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# help needed for power supply circuit using an IC voltage regulator

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#### shinn90

##### New Member
i need to get output voltage of -5V. Is circuit correct?

Assume that there is negligible ripple voltage across C1.
VC1(pk) = VI = -10V
Vs(peak) = -10V -0.7V - 0.7V

Therefore the secondary voltage rating of the transformer (in rms):
Vs(rms) = -11.7V/1.414 = -8.27 volts

electrical characteristics
Cout= 1uF , cin =0.33uF , Iout= 350mA, Vin = -10V

Partial credit!

First, in order to satisfy the no ripple assumption, Cin has to be HUGE, like >10,000uF!

The minimum allowed voltage at the regulator input is -5+(-Vdo), where Vdo is the DropOut Voltage of the regulator, typically ~2V.

Cin has to supply all of the output current for ~10msec (at 50Hz line freq), so the size of Cin depends on load current, and what the transformer secondary voltage is.

You have two variables which can be traded off; the size of Cin and the secondary voltage. Usually, you pick an off-the-shelf transformer which dictates Vsec, and then you compute the size of Cin.

Tell me what transformers you have available, and I will show you how to calculate Cin.

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