SPDT switch voltage drop

[syedaamna]

I have attached the schematic diagram. Why there is a voltage drop of almost 2.8V between source and SPDT switch?

 

[GGoodwin]

Hi syedaamna,

Switches are simulated using an ideal switch with a small series resistance (R_ON) and a large parallel resistance (R_OFF). The values of these resistances can be adjusted by the user. The default settings for Multisim Live are 100 μΩ and 100 MΩ, respectively. Current flow through the switch resistance causes a voltage drop between the terminals.

In your circuit, SPDT switch S2 is connected to 5 V, the LED is connected directly from the junction of the cathodes of the diodes (LED anode) to ground (LED cathode). Very large current flows from V2 through S2 and D2 because the resistances that can limit current are very small (100 μΩ from S2 and very small equivalent resistances from D2 and LED).

As I mentioned above, this large current causes a noticeable voltage drop as it flows through the switch even when the switch resistance is very small. We can place a probe to reveal this current

The current is 28.027 kA (28,027 A), enormous indeed. If we multiply this by 0.1 mΩ (100 μΩ) the result is the 2.8027 V that you are inquiring.There is no current overload fault simulation provided for diodes and LEDs so simulation will continue with the circuit treated as properly working.

The necessary revision for your circuit is to connect the resistor in series, not in parallel, with the LED

It can be seen from the image that with a resistor of appropriate value in series with the LED, the voltage drop across the switch is practically 0 V. Current through the LED is also reduced to just 17.706 mA.

Best regards,

G. Goodwin