I am facing to calculate the current consumption in a segment of a circuit as shown in the picture below.I have used the formula (showed in the picture) without considering the impedance of the capacitor C1. I have just considered the resistors components. Can any one tell me if the formula that I have done is correct to determine the current consumption.enter image description here Best regards,

  • \$\begingroup\$ Pro tip: Hold Alt while right clicking on V1 in your schematic. \$\endgroup\$ – winny Jul 2 at 14:06
  • \$\begingroup\$ Thanks. I didn't understand your answer. Is my question well explained ? \$\endgroup\$ – geek225 Jul 2 at 14:14
  • \$\begingroup\$ Yes. My pro tip is however to use the built-in tools of LTspice to get your power consumption value. \$\endgroup\$ – winny Jul 2 at 14:26
  • \$\begingroup\$ @winny: Thanks for your answer. However, using Ltspice to determine the value of current consumption is relatively simple. But my question is how to detreminate by calculation the value of current. And do I have to consider the impedence of capacitor or not in the calculation \$\endgroup\$ – geek225 Jul 2 at 14:30
  • \$\begingroup\$ Oh! That won't be so easy due to several non-linearities. Is this homework? \$\endgroup\$ – winny Jul 2 at 14:32

TL:DR Answer

Since the cap is across source Vac and not Vdc out, it is a rectified DC Voltage but no storage so the voltage is just a rectified sine but the current is not rectified as shown. (Looks like an drawing error)

We expect Ipk ~ 320V/50k= 6.x mA (Peak current)

Yes, you include Cap impedance, Zc=-j /2pifC =-j318 -j31.8 kOhms shunting the 2nd term in the denominator, resulting in a phasor current.

With Vpk(AC)/Vf (diode) ratio being so small, you can neglect this, which only causes a small apparent crossover distortion of +/-1.2V in the current.

You can compute Thevenin equivalent voltage and source Zs’ (at Bridge then ignore it) for the AC analysis, then load current is V’/100k+(V’-Vzt)/100k for some threshold voltage Vzk since it is drawing a few mA where the ZK Zener Knee Voltage is rated in the datasheet.

How to do step 1? Like any serial-shunt divider, except use Zc=-j /2pifC

schematic]([![enter image description here]1 )

Don't forget to add 0V symbol to DC output (-)=GND as I did.... to indicate V' properly


enter image description here

Gnd symbol=0V and is not earth GND which is connected to Neutral at source.

  • \$\begingroup\$ I thank you for your answer. I m not sure that I understand very well your meaning. So can you tell plz what modification does I have to do in the formula that I done to obtain the correct value of current. Thanks. \$\endgroup\$ – geek225 Jul 2 at 15:29
  • \$\begingroup\$ Thanks, However What's the value of V'. Is it Vin ?? and why you neglect R1 and R2 ? Thanks for your support. \$\endgroup\$ – geek225 Jul 2 at 15:39
  • \$\begingroup\$ Compare VIn with I to see effect of cap. On phase shift or phasor current. V’ is Across C with diode + R removed such that the Thevenin equiv is V’+ Z’ where Z’ is low 200//Zc. this depend how accurate you wish to compute sine current vs Vin sine voltage. then you apply load in my answer as another divider ratio \$\endgroup\$ – Sunnyskyguy EE75 Jul 2 at 15:40
  • \$\begingroup\$ 200 //Zc : How you have calculate the value of 200 ? Thanks. \$\endgroup\$ – geek225 Jul 2 at 15:49
  • 1
    \$\begingroup\$ You can also add any trace Docked to bottom or undocked and move, on screen to any Node V or any line or part for V,I,P,spectrum max,RMS,Min or duplicate existing PRobes and drag anywhere. Undo/Redo and much more with trace sample rate (options>other options) Scope trace is autoscale and you can choose MAX for full scale \$\endgroup\$ – Sunnyskyguy EE75 Jul 3 at 18:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.