virtual lamp

[greg1b]

I am a first year apprentice electrician and have recently installed Multisim v11.  I can't find info in the help files to answer these questions and am hoping for some help.  Why is a filament sometimes red?  Why do the lamps sometimes flash?

Thanks,

Greg

[Tom Sedlack]

It has to do with the way the simulator runs.  An AC sine wave starts at 0 volts (the lamp is off), starts ramping up (lamp is on but not with a red filament), and gets to the peak voltage (on with red filament) then starts back to 0 and repeats for the negative half of the cycle.  To see this, add a scope to your schematic and connect it in parallel with the multimeter.  If you lower the frequency of the AC power source to around 10 Hz, you will be able to easily see the 3 state changes and compare them to the voltage level on the scope.  Real world light bulbs act the same way but, because the filament does not shut on and off instantaneously, your eye can't detect the flicker.  It is also doing this 120 times per second (at 60 Hz, 60 on the positive half of the cycle, 60 on the negative half of the cycle. Much faster then the simulator

[greg1b]

Thanks Tom.  I somewhat understand your explanation but its not answering my questions.  I haven't designed a circuit other than similar basic circuits either with an AC supply or DC supply, resistors, and/or lamps.  In the same circuit I've noticed a lamp not lit, a dimly lit lamp, and a bright lamp with a red filament.  In another, a brightly flashing lamp with a red filament.  Without knowing the values of each, what can I immediately assume or suspect?

Thanks.

[Tom Sedlack · Accepted solution]

Greg,

Wire Multimeters across each of the 3 lamps.  You will see that each lamp has a different voltage across it.  If you compare the voltage across the lamp with the with the rating of the lamp, it follows what I mentioned before.  Voltage too low, the lamp is off. Voltage close to rating, the lamp is on but not red.  Voltage at or above rating, the lamp is on and red.  

The voltage drop across each lamp is based on the voltage setting and the wattage of the lamp.  Multisim appears to use a simple Ohm's law calculation to determine the resistance of each lamp based on the voltage and wattage.  The formula is R(ohms) = (Volts * Volts) / Watts.  If you replace each lamp with the calculated resistor, the voltage drop will be the same.

See the attached file to see the relationship of the voltages as well as the effect of the voltage along an AC sinewave on the flickering effect.

[greg1b]

Thank you, Tom.