Resistor question

Hi!

I was wondering what was the use of the resistor in the following circuit:

Thanks!

WB

It still is to limit the current into the base of the transistor. Else the controller output might get damaged, or in theory, the transistor.

@Arduino-NCC-1701
It controls the Matter/Anti-Matter mix.
Live long :grin:

2 Likes

The transistor base doesn't work on 5V?

no, it does not. It's not like a logic-compatible MOSFET.

Sorry, but I have no idea what a logic-compatible MOSFET is.

Is there a specific voltage it works on?

Thanks!

...and prosper :vulcan_salute:

Sorry, thought that might be the source of your confusion. It's another, bit more modern, way to turn a device on/off.

Assuming 'it' is a transistor, you may think of the base-emitter junction of a transistor as(more or less) a silicon diode. So somewhere in the 0.5-0.6 V range, it begins to conduct, and by about 0.65V it's full-on. The problem comes in when you connect that to your digital output and set it high; the transistor acts as a short circuit; thus, you need a resistor to limit the current or you'll fry your Arduino..

1 Like

Good one

No, not transistors like that. The work based on current.

What do you mean by work based on current?

Thanks for the help.

Well, I do know what a MOSFET is, just not a logic-compatible MOSFET.

What you said about a transistor, does this also apply to a MOSFET?

Oh wait a sec. A transistor is current controlled and a MOSFET is voltage controlled?

Hey you are smarter than you think.

Transistors like that have a parameter named Hfe, a magnification factor. Send a current into the base and the collector passes Hfe times that current. Check the datasheet for the transistor to learn its limits. alla gtransistors are different...

Thanks I think I get it.

So would a resistor be necessary if using a MOSFET?

Not needed but will not do any harm and may save your arduino in case you do something stupid...

Due to various technical things a 180 - 280 serial resistor is used between the controller pin and the gate. That's elegant but not not needed for life.
A 10 k resistor from output pin to GND makes it complete.

I tried calculating it, but it doesn't seem to work. Please correct me if I made a mistake.

The TIP120 is a transistor, from what I read, so current-controlled.

So I decided to look up the TIP120 spec sheet. Here's the link I found:

TIP120, TIP121, TIP122 (NPN); TIP125, TIP126, TIP126 (PNP) - Plastic Medium-Power Complementary Silicon Transistors (onsemi.com)

I looked at maximum base current. It said 120mA.

I then tried to calculate the resistance I would need so that I wouldn't surpass these 120mA.

So:

R=?
U=5v
I=120mA = 0.12A

R=U/I
R=5V/0.12A
R= about 41 ohms

so logically I would need about a 50 ohm resistor +- 10%

Then why is the resistor in the photo 2.2 kiloohm???

Did I do something wrong?

Thanks for clearing this up!

  • Typically a TIP120 has a HFE of > 1000
  • A TIP120 is a Darlington BJT.

Rb = (5V - 1.4V(Vbe)) / 1mA(good enough) = 3.6k use 3.3k

BTW

  • Always confirm Vce with a voltmeter to ensure the BJT is saturated when turned ON.