SCR Motor Speed Control made from salvaged parts

[Paul Jones]

I want to share some photos of a SCR motor speed control made from salvaged parts removed from surplus rack mounted electronics. In the early 1970’s my college roommate and I made several of these motor speed controls from surplus parts and aluminum used in rack mounted electronics we found in salvage yards. The one shown in the photos is still used in my home workshop (now controls my Unimat SL 1000 lathe (see http://www.homemadetools.net/forum/m...00-lathe-10111 )).

The salvaged printed circuit boards (PCB) and aluminum rack fronts and chassis were sold by the pound with no guarantee that anything would work. De-soldering the parts was easy using low heat soldering irons and braided copper wicks (we didn’t have the spring loaded vacuum pumps). The surplus electronics came from salvage yards in Seattle and the equipment was usually left in piles out in the rain or open-walled sheds. We always had a list of part numbers so finding best preserved parts took a lot of searching. There were always several junk yard dogs but kept caged by day and free to roam after closing. I found the salvage yards fascinating.

We saw an article in either “Radio Electronics” or “Popular Electronics” about making a 120VAC motor speed control using a 2N3669 SCR (Silicon Controlled Rectifier) rated for 16A at 200V. In the early 70’s SCR motor speed controllers were state-of-the-art so we had to have one. The most difficult salvaged component to find was the power resistor shown mounted on the right-hand side of the chassis in photo 3 (and better yet, this power resistor was adjustable). The aluminum box was cut from aluminum rack panels and chassis enclosures (hence some of the “extra” holes) and the stainless steel machine screws came from surplus rack components. We designed and etched our own printed circuit boards, and added an extra coat of solder to the copper foils. We thought it would be cool to have the PCB plug into an edge connector to make it easier to repair blown components (never needed). Even the blank PCB material was surplus.

[Hotz]

Speak seriously!!!

Because these guys with the name of Paul, are good ...

Very nice work ...

Paul Hotz ...

[kbalch]

Thanks Paul! I've added your SCR Motor Speed Control to our Electronics category, as well as to your builder page: Paul Jones' Homemade Tools. Your receipt:

SCR Motor Speed Control by Paul Jones

tags: PCB, soldering, electronics, controller

[kbalch]

This thread has been moved to the Must Read subforum. Congrats (and thanks) to Paul for making such a valuable contribution!

[Catfish]

Nice build Paul. Randy

[rossbotics]

That's cool Paul, I have a need for something like that on a machine tool I'm building, Maybe I'll make one instead of buying one,
Awesome build guy

[Paul Jones]

Thanks Doug. I am amazed how long the electronics have lasted considering everything was from used parts. I should add a fine mess screen to the side air vents to prevent any shorts from chips. Paul

[PJs]

Thanks Paul, great post. Enjoyed the "surpies" connection and that it still works. Something about the old school components that are built to last and you could push them pretty hard and would recover. The diodes and transistors look familiar but couldn't read the numbers...and etching boards was the Bom back then. The Surpies & penny a pound auctions at the Naval yards were the cheap and cheerful way back then, and probably where I got my attitude about it...doing so much with so little. I've used rack panels since then, even as an engineer, and love the versatility of them. Great build from 40+ years ago...Thanks for sharing your treasure! ~PJ

[Paul Jones]

PJ,
I thought I could help you with the part numbers and pulled the board from its edge connector. Some of the transistor cans are too corroded to read all the printed markings. The big SCR is a 2N3669, the four diodes are 1N2860A, the three transistors are 2N3754 (middle right), 2N26?9 (lower left),and unknown in upper middle position.
Paul

[Christophe Mineau]

Hi Paul,
This components were made for lasting, planned obsolescence was not yet a business strategy ...
Just for my understanding, what is the principle ?
Is it like chopping the mains sinusoid, triggering a thyristor at an adjustable phase ?