Turning off NPN Transistor, Frustration

Thread Starter

poopscoop

Joined Dec 12, 2012
140
I'm using Arduino to control a SPST N/O Relay. I want to run the relay coil on 12v, so I intend to use an NPN (2n4401) to use the 5v signal to switch the 12v source.

I've set up the NPN on a breadboard to test. I can draw a schematic if necessary, but right now its a 7812 on the 12v line, leading to a resistor/LED and the collector. A 7805 provides the signal on the base of the transistor, and the emitter is connected directly to ground.

Turning 12v on is no problem, but I can't turn it off. I have a resistor from base to ground, I've tried connecting the base directly to ground without the 5V supply connected. Nothing will turn off the LED, only dim it slightly.

What am I doing wrong? Is there a better way to control the relay through the 5v signal?
 

tracecom

Joined Apr 16, 2010
3,944
Assuming you are using the 5V output from the 7805 simply to supply base current to the 2N4401 (through a 10k or so resistor), I would guess you have the circuit wired wrong.
 

Thread Starter

poopscoop

Joined Dec 12, 2012
140
I'm obviously disconnecting the base from the 7805 (and touching it to ground) when I want it to turn off.

I'll post a schematic soon.
 

Evil Lurker

Joined Aug 25, 2011
116
If you are using an NPN transistor you should have a resistor in between VCC and the base to limit the current and voltage going into the base, not from the base to ground.

If you hooked the base directly to +5v without a current limiting reistor, then there is a good possibility you smoked your transistor, thats why you can't turn it off.
 

Thread Starter

poopscoop

Joined Dec 12, 2012
140
This works as shown. Connect R1 to the base of Q1 to light the LED. Disconnect it to turn it off; no touching to ground required.

This is exactly what I have. I'm going to assume I smoked this transistor in a previous project and didn't toss it. I'll try with a fresh set.

I appreciate the advice.
 

Thread Starter

poopscoop

Joined Dec 12, 2012
140
Transistors tested fine, but when I connected the LED's to emitter and the collector to ground everything worked fine.

I'm 99% sure I know how to read. There is some ambiguity on my pinout as to whether I'm looking at the transistor top-down or bottom-up. Evidently I misinterpreted.
 

Papabravo

Joined Feb 24, 2006
21,159
When the base emitter junction of a transistor is "forward biased" it wants to be about 0.7 V above the emitter. The base resistor allows this to happen by dropping the voltage from whatever is on the other side (5.0V) in this case down to 0.7 volts which is where the transitor wants the base to be so the emitter can be at GND.

It is not uncommon to add a resistor from the base to GND to make sure that if there is no driving source on the other side of the base resistor that the base will eventually return to GND (No cuurent flow <==> No voltage difference)

<==> means each of those conditions implies the other one.

In your example if you have a 10K base resistor then it should have:
(5.0 -0.7) / 10K = 430 μA

If the transistor has a β of 100 then the collector might supply a current of 100 * 430 μA = 43 mA

This of course will be limited by the LED and the series resistor, but at least you have an idea of what current might be avalable.
 

Thread Starter

poopscoop

Joined Dec 12, 2012
140
I misread the pinout drawing and had the collector and emitter mixed up. It is now wired correctly, with the emitter to ground and the collector attached to the load.
 

Shagas

Joined May 13, 2013
804
If you are using an NPN transistor you should have a resistor in between VCC and the base to limit the current and voltage going into the base, not from the base to ground.

If you hooked the base directly to +5v without a current limiting reistor, then there is a good possibility you smoked your transistor, thats why you can't turn it off.
Check your collector-emmiter with a multimeter for continuity .
If it buzzez then use a new transistor :)
 
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