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.

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