Multisim and Ultiboard

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why doesn't the LED light up in multisim?

ThatDood_2-1628431598122.png

The simulation is running but the LED on the right does not light up. The forward voltage for the LED is 1.83V which is the LED implemented into Multisim already. Using Voltage divider, The voltage across the resistor on the right is 1.875V, higher than the forward voltage of the LED and on the left is 1.25V which is lower than the forward voltage of the LED. For some reason they both do not light up. I tried increasing both resistors to 4k ohms but it is still the same

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Message 1 of 3
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Hi ThatDood,

 

 

The battery cannot supply enough current to make the LED sufficiently glow. The hindrance here is the high resistance of the thermistor. Increasing the value of the shunt resistor to 4 kΩ is inadequate, note that by removing it (increasing the resistance to ∞) the current could not come too close to 1 mA. Check the On current (ION) of the LED from its settings.

 

If you want to make this simple thermal sensor work you have to find a way to lower the resistance that limits the current from battery. Otherwise you need to find alternative configuration. Another important concept to consider when trying to make this circuit work is self-heating. Ideally, the change in resistance of the thermistor should depend only in the change of the temperature of the surrounding or the object the thermistor is having contact. If you supply the LED current through the thermistor, the transducer itself will heat up due to current flow. This will have significant influence in the change in resistance of the transducer thus causing deviation of its response from surrounding temperature.

 

Of course you may also consider using transistor(s), op amp, or comparator.

 

 

Best regards,

G. Goodwin

 

Message 2 of 3
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можно чувствительность светодиода настроить, для того чтобы схема заработала, хотя на 0,1 миллиампер я такого не встречал😀

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Message 3 of 3
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