Simple LED Circuit Simulation problem?

[Trevor33]

Hello, and I recently started to use Multisim so that I can design circuits using schematics and then trying them out on the protoboard. However something is confusing me from the simulation. 

So on a real protoboard with 5V and a 330 ohm resistor and ground, I can connect a green LED and it will turn on. Obviously if I take out the resistor my led will burn out and it won't work anymore. 

On Multisim, If I add a 5V VCC, a green LED and ground (no resistor), and when i start the simulator, it turns on. I'd expect the LED to be off since a direct voltage of 5V is not supported for an LED. 

Is the simulator suppose to behave this way or how will I be able to tell if some circuits got burned? 

[johnsold]

I do not know how Multisim's LED is modeled. Many models do not include "burn out" factors. Since diodes have exponential current-voltage relationships, you may find that the LED is drawing several amperes at 5 V.

Most simulation models work well when the devices are operated within their normal ranges but may not be at all realistic when outside those ranges.  For example some op amp models can produce output voltages far in excess of the power supply voltages, which real physical circuits cannot do.  It is important to determine the limitations of the models when evaluating the quality of a simulation.

In many cases the only way to detect burnout is to calculate the power dissipated in the device and compare that to its rating.

Lynn

[Trevor33]

There isn't a circuit simulator that actually warns about frying a component or something similar to this function? From what I'm learning I will have to manually check the circuit currents in the simulator to see if they are within range in their datasheet. 

[johnsold]

I think a subset of the Multisim components do have the capability you want, but I do not know any details. I do not have Multisim but am a pretty good circuit designer, so I try to contribute here when I can.

Lynn

[stclairg]

I have to say the modelling for diode is awful.
I registered to figure out the math for my circuit for LED stair lighting using 8W/m 24v LEDs in parallel for the 18 steps that I planned to wire. A quick Excel calculation tells me that this should have been 144W and was getting 549A (at 24v) out of my main voltage source and 30.453A across each diode in multisim. It took me ages of fiddling with settings before googling and finding this issue and eventually swapping all of my LEDs for lamps to get to the numbers I had on paper - now verified. I should have gone to a different product, this was a terrible experience.I thought there was an issue with the "motion sensor" part of my circuit so spent ages re-wiring that over and over to end up at the same result each time, even have the diodes lit when they should all be off! The diagram for this is here https://www.multisim.com/content/uSCDXWZ95HsJCoJ39V2EQX/stair-lighting/

Corrected version using lamps - https://www.multisim.com/content/MQ4MC5P6QK5JXArfrHruLd/stair-lighting2/