Current flowing through transistor even when base is shunted to ground.

[EasyGoing1]

Re: Current flowing through transistor even when base is shunted to ground.

It's the same reason you don't connect an LED directly to a power source without a resistor or some other type of circuit to limit current.

Now THAT makes a lot of sense. I've noticed that resistor values make a huge difference when choosing one for LEDs. I was surprised when you said anything from 500 to 5k should work fine ... I guess bias current in a transistor isn't as sensitive in terms of results as a current limiting resistor is for LEDs?

[EasyGoing1]

Re: Current flowing through transistor even when base is shunted to ground.

Transistor is now damaged and leaking due to missing base resistor. Nano's output voltage should be low enough to keep it in cutoff.

Probably should have used a 2N7000 if you really don't want to put a resistor there. That is also to possibly protect the Nano's output, too.

I try to be quick when I'm soldering, but I did consider the possibility that I damaged it when soldering it up... I don't have a 2N7000, but I do have these NPNs that came in a transistor kit that I purchased:

BC337
2N2222
S8050
C1815

And these PNPs:
BC327
2N2907
2N3906
S8550
A1015

[momaka]

Re: Current flowing through transistor even when base is shunted to ground.

I assume you mean a resistor between pin D3 and the base of the transistor. What should its value be, and what does that resistor do?

It limits the current going into the Base. It's the same reason you don't connect an LED directly to a power source without a resistor or some other type of circuit to limit current - the diode/LED will draw a lot of current and burn out.

For the purpose of driving those LEDs with that transistor, a 1-KOhm resistor should do fine (though I imagine anything in the range of 510 Ohms to 5.1 KOhms would work OK too.)

[EasyGoing1]

Re: Current flowing through transistor even when base is shunted to ground.

Also, looks like the OP is driving those LEDs pretty hard... a few hundred mA?

The LEDs are rated at 3V @ 150ma each. When I turn them on, they only stay on for 200ms then off for 100ms and that repeats a few times, then they stay off. They aren't turned on for very long at all. I used an online LED array calculator to get the layout of the LEDs and the value of the resistor (6.8 Ω). The actual supply voltage is 9.8V.

[EasyGoing1]

Re: Current flowing through transistor even when base is shunted to ground.

^ This.

Although BJTs require a relatively significant amount of current to bias (as in, it's unlikely that stray external noise / electrical signals can turn it On), it's still a good idea to include pull down / pull up resistors (depending on BJT type and/pr desired operation.) And for MOSFETs, this is practically required. Leaving a Gate pin floating can make a MOSFET self-bias from stray noise.

Also, as I learned in digital electronics classes the hard way, never leave input pins on a CMOS chip floating - that too, is asking for trouble. Heck, even for TTL chips it's a good idea to always tie down inputs to something.

On the Arduino Nano, when you designate an input pin as INPUT_PULLUP, it assigns a pullup resistor to the pin internally. That keeps the pin idling at 5V until you do something to it that pulls it down ... the output from the motion sensor in my case pulls it down low enough to trigger the interrupt which I use as the condition to fire the LEDs.

[EasyGoing1]

Re: Current flowing through transistor even when base is shunted to ground.

You need to have bias resistor for he Base of the Transistor.
When is the Voltage at the B-E when the D3 is LO?

I assume you mean a resistor between pin D3 and the base of the transistor. What should its value be, and what does that resistor do?

The B-E voltage when D3 is low, is 0.002 according to my Fluke meter.

[momaka]

Re: Current flowing through transistor even when base is shunted to ground.

A wiser approach might be to add the resistor AND a pulldown at the base to cover the case when the MCU is "in reset" and the pin may float before the software has a chance to configure it as an output -- assuming that is a possibility (I don't use arduinos)

^ This.

Although BJTs require a relatively significant amount of current to bias (as in, it's unlikely that stray external noise / electrical signals can turn it On), it's still a good idea to include pull down / pull up resistors (depending on BJT type and/pr desired operation.) And for MOSFETs, this is practically required. Leaving a Gate pin floating can make a MOSFET self-bias from stray noise.

Also, as I learned in digital electronics classes the hard way, never leave input pins on a CMOS chip floating - that too, is asking for trouble. Heck, even for TTL chips it's a good idea to always tie down inputs to something.

[Curious.George]

Re: Current flowing through transistor even when base is shunted to ground.

i dont know the issue, but you may have damaged the transistor - you need a resistor between the base and the mcu

That would just serve to limit the current through the BE junction. Presumably, the output impedance of the MCU is high enough that the "resistor" is actually part of the MCU.

(The OP would note that the output of the MCU swings between 0V and ~0.6V as the BE junction clamps Vo(h) to that level.)

A wiser approach might be to add the resistor AND a pulldown at the base to cover the case when the MCU is "in reset" and the pin may float before the software has a chance to configure it as an output -- assuming that is a possibility (I don't use arduinos)

Note that you want to size the pulldown so that it "soaks up" any leakage current from the MCU.

Also, looks like the OP is driving those LEDs pretty hard... a few hundred mA?

[eccerr0r]

Re: Current flowing through transistor even when base is shunted to ground.

Transistor is now damaged and leaking due to missing base resistor. Nano's output voltage should be low enough to keep it in cutoff.

Probably should have used a 2N7000 if you really don't want to put a resistor there. That is also to possibly protect the Nano's output, too.

[budm]

Re: Current flowing through transistor even when base is shunted to ground.

You need to have bias resistor for he Base of the Transistor.
When is the Voltage at the B-E when the D3 is LO?

[stj]

Re: Current flowing through transistor even when base is shunted to ground.

i dont know the issue, but you may have damaged the transistor - you need a resistor between the base and the mcu

[EasyGoing1]

I made a small project where a motion sensor triggers an Arduino pin and when the pin from the motion sensor goes high, another pin that's connected directly to the base of a 2N3904 goes high, which switches some LEDs through a 10V source.

When I breadboarded the project, it worked perfectly. But now that I have it soldered onto one of those generic project PCBs when the circuit is idling and there is no motion triggering the pin, the LEDs light up ever so slightly, indicating to me that the transistor is still allowing current to flow from emitter to collector even when the base is at 0 volts. I even tried shunting the base directly to ground and the behavior did not change.

Any idea what the problem might be?

Here is the schematic: