Electronic – Voltage drop across LED in open circuit


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I've searched the forums but can't find an answer for my issue. I've got a simple LED circuit powered by 5V DC. When the switch is closed, I've got 2.8V across the resistor, 2.2V across the LED, and 0V across the switch (as expected).

However, when the switch is open, I get 3.45V across the switch but no other noticeable readings (0V across resistor and 0V across the LED). Where did the other 1.55V go? If I shunt across the LED, I get the expected 5V across the open switch connections. How is there a voltage drop across the LED in an open circuit and why can't it be measured with a DMM?


simulate this circuit – Schematic created using CircuitLab

Best Answer

It comes down to the LED still conducting current when it is not illuminating. Take a look at this characteristic from an OMRON LED. It's very rare to see stuff like this in data sheets: -

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This shows the volt drop (about 1.3 volts) across an LED when 1 uA is flowing and it's not unreasonable to expect all LEDs to behave within a ball-park or two of this. So you are seeing a volt drop across the LED due to the current through the meter's inputs when placed across the open switch. If the meter's input resistance is 10 Mohm then there is about 0.345 uA flowing and there will be probably over a volt dropped across the LED should it be the device in the picture above.