0
\$\begingroup\$

enter image description here

If I simulate this circuit in Multisim everything works fine, but in reality the LEDs burn, if I put a 1k resistor in series with the capacitor the LEDs no longer burn

I think the LEDs are burned by the initial charging current of the capacitor that must be high and while the capacitor is charging the voltage drops all over the LEDs and they burn

I have tried all the possible simulations in multisim and at the moment of time T = 0 only marks 10 ma of maximum current, this cannot be possible, in T = 0 the current must be very high because in the circuit there is no resistance

Thank you

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Try starting SIM with 90 phase shift to get peak voltage across cap with 0 ESR then you will see infinite current \$\endgroup\$
    – D.A.S.
    Commented Feb 16, 2020 at 10:58
  • 1
    \$\begingroup\$ Um, also, if you do this in reality, you're living a very dangerous, and thus potentially very short, life. \$\endgroup\$
    – Marcus Müller
    Commented Feb 16, 2020 at 11:15

1 Answer 1

Reset to default
0
\$\begingroup\$

It only works fine in the simulator because with a phase angle of 0° it starts at 0 V. In reality when you switch on the mains it could be at any point in the waveform. Change the phase angle to 90° or 270° to see what would happen if it was switched on at peak voltage, and then you will know why your LEDs die.

But even if you could somehow ensure that your circuit was only switched on during zero crossings, operating it without series resistance is a bad idea. At 50 Hz the capacitor has sufficient impedance to limit the current, but at higher frequencies it doesn't. AC mains often has higher frequencies impressed on it due to inductive devices such as motors and transformers being switched on and off, 'ripple' control signals etc. The LEDs might survive a few spikes but will eventually die.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks ..., it is correct if I put 90 degrees the initial current is very high, in multisim desktop it works in the online version it does not work for me, maybe I do something wrong I know the risks that you indicate, but I was just doing a small experiment, trying to light two LEDs with the minimum power dissipation, thanks your response has been very helpful \$\endgroup\$
    – Mario
    Commented Feb 16, 2020 at 20:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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