 ### Network # current and voltage in power supply circuit

#### surya.rai

Nov 13, 2022
3
i have the following power supply circuit which i am simulating in proteus. how to calculate the current through the resistor R3? as i understand, the primary input is 220V rms and peak of 311.12V, f=50Hz. the transformer has turn ratio 15. at the secondary before the diode the peak is Vs,in =311.12/15=20.74V, the peak voltage output from the diode D4 is, Vs,out = Vs,in-0.7=20.74V-0.7V=20.04V. And therefore the current through R3 should be, Id = (20.04V-12V)/1kohm=8mA because the zener diode D6 has zener voltage of 12V. The load current should be around IL = VL/R4= 12V/1.2Kohm=10mA but it is not so.
The ac voltmeter and the ac ammeter in the picture above are showing rms values.

So why is my calculation wrong?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,337
Check the waveform of the voltage across C2. You will notice that your assumption of a steady 20.04 V is wrong. There is considerable ripple on this voltage. Your "voltmeter" across C2 shows this by indicating only 13.8 Vrms.

#### surya.rai

Nov 13, 2022
3
Check the waveform of the voltage across C2. You will notice that your assumption of a steady 20.04 V is wrong. There is considerable ripple on this voltage. Your "voltmeter" across C2 shows this by indicating only 13.8 Vrms.
i noticed that the secondary rms voltage should be 14V(Vs = Vp/N=220V/15=14V) but the ac voltage meter shows 10V?the transformer setting is below, L1=N^2*L2, if L2=1mH then L1=225mH so N=15 Moderator
Nov 17, 2011
13,337

#### surya.rai

Nov 13, 2022
3
first of all the secondary rms voltage is not correct, if this incorrect the rest of the circuit value are wrong

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,337
I'm afraid the secondary RMS voltage IS correct. The 100 µF capacitor is way to small to buffer teh voltage significantly. That is why I asked you to look at the waveform of V2 with a scope (even a simulated one).

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