SCR Motor Speed Control made from salvaged parts

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.

Speak seriously!!!:D

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

Very nice work ...

Paul Hotz ... :hattip:

:cool:

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:

<div id="blocks"> <div class="block b1 pngfix"> <div class="bimg"> <div> <a href="http://www.homemadetools.net/scr-motor-speed-control"><img src="http://www.homemadetools.net/uploads/121268/scr-motor-speed-control.jpeg" /></a></div> </div> <div class="head pngfix"></div> <div class="left pngfix"></div> <div class="right pngfix"></div> <div class="blockover b1 pngfix"> <div class="title"> <a href="http://www.homemadetools.net/scr-motor-speed-control">SCR Motor Speed Control</a> <span> by <a href="/builder/Paul+Jones">Paul Jones</a></span> </div> <div class="tags">tags: <a href="http://www.homemadetools.net/tag/pcb">PCB</a>, <a href="http://www.homemadetools.net/tag/soldering">soldering</a>, <a href="http://www.homemadetools.net/tag/electronics">electronics</a>, <a href="http://www.homemadetools.net/tag/controller">controller</a></div> </div> </div> </div>

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

Nice build Paul. Randy

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

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

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

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

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 ?

Christophe,
I wish I had kept the schematic but I am pretty sure it is "phase angle control" using thyristors for controlling the phase angle or trigger angle thus allowing the output RMS voltage of the load to be varied. I think a more efficient method would use TRIACs instead of thyristors but I don't think these were available in the early 70's.
Thanks for asking, Paul

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Originally Posted by 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

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Thanks Paul for specs...I thought those diodes might be mil spec. and they were. Also that TO-3 SCR still has an equivalent out there but wonder if they'll last 40+ years¿ Shouldn't be hard to find everything based on these numbers and still have a big box of 1/2 & 1W resistors. Loved the heatsink too by the way! Thanks again for the post! ~PJ

PJs - thanks for the spec confirmations. Yes, the heat sink was cut from a piece of scrap aluminum sheet and the corners thinned a little to allow bending without breaking. It is crude but effective. Years ago, I would use the motor speed controller for a one-speed saber saw for cutting metal and the controller would become extremely hot but never burned out. Now I use it for the Unimat lathe which has very little load on the controller so that is probably why it has lasted so long (see http://www.homemadetools.net/forum/m...00-lathe-10111 ). Thanks, Paul

congratulations Paul, but, do you send-me eschematic diagrans?

Paulo,

I no longer have the schematics or the original construction article that I used to fabricate the SCR motor speed controller. The circuit design came from an electronics construction article in either Popular Electronics or Popular Mechanics sometime in 1968 or 1969. If you can find the archives to these magazines, I am sure you will find the details on how to construct this. The speed controller has been very reliable for now close to 47 years and still in use every week for my Unimat lathe (see my publication http://www.homemadetools.net/forum/m...00-lathe-10111 ).

Thank you for your interest,

Paul Jones

These are a lot easier to make today with triacs and diacs. http://i.imgur.com/GE3ZXqP.png Here's a commercial one I repaired that is almost that exact circuit I'm making a holder for it in this image https://i.imgur.com/BWiLc5Z.jpg It has an extra resistor and cap for inductive loads. But if you use the right triac you don't even need those parts.

I replied to this post before so this would have been a duplicate comment.

Yes,
And obviously, a simple vacuum or hand drill speed controller can be salvaged or even an halogen light dimmer, I have tried it already,
but for a real use of a washing machine universal motor in a shop made project, it is not really convenient, as it does not drive
at constant torque . As soon as you apply load, the speed drops and you have to readjust.
The only way to do it properly, according to me, is to regulate in close loop, using the signal from the tachometer, just like what does the washing machine control board.
But this takes more than a few components and requires a micro controller.
I already found on eBay a Russian seller who sold PCBs and instructions for such a controller, but I never managed to arrange the payment, there was something wrong at the time between Paypal and Russia, maybe it has been solved since.
One day, if I only had time, I may try to make one with an Arduino, I think I it is doable, but no more time ...

But if someone has already done it, it would be really nice to share :)

Nice work, brings back memories! Back in my youth I did a lot of salvage to build things like a control panel to launch my rockets with. This was most likely the early 1970's, I remember becoming obsessed with linear regulators and built a bunch of power supplies and then tried to blow a few of them up. Those regulators where tough. Unfortunately most of the surplus I got was second hand form junk dealers with a big markup, even those sources dried up after a bit. At this point I don't know of any surplus shops in the area.

Hi wazard69,

It is more difficult to find surplus electronics stores these days let alone electronics junk yards. Locally we have the store Orvac Electronics in Fullerton, CA (see Catalog ) that used to have a large section of the store devoted to surplus electronics (the smell of the old electronics would bring back memories) but most of the materials are gone compared to what they used to have. Back in the late 1970's to early 1980's I built S-100 bus computers with 8080 and 8085 CPU chips as a hobby and bought many surplus parts from Orvac and several other stores in the area. I still have the computer with the S-100 bus backplane but hesitant to power it up with its old 12 amp power 12V power supply because the old large can capacitors could explode. The capacitors were pretty old when I bought them in the late 1970's but it would be fun to see the computer come alive again.

I remember it was such a thrill the first time I saw the blinking cursor indicator on the screen of an old 9" CRT monitor when the 8080 2MHz CPU computer started up and I gave it the command to clear memory and display all the RAM memory showing all zeros in hex. The first 8080 and later a 8085 (a 4 MHz CPU) could only address 64 Kbytes of memory but this type of static RAM drew several amps of power, needed a big fan for cooling, and you really only address 58 kbytes of real memory because the 4 Kbyte BIOS resided in a ROM address in the upper memory address space. Nonetheless, I eventually improved the computer capabilities and installed a 5 1/4 " floppy disk drive and ran a FORTRAN compiler and link editor on the computer. It was an amazing little computer in its day and built from S-100 bus board kits and surplus parts. As a hobby with the S-100 bus computer I programmed in 8080 machine code, Assembler, and FORTRAN (I had been using FORTRAN since 1968). It also ran LISP and a BASIC interpreter and but the compiled FORTRAN was always much faster to run.

Regards,
Paul

If you want to buy parts there has never been a better time for that than now. Today we have the Internet. So you can get parts shipped right to your door cheap. Mostly I buy direct from China. There's some waiting involved with that but the prices can't be beat. I'm patient. Right now I could get 10 more of the triacs I used in the dimmers I made for $1.64. They're probably counterfeit but they still work just fine. https://www.aliexpress.com/item/10PC...844754301.html

Paul Jones, It would be appreciated if you'd share of provide a link for the schemagic for this project.

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Originally Posted by nhengineer

Paul Jones, It would be appreciated if you'd share of provide a link for the schemagic for this project.

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Just looking at the circuit board I'd say there are better schematics for making SCR phase fired converters today. We have higher performing parts we can use now. I found this circuit works well http://i.imgur.com/GE3ZXqP.png That's all you really need. It can even handle inductive motor loads. I've run a 600W router on one that I made. You can even get a heavier triac for larger loads. Here's the same circuit with a snubber http://i.imgur.com/iiuSn8H.png Though with the right triac you don't really need a snubber.

Thank you.

Nice work. I am having a little trouble translating the solder "circuit" into a working diagram. Would you be able to post a circuit diagram with a list of components so I and others could build one too? Thanks so much - Nick

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Originally Posted by Nick79

Nice work. I am having a little trouble translating the solder "circuit" into a working diagram. Would you be able to post a circuit diagram with a list of components so I and others could build one too? Thanks so much - Nick

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Here is a schematic to a circuit that works fine and is easy to build http://i.imgur.com/GE3ZXqP.png
You want to get a "snubberless" triac for inductive loads like motors. I used a BTA24-600BW and a DB3 diac with the dimmer I built and it worked fine to speed control a 600W router here. This is what the circuit I built looks like https://i.imgur.com/M7WENhG.jpg
Quite frankly I wouldn't build the original posted circuit today unless I had a time machine and went back 40 years in it. Then it wouldn't be today though.

Chocolate out of sh....... The only way

Paul,

Neat write up. It was an SCR that got me into a career path that served me well. In June of 1967, with my fresh AA in Electronics Technology, I went forth seeking a job that paid more than my gas station one. It was a boom time for electronics, so I hit every company in Costa Mesa and Newport Beach that had an opening. First question was always, 'what's your draft status?' As soon as I said 2S, I would get come back when you're not draftable. Then stopped in at Borg Warner Controls in Santa Ana. Asked about a tech position, their question back was a new one. Do you know what an SCR is? Why yes I do.

The last week of class our instructor came in and held up this odd little object. This is an SCR and it can control 5 amps, had a hex base with a 1/4" stud. We were in awe, having been warned, now this will run at 1 amp and can hurt you for two years. He described how it worked in a circuit.

So the Borg Warner guy said draw the circuit, which I did. You're hired, start next Monday. I too naive to realize that was too easy. The next Monday when I arrived, I found out the SCR's in the equipment they were building had 3/4" studs and controlled up to 500 amps. The core equipment was motor speed controls using 3 phase motors for process lines instead of DC motors. Cheaper and a lot cleaner. They also had just gotten a contract from Comsat for Uninterruptible Power Systems to be used at their satellite base stations. Single 125 kVA modules, 480 volts, 3 phase.

Turned out I enjoyed the work and at the time, there were probably less that 500 techs world wide working with this type of equipment. We used joke that we were basically Edison linemen with a solid state background. While working for Borg Warner, later Emerson Industrial Controls, I did jobs in Mexico, Canada, Brazil, then in 1976, the Soviet Union. We did the motor controls for the Kamaz truck plant and I was the field engineer doing the start up. Then while working on a UPS at Hughes in Fullerton, they asked me to come to work there. Went from electrician to plant engineering to systems engineer there and it all started with an SCR.

Hope I didn't bore you with the tale, but you can see why I have a very fond bond with SCR's.

Cheers,
Les T.

Problem with the cheap Chinese knockoffs they tend to fail.I order a lot off MOSFETs and similar components for inverter charger repair jobs and then have returns due to component failure,not a happy experience when sailing way off grid.When taking a job I now cost Chinese/original with risks, advantages .Latest 20N60 batch received where second hand with new legs soldered on,the ingenious skullduggery will never cease to amaze me.