why common GND needed for transistor switch to work

Thread Starter

MaxMix

Joined Dec 1, 2016
3
Hi all, a new member here and new to electronics too...
I'm a bit puzzled by the behavior of my 2N3904 transistor switch - probably missing something very fundamental.
I have a device with a digital pin terminal and I'm trying to pull down voltage on the control pin (5V). If I do it just by shorting it to the ground on this terminal it works fine (trigger received). When I try this with 2N3904 (pin@5v to C, E to device ground and external 5V to B; taking 5V from Arduino) it works only if have the common ground (device GND linked to Arduino GND). If I don't connect to Arduino GND the transistor is able to drop only 1.2V (5 -> 3.8V) and the trigger is not taken.
I'm not sure if I understand why this happens from the perspective of a mechanism....
Could someone explain that to me, please?

1.png 2.png
 

MaxHeadRoom

Joined Jul 18, 2013
23,342
Because you are attempting to couple two separate circuits and there is no common reference point.
You would need something along the lines of Opto isolation for it to work otherwise.
Max.
 
Last edited:

djsfantasi

Joined Apr 11, 2010
7,750
That is because the transistor needs to be able to reference the voltage/current on the base to some common reference point. Unless you connect the grounds, the signal from your device is meaningless o the transistor, because it has nothing to reference it to.
 

Thread Starter

MaxMix

Joined Dec 1, 2016
3
Cheers for that. Yes, I sort of get the headline... but I'm just trying to figure out why 1.2V were indeed dropped at all.
Would it mean that it so just happened that there was a tiny potential difference between Arduino 5V and Device's GND (I actually measured it at 40mV) and this allowed a minuscule BE current which opened the CE just a bit to flow a very small current and drop 1.2V ?
 

Attachments

cmartinez

Joined Jan 17, 2007
7,360
Hi all, a new member here and new to electronics too...
I'm a bit puzzled by the behavior of my 2N3904 transistor switch - probably missing something very fundamental.
I have a device with a digital pin terminal and I'm trying to pull down voltage on the control pin (5V). If I do it just by shorting it to the ground on this terminal it works fine (trigger received). When I try this with 2N3904 (pin@5v to C, E to device ground and external 5V to B; taking 5V from Arduino) it works only if have the common ground (device GND linked to Arduino GND). If I don't connect to Arduino GND the transistor is able to drop only 1.2V (5 -> 3.8V) and the trigger is not taken.
I'm not sure if I understand why this happens from the perspective of a mechanism....
Could someone explain that to me, please?

View attachment 116234 View attachment 116235
Remember that an NPN transistor works when a current flows from its gate to its emitter. In your circuit, if you don't connect the emitter to the ground of your arduino, then that current has nowhere to go.
 

crutschow

Joined Mar 14, 2008
27,394
You missed the fundamental rule of electricity.
Any current that flows has to have a return path and all voltages are relative.
Thus there can be no one-way current flow and no voltage without a reference point.
Often those are both provided by the common/ground connection.
 

BobTPH

Joined Jun 5, 2013
3,474
That is like trying to light a bulb by connecting one lead to the positive terminal of a battery. If you can figure out why that wouldn't work, you can answer your own question.

Bob
 
Top