Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Calculating base resistor on transistor?

Status
Not open for further replies.

R2-D2

Member
I just wanted to be sure of the right value.
I have 4 LEDs hooked up to a 3904 transistor. Each LED is lit at 3V, 20mA - I figure the LED resistance is 150Ω while lit - I guess. Series resistor 120Ω. Source is 15 volts. Load to the 3904 collector is therefore [120Ω resistor + (4*150Ω LEDs)] = 720Ω?

So, my base resistor is RB = 0.2 × RL × hFE , about 30k? (hFE is about 200)
Right?

Transistor Circuits

Thanks!
 
I just wanted to be sure of the right value.
I have 4 LEDs hooked up to a 3904 transistor. Each LED is lit at 3V, 20mA - I figure the LED resistance is 150Ω while lit - I guess. Series resistor 120Ω. Source is 15 volts. Load to the 3904 collector is therefore [120Ω resistor + (4*150Ω LEDs)] = 720Ω?

So, my base resistor is RB = 0.2 × RL × hFE , about 30k? (hFE is about 200)
Right?

No, you're not trying to use the transistor as a linear amplifier, only as a switch - you want the transistor to be switched hard on, so use a much lower value - 1K should be fine.
 
Thanks Nigel.
Yes, I am using it as a switching transistor from a 555 timer circuit.
 
Then why not just connect the LEDs directly to the output of the 555 which can supply up to 200mA?
 
Then why not just connect the LEDs directly to the output of the 555 which can supply up to 200mA?


Oops, I didn't look at the circuit right. Rather, the LEDs are connected to an OR gate output (4071).
 
Then it needs a transistor.

Please post the schematic, as it's CMOS, you might not even need a base resistor, especially if the supply voltage is low and the output isn't connected to anything else.
 
schematic posted

Please post the schematic, as it's CMOS, you might not even need a base resistor, especially if the supply voltage is low and the output isn't connected to anything else.

Part of the circuit...
 

Attachments

  • Dual input LED ci&#1.jpg
    Dual input LED ci&#1.jpg
    44.1 KB · Views: 278
Please post the whole circuit.

You might be able to save an IC by using diodes for the OR gate.

What's the supply voltage to the logic gate?
 
The schematic shows one of sixteen of those branches of LEDs. That's why I'm using four of 4071 OR gate CMOS chips (with sixteen transistors). The interconnections between outputs of the dual timer circuits into the OR gate inputs is a bit complex. I am applying 15 volts to all 4071 and the 556 timer.

I built one branch (as shown in schematic) on a breadboard with the timer circuit and tested it without a resistor to the 3904 transistor base. To my surprise, the transistor did not get hot without its base resistor. Then I added 1K.....then 10K to its base and LEDs were just as bright, no change in their intensity. Any higher than about 10K to its base then the LEDs begin to get dimmer. I think this is because there's more voltage drop across the transistor (C-E) as its base resistance increases which allows less voltage across their corresponding LEDs. I might just go with a 1K - seems to work fine.
 
Last edited:
Why people insistis in explaining the circuit instead of showing it? A recurrent thing in all forums.
 
You can remove the OR gate ICs and replace them with cheaper diode gates.
 

Attachments

  • diode OR transis&#116.PNG
    diode OR transis&#116.PNG
    721 bytes · Views: 188
Yes, I'll do that. Definitely cheaper. Thanks for that idea and schematic.
 
Except I made a mistake with the value of the base, resistor, it should've been 10k.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top