SCR Motor Speed Control made from salvaged parts

[Paul Jones]

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

[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 ?

[Paul Jones]

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

[Paulo Bonini]

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

[Paul Jones]

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

[pfredX1]

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.

[pfredX1]

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

[Christophe Mineau]

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

[wizard69]

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.

[pfredX1]

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