Maker Pro
Maker Pro

current and voltage in power supply circuit

surya.rai

Nov 13, 2022
3
Joined
Nov 13, 2022
Messages
3
i have the following power supply circuit which i am simulating in proteus. how to calculate the current through the resistor R3?
1.png

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
12,722
Joined
Nov 17, 2011
Messages
12,722
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.
Read e.g. this article on choosing smoothing capacitors and try different values for C2.
 

surya.rai

Nov 13, 2022
3
Joined
Nov 13, 2022
Messages
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.
Read e.g. this article on choosing smoothing capacitors and try different values for C2.
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
2.png
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,722
Joined
Nov 17, 2011
Messages
12,722
Repeat: Check the waveform, read the linked article.
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,722
Joined
Nov 17, 2011
Messages
12,722
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).
 
Top