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2.4 – Motor Speed Control (part 4 – testing time)

When you have completed the basic circuit, look it over a lot’ – check for boo boos and out of place solder. Check each track, to ensure it is not accidentally connected to any other.  

If all is well you can test it with a 60W incandescent light bulb. It has to be this old type – new compact fluorescent ones will not like the chopped current.

Turning this baby on is one of those exciting moments where things should work nicely.

By using the variable resistor you should be able to vary the light from completely off all the way through to full brightness.

Be very careful. Remember that as soon as you plug this little thing in it will have uninsulated mains voltage all over the place, including over the heatsink. Make absolutely sure the workspace is clear and safe and your not going to have it accidentally knock over or into something conductive. Test it briefly then disconnect from the mains (well ok, you can play with it a little bit, but remember those connections are live, and the crock clips don’t exactly meet domestic wiring regulations). Clear away as soon as you’re done so no stray or inquisitive fingers can easily plug it in.

Hooray! It varies nicely. Light goes bright, light goes dim. Bright, dim. Dimmer, Off. FULL BLAZING light. Ahhh – most satisfyingThe BTA triac tested with incandescent bulb

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Responses to Motor Speed Control (part 4)

  1. Rob

    Hello there, great post lad, I have just aquired the motor and was wondering could you possibly go into more depth about the two dc currents running through the motor to make it a shunt please

    kind regards


  2. Bongo

    Hay Rob. Well basically you provide the field coils of the motor with a separate rectified current (DC) of say 9V, so that it acts more like a permanent magnet. Wire it up as in the diagram making sure that the current from the triac is also rectified and going through the field coils and then the armature.

    By having both the triac derived current and the current from a separate transformer going through the field coils, you achieve better speed regulation, torque and lower peak starting current. Does that help?

  3. Barry

    Sorry just one other question. Is the 220k Pot safe to handle while this unit is in operation?. How much voltage is seen at the 220k pot?

  4. Bongo


    The current should be low(ish)- 0.05A, but it will still be at mains voltage and frequency (240V 60Hz). So in other words, the terminals of the Pot are certainly worth avoiding! Take super care while prototyping and when you finalise you project you will NEED to ensure they can’t be touched accidentally.

    Also remember the whole of the triac’s heat sink will be live so no touching to see how hot it is getting without unplugging and double checking.

  5. Barry

    You can get an insulated triac that won’t leave the heatsink live. BTA is insulated and BTB is not, from what I understand. Your one should be insulated.

  6. Bongo

    I am fairly sure you can get a version that is insulated, yes… It is possible that this one is – I didn’t actually test it, was just assuming. If you need an insulated one use the meter to test it first, don’t trust the datasheet ;)

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