Thanks for the info, I'll keep those calcs in mind as I design this thing. Unfortunately, I can't use a wall wart normally because I need it all to fit into a neat smallish enclosure. But, fortunately, it's for my parents camper to monitor their current consumption because they only have 15A service there, so at least I won't burn down my apartment :o) However, I'm still considering just cracking a Wall wart open and using that transformer,cap,diodes as is to save some pain in the rear. I have a couple of these from old cordless phones that would be perfect cuz their small and are rated for some adquate current, voltages. Anyway, thanks again for the pointers, Peace MJ Brush ----- Original Message ----- From: "Jose Da Silva" To: Sent: Wednesday, June 02, 2004 4:13 PM Subject: Re: [EE:] Transformerless PS w/o Zener? > On June 2, 2004 12:02 pm, Matthew Brush wrote: > > >Here's my question, is there any way to make it work without the Zener > > diode? Not that it's hard, but I'm trying to make this thing using > > stuff I already have, and I don't have any Zener diodes (damn Canadian > > RadioShack is going to the dogs). I know they're probably like $0.15 at > > digikey, but then there's the min. order fee $5.00 and then shipping > > $8.00 so a handfull of zeners would cost me almost $15.00. I really > > don't have enough money right now to order more stuff to make it > > worthwhile. > > The transformerless power supply is a current supply, not a voltage supply. > You will need the zener. If you don't have a zener, you can put several > diodes in series (0.7 volt drop each). > > >So is there any other way of doing this, like maybe a resistor divider or > > some such thing, fed into a regulator? I really don't need much > > current, just enough for the pic to wake up, take a sample and go back > > to bed (and then the LEDs), maybe less than 100mA?. Do any of you gurus > > know any good tricks on the cheap that'll do this? Or should I just > > hack up a Wall Wart and use that transformer (might fight into the > > outlet box)? > > The transformerless power supply is a bit of a parasite on power. > This isn't exact, but will give you a quick-n-dirty rough idea. > Your circuit may take perhaps 100mW, but the drop down will be high. > > To figure on what you need, here's an idea.... > Suppose you need 10mA and because you are going for the half/wave supply, > then you really need 20mA because you use only 1/2 a wave, or 50% duty > cycle. > Xc = deltaV / I = 240V / 20mA = 12000 > > C = 1 / (2 x PI x f x Xc) = 1/(2 x 3.141 x 60 x 12000 ) = 221nF > > You're circuit may only need 10mA x 5v = 50mW > But you're circuit will need 20mA x 120v = 2.4W to operate. > If you went for the full wave idea it would be 1.2W to operate. > > >Another quickie, can I use a little 600:600 audio transformer from a > > telephone to provide some isolation with the "transformerless" power > > supply? > > Just get yourself a walwart. > If your house burns down and it turns out it was due to this circuit, say > bye-bye to your insurance claim. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.