Quote Originally Posted by Melanie View Post
The efficiency of a Transformer at 50/60Hz is dependent on the amount of iron in it's core... once you drop below about 1.5VA (which is just over an inch cube with an EI30 core) the amount of iron is so small that half the power consumption is used just to heat up the core and windings the efficiency plummets to only 50% or less.
Well, I'm sure those little transformers aren't the greatest for efficiency, but I don't think the medium small ones (6v@83mA) I've been using are all that bad.
I've got one sitting here on the bench for days with a light (10-15mA?) load on it, and it's only barely warmer than it's surroundings. It can't be dissipating very much energy. Maybe one of these days when I'm feeling less lazy I'll attempt to measure the efficiency on one.
It certainly doesn't heat up like cheap chinese wall wart transformers do.

But how efficient is a xformerless supply if it's not "precisely loaded"? Input current is always the same regardless of load! And unless that fancy X2 cap is sized exactly to the needs of the circuit, it looks like that zener is gonna be dissipating a good dose of heat.

If your load draws a constant current and the cap is sized correctly I can see how it could be very efficient. If the load varies over a wide range as the PIC switches stuff on and off, but the power supply draws the maximum current all the time, then I don't see how that can be very efficient...



steve