SCR power control for electric blanket

[soldar]

I have several LIDL electric bed warmers where the power control thing on the cord has failed. (BTW, I believe their repair might make be interesting for Big Clive but I know of no way to contact him. If anyone knows you can tell him I would send them to him.)

So I thought I would make my own power control. I measured the resistance of the blanket and it is too low to connect directly to 230V. I suppose I could design a simple astable with a duty cycle of , say, 1/3. 3 seconds on, 7 seconds off. That should work reasonably well but it would require separate low voltage supply.

Another option I considered was using one of the many unused triac "dimmers", modifying it so the max power was about 80W. But i did not go that route. Too easy.

I have a box full of *really* old SCRs and decided to have some fun. After some tinkering I have a working power control. It has a switch with three positions, High, Medium, Low which produce about 75, 50 and 30 W respectively. I preferred that rather than a continuous range with a pot.

I played with the values of the components and determined the best values by trial and error.

Because it is an SCR I rectify the voltage first. The capacitor is charged through the three resistors in series. When the voltage reaches a certain level the two transistors conduct and fire the SCR. It is very simple.

It works reasonably well. It seems it needs to warm up and stabilize before settling on a give power. I suppose the transistors are to blame.

Nothing special. Just a fun project.