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I have a 2N3904 NPN transistor. I'm feeding the base 5 V. The transistor turns on just fine and it shorts the collector and emitter. However, when I turn my meter on volts, I read around 1- 1.5 volts across E and C. I am using this transistor in a project as a sinking output. Why am I reading this voltage and can I get rid of it?

I'm controlling the transistor with an Arduino. I'm writing HIGH for 4 seconds, then LOW for 4 seconds. I'm only showing 1.5 V across the E and C when the Arduino writes LOW. What is going on?

Here is a drawing:

Drawing

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    \$\begingroup\$ If your Vbe is 5V, your transistor is destroyed. I think it's more likely you have a resistor you didn't tell us about somewhere; draw a schematic if you want any meaningful help. \$\endgroup\$
    – Hearth
    Commented Mar 14, 2019 at 19:26
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    \$\begingroup\$ A schematic would be good, so we can at least understand your intent. My suspicion is that you've neglected to add a base resistor and instead are feeding 5V directly to the base with no current limit. This will saturate the B-E junction and kill your gain. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Mar 14, 2019 at 19:26
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    \$\begingroup\$ The OP writes that the 5 V is applied to the base. This does not tell me that the Vbe is 5 volts, but that a 5 V reference is tied to the base. There's no good information on the rest of the circuit. \$\endgroup\$
    – jonk
    Commented Mar 14, 2019 at 20:39
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    \$\begingroup\$ Am I correct in reading your drawing in saying that there is nothing connected to the collector other than your multimeter? And you have red to collector and black to emitter? Is it digital or analogue? Make and model? \$\endgroup\$
    – Transistor
    Commented Mar 14, 2019 at 21:48
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    \$\begingroup\$ So you're reading a point of high impedance, connected to a diode, with something that's high impedance. You may just be rectifying stray line-frequency radiation in the room. \$\endgroup\$
    – TimWescott
    Commented Mar 14, 2019 at 22:58

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I'm only showing 1.5 V across the E and C when the Arduino writes LOW. What is going on?

When the Arduino writes LOW the transistor is turned off, so it is not shorting between the Collector and Emitter.

However this is only true when the voltage between Collector and Emitter is positive. If it is negative by more than ~0.5 V the transistor will turn on in 'reversed Emitter follower' mode. If an AC voltage is applied then it will act as a half wave rectifier and the meter will show the average rectified voltage.

"But I am not applying an AC voltage!", you say. But you probably are. Your body is effectively a large conductive plate with a typical capacitance to ground of ~100 pF. Voltage will be induced onto it from the electric fields put out by mains wiring. Depending on the strength of this EMI, just holding onto the meter lead could induce several volts onto it.

Normally you don't notice this induced voltage because the coupling capacitance is very low, so at mains frequency its impedance is very high. Therefore - unless the circuit you are probing also has very high impedance - the coupling capacitance will drop almost all the voltage due to its higher impedance, and what's left will be below the threshold for rectifier action. Without rectification the multimeter will show close to 0 V because it is designed to reject AC on DC ranges.

The circuit below represents the situation.

schematic

simulate this circuit – Schematic created using CircuitLab

Here's the result of simulating the above circuit in LTspice. The average voltage of this waveform (which the meter would read on DC) is 1.5 V.

enter image description here

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