a little help with capacitive power supply values

[Melanie]

> What's the potential of the neutral line when it's tied to earth ground?

Actually it can be several volts.

Consider a few kW load at the end of the wire (very easy if you consider the kind of loads encountered in a building). The wire has a Resistance depending on it's material and cross-section and length.

If you connect your DVM between the Neutral Line and Earth at your wall socket with no load on that circuit (remembering it could be connected to several outlets), then you probably will get no voltage, but as soon as you start to switch on appliances you will begin to register a voltage. Now load up with a few kW (say an electric room heater) and report back with what you find...

[Dennis]

Hi guys

Dave .. thanks a million for the extra info :-) , it is the appnote which is confusing me .. The values for R1 and C1 seem to arrive out of nowhere, unless I'm reading it wrong ?

Melanie .. thanks for the extra info too !
There ain't now way I'm gonna be measuring much after I've done the right calculations especially not when under a real load :-) ....or should I .. have you ? Have you taken pics? Video ?
And as for switch on , I will do it using a push-button and a broomstick ;-)

Kind regards

Dennis

[Pic_User]

This is an interesting idea.
Transformerless Power Supply of 1A at 5Volts!
Thanks to Ioannis!
http://www.picbasic.co.uk/forum/showthread.php?t=12249
-Adam-

[Dennis]

Adam Thanks for that :-)
Saw it when it was first posted ... I don't want to use a bride rectifier or transformer and 1 amp is way more that I need :-)

All I want the pic to be able to do is act as and intelligent switch

AC IN --->> PIC >>> LOAD

Make sense ?

Kind regards

Dennis

[amgen]

the capacitor reactance (resistance for ac voltage) is
1/(2 X pi X freq X Cap(micro-farad)) ohms,

for 50MA from power supply, 220V/.050 = 4400 ohms....
the required resistance provided by cap for 50HZ at 220V.

then,
Cap= 1/(314 X 4400) equal aprox .7 micro-farad cap (>250 VAC rated)
works out to about .15 micro-farad per 10MA.
See attached diag.

[Dennis]

Amgen

Thank you that's exactly what I'm asking for !

I have been through the appnote and what's confusing me is how you got he value for the resistor R1 ??
You said ....

the capacitor reactance (resistance for ac voltage) is
1/(2 X pi X freq X Cap(micro-farad)) ohms,

For 50MA from power supply, 220V/.050 = 4400 ohms....
the required resistance provided by cap for 50HZ at 220V.

How did you get the value for this ...

Cap(micro-farad)) ohms

Kind regards
Dennis

[Melanie]

Just Transpose the formula that you've got... if...

Xc=1/(2 pi F C)

then...

C=1/(2 pi F Xc)

Xc is the AC Resistance (Reactance) that you want in Ohms, C=Capacitance in Farads and F is the Frequency in Hertz.

Say you wanted 50mA at 110v 60Hz (way too high for driving a PIC and a TRIAC say for Lamp Control - more like 15mA required), then V/I=R (where in our case R is actually Xc)...

110/0.05=2200

So we know that Xc must equal 2200... therefore applying the transposed formula...

1/(2 x 3.14 x 60 x 2200) = 1.2uF

Theoretically, that's great... but practically you'll discover that Capacitors don't come in all the myriad of values that Resistors come in, so you probably only have easilly available 0.1uF, 0.22uF, 0.47uF and 1uF. Work out which one best suits your current requirements and design your circuit to match it. I would strongly discourage you from desgining Transformerless Power Supplies with Supply Capacitor values greater than 1uF.