>> Transformer specs: >> Primary: 110v at 0.5a or 220v at 0.3a >> Secondary: 24v at 20va(.85a) or12v at 20va(1.7a) >> He has 220v coming in with a green ground wire but no neutral, and the >> circuit board is using both 24v and 12v. > > Rather than wind your own, one could take apart a suitable 220->24V > transformer (common furnace part?) and center-tap it. Perhaps less > work, but still a lot. One might also consider analyzing the circuit > to see what it is that wanted the 12V in the first place, perhaps its > needs are a lot less than the 24V, which usually runs 'everything' > in a HVAC control. > > It may be that the 12V is just a feed for a processor power supply, > which could be supplied by a linear voltage regulator or something > like that. Small, is my point. Processors don't usually run on > 12V, so it's not like you'd need to hit that exactly anyway, were > that the case. Depending on the circuit. > > The 'troll' under the stairs has two 12V transformers, because > it needs both 12V and 24V. The 24V is for the sprinkler system > valves, the 12V was immediately regulated down to 5V for the > processor. I only did it that way to not burn so much voltage > as heat. (It's a linear regulator. Later I also added some > significant 12V loads too, for low-voltage walkway lighting. > That was OK, because the two 12V transformers were in fact > from low-voltage lighting kits. But that is irrelevant.) I > could have run it all from 24V, and either put a bigger heat > sink on the regulator or used a series dropping resistor to > knock it down that way. > -- Jim ____________________________________ IIRC back when I worked with electronics, a linear voltage regulator was several components on a small board. Has that now been combined into one chip? Probably couldn't afford to generate too much heat in this unventilated box. Gerry
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