NPN transistor switch help

Thread Starter

live4soccer7

Joined Jun 7, 2008
88
Thanks for the image. Well it's not a problem of having the leds light up and being bright. It's having them turn off. I've heard a few people say a 1k resistor should be used, but it seems when you do the math to find which value should be used it is a lower value around 150-180. If 1k should be used then there is a concept or something I'm missing. Can someone please enlighten me in this situation?
 

Wendy

Joined Mar 24, 2008
23,415
It revolves around collector current. What resistors are you using for the LEDs, and what is the voltage across them? While I'm asking, what is the real power supply voltage?

If you have 20ma (typical) through the Collector, you should have 2ma through the base. If you have 3.6 V for the power supply then you have 3.0V across the base resistor.

3.0V/.002A = 1500Ω

Since you have 4 legs you may have 80ma through the collector, in which case you would plug in these numbers.

3.0V/.008A = 375Ω ≈ 390Ω or 360Ω

The actual β will be higher, so a larger resistor will work. But when you design something like this, you go with worst case, so it will always work.

Turning the transistor off is easy, either open the base up or ground it. No base current, no collector current.
 

Audioguru

Joined Dec 20, 2007
11,248
He didn't use a current-limiting resistor for the LEDs so the poor little transistor conducted way too much current and got fried. Now it cannot turn off.
 

Thread Starter

live4soccer7

Joined Jun 7, 2008
88
I think audio may have it right. I didn't think I needed one because the 3.15v for the always on that was supplying the leds was a pretty good value for leds that are run from 3.0-3.6. I think I'll set up a resistor in front of the leds for .15v and 80mA. That way there is 3v going to the leds and then I will still run the 150ohm on the base because that should come out the same.

.15/.08 = 1.875 ohm

I'll have to see what kind of resistors I have at that value. I know I have a couple 1ohms. Hopefully this will solve the problem.
 

Thread Starter

live4soccer7

Joined Jun 7, 2008
88
definitely just tried to setup the same setup, but with a 7.5ohm resistor before the leds. Still the same results.

I don't know why it wouldn't turn off because there is no voltage to the base when it is off and with the 7.5ohm resistor on there I can't see how it would fry the transistor. Any ideas?
 
Last edited:

Wendy

Joined Mar 24, 2008
23,415
You must have a larger resistor, it sounds like what several people said at first may be right, you need more voltage on the power supply. This is so you can actually control the current to the LEDs. Having a voltage that is 0.1-0.2V voltage greater than the Vf is not enough. The resistor for LEDs is not a luxury, but an absolute requirement. Reread the chapter about LEDs in my article.

LEDs, 555s, Flashers, and Light Chasers


From the sounds of it the only reason the LEDs are alive is the transistor threw itself in front of the bus. Unfortunately, they are probably damaged too.

BTW, the article covers transistors too.

Is this voltage from a coin cell perhaps?
 

Thread Starter

live4soccer7

Joined Jun 7, 2008
88
actually I have solved the problem. All of our calculations were right. The device is usb powered or battery powered and I swear I checked the switch voltage in both cases I found it is not switched when it is being plugged in via usb. It works perfect with just the batteries powering it. I've been hunting for a switch voltage, but no luck yet for the USB power.
 
Top