# proteus dc voltmeter and ammeter results are not accurate / non existent!

I'm a beginner to Proteus and was just exploring simple circuits and trying out different things on the simulator. I built this simple circuit with a potentiometer and an led and measured current and voltage on different points. They just don't add up! (i.e. make sense with theoretical readings)

Firstly, why is there no reading on the ammeter on the bottom-left? The 1K resistance is a load which has a voltage drop of 2.55v and thus there has to be a current through it! (0.00255A according to calculations, which is 2.55mA)

Second, the series resistance of the LED is set to 1 Ohm, so if 2.45v drops across it, the current should be 2.45A which is ridiculous! Anyways the ammeter connected to it shows only 0.25A (255mA when set to mA measurement) which I don't understand... Is there something wrong with my calculations with the Ohm's Law? Thanks in advance for any help :)

EDIT: Please ignore the second part of the question as I forgot at the moment that LEDs where NON-Ohmic conductors....

Please Note: There are no warnings or errors in the message section of the simulation errors tab.

• Would work better if the LED were connected in series with RV1 instead of connecting directly between +5V and ground. LEDs and diodes do not follow Ohm's law, they instead have an exponential relationship between current and voltage. This is why we can approximate LED forward voltage by a constant 2V with a series resistor - not an accurate model, but good enough for normal operating conditions. A real LED will briefly burst into flames if put directly across 5V. – MarkU Aug 11 '17 at 0:05
• But to vary it's brightness it has to be connected to the suffer terminal of the pot roast it would just be like a 1K resistor in series and I forgot that LEDs are NON-Ohmic conductors sorry.... But what about the ammeter at the bottom left?? Why does it not show the appropriate reading? – Vishal Dhayalan Aug 11 '17 at 12:23
• RV1 is a variable (3-terminal) resistor then? However as shown at 100% of full scale, the effect is the same as if D1 was connected to VCC directly. I do see what you mean about the problem with the ammeter in series with RV1. If there is 2.56V across 1k then there must be 2.56mA flowing through the 1k RV1. (If the VCC supply was not being crashed by the LED, then it would be 5V across 1kOhm with 5mA of bias current through RV1.) – MarkU Aug 11 '17 at 19:46