> One more question about the circuit below, what would a suitable value be
for the resistor between the rectifier diode and GND?
>

The resistor drops the voltage from the value on the input capacitor to 5
volts.
It should be sized such that it will drop slightly less than that voltage
when the output circuit is drawing its peak current. Sizing therefore varies
with expected peak load. The 5v1 zener will pass the small extra current.
With this arrangement the zener passes all the current when there is no
load.

A typical example.

Input = 24 VAC
DC voltage on input capacitor = 24 * 1.414 = 34 volts.
(Mean value will be somewhat lower due to ripple. Amount of ripple will
depend on load current and size of input filter capacitor).

Say load current mean peak = 20 mA
(SHORT surges above this value will be handled by the 0.1 uF cap on the 5v
output. If you are expecting substantial surges you should also place an
electrolytic (say 10 uF) in parallel with this 0.1 uF. The higher the surges
compared to mean peak the larger the cap should be.

R MAX = (Vin-Vout)/Imean_max = (34-5)/0.020 = 1450 ohms.

This value resistor will provide 5 volts at 20 mA for 34 VDC in. To allow
for real world results it should be smaller than this.
eg 1200 ohms provides up to (34-5)/1200 = 24 mA and 1000 ohms provides 29 mA

The resistor power dissipation (essentially constant) is (Vin-Vout)^2/R
At 1000 ohms this is 29^2/1000 = 890 mW.
Use a 2W resistor or better.

The zener dissipation max (which occurs at no load) is Vz^2* I
= 5 x .029 (for 29 mA) = 145 mW
Use a 500 mW zener or better.

The formulae above may be used for other values of Vin, Vout, Imean_max to
size resistor value, resistor power rating, zener power rating.



        Russell McMahon

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