a base of zero volts. Now, an SCR will provide a variable "clip" of the sine way... Think of it this way... The voltage wills start to rise and then at, say, 10 volts (or minus 10 volts) it will clip. So if you drew this on a chart, the line would suddenly go horizontal until the voltage got back to 10V (or -10V), at which point the SCR would turn on again. Now, the net result is that the light bulb only gets 10VAC applied to it so it will be very dim. Of course, that 10V is just an example. Turning the dial changes the clipping voltage and, with that, the intensity of the bulb.

So, that is what an SCR does.

Alan

Here's a photo of the SCR I need. Frankly I don't see how it works....just looks like a few layers of material sandwiched together

Yep, thats a Silicon Controlled Rectifer. Big Ass Electronic Switch. Looks like two hockey puck style SCRs clamped between those two buss bars. There are two sets of red and white wires. This is the indicator of two SCRs.

Pictures of the SCRs themselves about halfway through this page:

http://www.darrahelectric.com/articles_trouble.htm

Very common SCRs. Looks like the lock they are attached to is water cooled. You can see the flared fitting peeking out on the left.

The question is are the SCRs in parallel with common cathodes and anodes or are they parallel with anodes and cathodes. If it is the latter it forms a TRIAC. This would make sense as this would control the power and switching of the main welding transformer.

A couple better pictures and you could make one. From the size of the unit I would guess the SCRs are around 200 to 300 amps. Voltage could be anything up to 1kv.

Its easy enough to identify a faulty SCR (Thyristor) - and replace it.

But to replace one that isn't there calls for a lot more expertise - needing to know what frequency its switching at , how much voltage the gate is getting from the control circuit (through the green connector), and of course how its to be connected as macona mentioned .

( ages ago When I worked on DC Drives of Printing machines we just needed to check a faulty transistor with a ohmmeter ( and replace it). meter shows almost 'open' between cathode and anode and a small but definite ohms like 20 or 30 ohms between gate and cathode- the faulty ones usually show a short between gate and cathode , or very low ohm between anode and cathode)
But we wouldnt know what thyristor to put if the thyristor wasn't there.

I can see a lot of stickers all round the interior of the panel - anything that mentions the power , frequency etc??

Another thing I noticed in the photo of the absent unit is that the thysistor 'nut' has red paint on it - could mean some definite tightening torque for the ( anode or cathode)
( and There is one thing to remember while getting thyristors - one end has the flexible lead with the connection lug at the end the other side is the nut and bolt end- YOU CAN GET THYRISTORS WITH ALMOST IDENTICAL NUMBERING BUT WITH THE ANODE AND CATHODE INTERCHANGED )- I found this out the hard way !! -- so look carefully at the thyristor symbol etched on the side of the thyristor - the direction of orientation of the symbol..


Sudtechcnc.