Minimum Power - Transformers

[hightower]

I'm trying to fathom the science behind why certain transformers need a certain power in order to work. What I've got is this, and wondering if someone can confirm my thinking:

12v output, 30W load - resistance of 4.8ohms

12v output, 2.5W load - resistance of 57.1ohms

Is it because as the power reduces, the resistance in the circuit increases and it gets to a point where there is too much resistance for the circuit to work? So then:

12v output, 6 x 2.5W load in parallel - resistance of 9.52ohms

...which might be little enough resistance to allow the circuit to work again?

Am I on the right lines here or do I have the complete wrong end of the stick?

Thanks,

 

[telectrix]

bear in mind that the impedance may well be inductive. but you're on the right track.

 

[Richard Burns]

The design of modern electronic lighting transformers is a filter, rectifier, switching, transformer electronic circuit that has to reach a point at which the circuit oscillates, however if the current is too low then the circuit will not reach the oscillation point and will not properly "transform" the power.
The transform is in inverted commas because the circuit actually operates at tens of kHz on a DC supply internally so is not what one might call a normal transformer. The output of a normal lighting transfomer is a very high frequency pulse with a very approximate ac like waveform superimposed, an incandescent lamp will not care as it will only "see" the total power it can consume, but for electronic lamps the waveform is abysmal for control.

 

[hightower]

If only you had one of your patented diagrams RB to help it sink. ....

 

[telectrix]

sink diagram? by all means, have one of mine:

 

[Richard Burns]

May be a bit beyond me to draw a diagram of the oscillation of an electronic transformer and the associated circuit!

 

[Richard Burns]

But anyway I have had a go at it.

This is my idea of what is happening, but it has been a long time since I have done electronic circuits and I may be incorrect, so be prepared for someone to come along and say it is rubbish, but it should give you an idea of what may be happening.


and that waveform is really hard to draw!

 

[Marvo]

Nice waveform drawing, must have taken you a good few minutes.

HT, I'd suggest you refer to and think of electronic transformers as 'power supplies' or 'drivers' rather than transformers. Switch mode supplies operate and behave nothing like inductive transformers which will work under any load regardless of how small and don't have a minimum power requirement.