big resistors, zener diodes, and transistors?

Thread Starter


Joined Dec 31, 2007
ok, these are hopefully somewhat easy questions. I'm fairly new in electronics so I'm always afraid of my own calculations and what I've learned by reading. It's easy to mix things up, I thank everyone for any help ahead of time.

First question,

I'm replacing a 21W 12v lamp with a resistor (I know this will be a very large resistor), These are my calculations, am I correct?


21W/12v = 1.75 A
12v/1.75A = 6.86 Ω

so the resistor should be >= 21W and 6.86Ω

The closest I could find was 50W and 6.81Ω, which will work, right? It would be pulling 1.762A instead of 1.75A, close enough for a lamp.

ok, that was the only question I think I could completely answer myself.

Second question,
The correct base resistor for the transistor always confused me. Online, I always see a 1KΩ resistor used, so is my assumption correct that you can always use a 1KΩ base resistor to fully saturate the transistor? Does the entire circuit need to use the same voltage for this to work or will it work regardless? Such as if the device in the image below used 12v, and the 5v logic from the IC was the same, would it still work properly? I've read so many different things on them, im completely confused.

I have used transistors in the past with the 1KΩ resistor trick, but I just wanted to be sure I was doing it right.

Third and final MONSTER question.

I may need to create a separate thread for this one.
I really appreciate you sticking with me thus far.

This is a zener diode regulator question (you'll see)

I'm driving rows of LEDs from an IC. I'm using an unstable 12v source voltage (12v - 14.5v) which is fine for the IC as it works off 5v so I'll just use a regulator but I was hopping to use the least amount of transistors to power the LEDs

There are 5 LEDs in each row, and 16 rows. The rows are divided into four sets. Each set will turn on independently of each other. There will never be more than one set on. Below is an image of what i'm talking about. Color coded for each set.

So my problem is the power. I can figure out what size resistor to use so I don't burn out the LEDs but I don't know how to supply power to them.

as stated above, power is 12v - 14v fluctuating. I wanted to use a zener regulator circuit but if there's something better, i'll listen.

Each LED is 20ma, 2.1v forward voltage. So each row will require 100mA. As there will never be more than 4 rows on at a time, the max current draw is 440mA (400 + 10% added for safety). voltage will need to be 10.5v as they'll be running 5 in series. Would it be possible to source 10.5v with a zener circuit? I've read that your source voltage must be 2v higher than your desired output. This would put me 1.5v below below my source which wouldn't work.

Do I have any chance? lol
I don't mind running multiple regulating circuits, or even individual for each set if I have to as I realize there is alot of current going thru. But I really don't want to have to break the rows of 5 up [if possible of course] (easy to solder if in a row, I don't want wires hopping everywhere).

Thank you everyone for all the help! I really, truly appreciate it. I hope I made this as clear as I could, I've been making images and collecting data for the last three hours, I tried to make this as simple as I could.

Thanks again.


Joined Feb 26, 2008
Question one looks spot on.

Question two:
You are limited by the gain of the transistor (Beta).
1k will work for a small load.
A smaller resistor will allow a larger load.
A much larger load will need a darlington pair, or a mosfet.

Question three:
Placing the LEDs in series doesn't raise the current.
It's still just 20mA through each string of 5.
I'm still reading through the rest.


Joined Feb 26, 2008
Look for a low drop-out regulator (LDO).
They need less than 2V extra to work.
A LT1117 would work; only 1.3V drop worse case (800mA, below 0*C)
Last edited:


Joined Mar 24, 2008
Have you read this?

LEDs, 555s, Flashers, and Light Chasers

One of the things to remember, LEDs vary their voltage drop according to color. Define the LEDs and the resistors define themselves. Most LEDs take 20ma, thought their are a lot of exceptions. You can find LEDs that take .7A, or even over 1A.

Thread Starter


Joined Dec 31, 2007
thank you both very much for the help.

Placing the LEDs in series doesn't raise the current.
It's still just 20mA through each string of 5.
Good to know. So when putting LEDs in series you only need to add the needed voltage together which I why I need 10.5v @ 2.1v x 5 LEDs ?

Thanks I would have been messing things up royally, lol

as for power, I've decided to go with a LDO like you said. I'm going with 10v as I can't find a 10.5v that suits what I need (unless I want to buy 2K of them). Unfortunately I'll have to create rows of four now, I guess it's not the end of the world.

Here's the new circuit (so far):

I'll never be pulling more than 100mA from the LDO so I was able to get real cheap ones which I think are actually less than a zener diode, lol.

The Final problem I have are the transistors, i'm a little confused how to properly calculate R1. Again, this is just something I've read about too many times and everyone seems to have a different opinion on how to use transistors.

so, is this correct?

my load is 20mA (4 LEDs)
transistor has hfe of max 300 (this is also called beta?)

this is what the datasheet says:
Symbol | Parameter | Test Condition | Min. | Max.
h fe 1 | DC Current Gain | VCE=1V, IC=5mA | 45 | 300
h fe 2 | DC Current Gain | VCE=1V, IC=100mA | 85 | 300
h fe 3 | DC Current Gain | VCE=1V, IC=800mA | 40 | 300

What am I supposed to use as hfe?

would this be a popper formula to get R1?
R1 = Supply Voltage / ( Maximum Current Required / Minimum HFE * 1.3 )

supply = 10v
max current = 20mA
hfe = 40 ???

R1 = 10/(0.02/40*1.3)
R1 = 15384.615
R1 = 15Kohm resistor

But if I used something like a 10Kohm it would work too and I wouldn't melt the transistor, right?

I'm sorry for my great confusion but there are no two sources saying the same thing so It's all very confusing. Again, thanks so much for the help.


Joined Jul 17, 2007
You're using the transistors as a switch. You basically want to get the transistor into saturation. Usually, somewhere between 1/20th and 1/10th of the collector current is enough to saturate the transistor. Split the difference and go for 1/15th, or 20mA/15=1.333...mA

If you're driving the transistors' bases from a 0-5v signal, you'll need to determine what resistance will give you 1.333... mA current through the base when 5v is input.
I=E/R; conversely R=E/I, so R=5v/1.333...mA = 3.75k Ohms. You could go with 3.3k or 3.8k.