Tuesday, August 02, 2016

MERG speed control part 2

Half an hour's relaxed work while listening tot he radio and look what I have - a working speed control!

4 pages of instructions lead the builder through the job. Starting a with a circuit diagram, we move on to a parts list with 7 items on it and then on to the exciting bit, construction. 

A nice clear diagram shows the top view of the veroboard. All you do is poke the legs of each component in turn through the correct holes, solder to the copper strip on the back and by the end, all should be fine. A couple of components (diode and the big capacitor) must be fitted the right way around and this is covered in the guidelines. I like the way you are also told when it doesn't matter. 

I nearly went wrong by putting the 12v input and + output on the same track. They should be on adjacent tracks, which makes sense if you think about it, the input power runs through the circuitry before reaching the output. 

Soldering is a little fiddly. The correct way to do this is hold everything, heat the join and introduce the solder. That's a 3-handed job but you quickly work out how to hold things. That's part of the skill!

With such a simple circuit, it's well worth having a cup of tea after building and then re-checking every component with the plan. That's how I spotted my error with the input and output wires. 

After this, a dose of 12V in should see a voltage out that varies when you turn the knob on the variable resistor. I used my simple meter to check this and then tried it with a Bachmann Junior loco. Despite the lowest voltage being 1.2V (a limitation of the regulator) the loco stopped and started as required. If you have high-efficiency coreless motors this might be more of an issue, but then you should stop being tight-fisted and pay more than 2 quid for a controller!

Anyway, a lovely little project. I fancy building it into a battery electric loco for the garden railway one day. But then I also bought the shuttle unit and it could drive that too...


Anonymous said...

Easy enough to take the output down to 0v by adding 2 diodes (1N4002) in series with the output.

Brian G.

Phil Parker said...

Thanks Brian. I'll add them to my shopping list in the future. I've had a few ideas for this circuit and feel the need to order another one...

Frank Collins said...

Hi Phil, Congratulations on joining MERG, I joined up just over a month ago and it's worth the membership just for the discount on their kits and components.
I suppose I have a bit of an advantage as my job before retirement was Electrical Technician and electronics hold no real problem to me.
Some of the "Pocket Money" kits are really worth their weight in gold, I'm particularly interested in the servo motor controller as I have a few points and signals to operate on a future project.