Transformer

Are you sure the transformer is supplying DC? Stuttering sounds like the symptom of feeding AC to a DC motor. A transformer for lighting wouldn't

it may be outputting hf ac. Rectifying that could produce too many volts.

NT

a 12v DC motor but it doesn't drive the motor.It just stutters but doesn't turn.

Most lighting transformers output AC. Are you using the word 'transformer' correctly, or is it actually a boxed PSU?

Bill

Not if you omit smoothing caps..

Just needs a bridge rect that's all.

If its a crappy 12V DC permanent magnet thingy. Like a toy train or summat. Never had smoothing caps in those old controllers, not scalextric either.

that would increase motor current. AIUI 12v dc motors aren't designed to work that way. I'm sure you have more experience re whether that matters in practice.

NT

Low volt halogen lighting is AC. Your motor DC.

You need to rectify the output of the transformer to get DC. But the resulting DC voltage might be quite a bit higher than 12v.

That makes sense. When it said 12v down from 240volt ac, I assumed it was 12v DC. My bad!

The output waveform of lighting "transformers" looks pretty horrible. The one I looked at (from TLC) delivers a square wave at around 25kHz modulated by full-wave rectified 50Hz.

John

That sounds like a SMPS rather than transformer.

You might say it's lamentable.

Not really. and the motors are not designed to do anything much. They are really naff. But they do have inductance!

thats yer electronic ones.

I assumed a toroidal.

Plenty of older true transformers for low voltage lighting ain't toroidal. Assuming you know what a toroidal transformer is.

DC motors are often driven with a chopped supply. It is used to give them a good torque at low speed.

DC, are you sure? These are merely ac normally. You need a bridge rectifier. Of course there used to be some motors with field coils as well as a wound armature, and these could e made to run on AC. Brian

Yes and remember if you use full wave rectification a deal more than the actual ac voltage will be the DC result. You cold also smooth it with some capacitors once its dc, but it should run but rather faster than you anticipate. Brian

Are you certain of that? I have assumed that DC motor speed is generally dependent on the average voltage. An extreme example would be what happens if you put a huge hunk of inductance in series with the motor to provide constant voltage and current.

Average = peak x (2 / pi); for 12V rms = 12 x (sqrt(2) * 2 / pi) = 10.8V ave

If you're smoothing with capacitors, then you are entirely correct.

Just my thoughts.

Torque for a DC motor is purely dependent on current.

Chopping a supply at start-up is more a means to reducing the current down to more manageable level and to promote motor and control gear life expectancy.