Please help clarify use of resistor in this circuit

[Degus]

Hello everyone. I'm trying to teach myself how electronics work and I've ran into some questions with this circuit. This is from Make:Electronics experiment 10. I understand R2 is resisting the flow to the gate so the transistor doesn't fry. I also understand R3 will restrict flow to the LED so it doesn't burn out. But what is the

purpose of R1?

 

[jim hardy]

But what is the...purpose of R1?

R1 limits the current drawn from 12V supply if you should happen to make a wiring mistake around the transistor..
Good design is "Beginner-proof"

What would happen if you changed R1 zero ohms and changed R3 to (680+180) ohms?

Can you figure transistor current in both cases ?

Try it.



...

 

[Degus]

@jim hardy thanks for your quick response! I currently don't have access to the bat cave so i can't physically experiment till tomorrow. But...I don't see any difference from having two resistors that add up to 860 and having one resistor at 860 for the LED. I noticed you asked for the current at the transistor...if the circuit is closed and charge is flowing through emitter and collector...wouldnt both resistors be the same resistance for the current flowing through the transistor?

also, you mentioned R1 limits the current drawn if i made a mistake with the wires...i am still failing to see why you would need R1 and not just increase the value of R3.

Thank you for your time kind sir.

 

[jim hardy]

But...I don't see any difference from having two resistors that add up to 860 and having one resistor at 860 for the LED.

you are right. V = I X R

also, you mentioned R1 limits the current drawn if i made a mistake with the wires...i am still failing to see why you would need R1 and not just increase the value of R3.

If a beginner were building this from a schematic instead of from a physical layout drawing,
and connected D1 not where it belongs but from wrong end of R2 oops R3 _jh to V-
when he threw S1 , what would limit current?

Observe all 3 transistor leads are connected to resistance.
I can't see any single wiring mistake that would wreck that transistor . It'd even survive being put in backward.

 

[Degus]

@jim hardy ok. I think I'm following you. So it's safe practice to have resistors hooked up to base, collector and emitter. Thank you much for helping me out!

 

[davenn]

@jim hardy ok. I think I'm following you. So it's safe practice to have resistors hooked up to base, collector and emitter. Thank you much for helping me out!

its not so much about safe practices as it is about the circuit design

not that it's wrong or won't work but your circuit is done a little differently to the normal way.
to use a transistor to light up a LED you would have the current limiting resistor both in the collector lead of the transistor and the emitter would go straight to negative

For a standard 2.5V, 20mA LED, that single resistor let's still call it R1 would be on the region of 1000 Ohms ( 1k)
( That value can be experimented with) R2 in the base lead could be any value from 1k to 10 k

cheers
Dave

 

[jim hardy]

Dave's exactly right, we most often use the transistor as a simple switch, or "Pull-Down" .

Looking at the textbook page that Degus posted, i said to myself
"That author has aimed this at beginners. Why did he make a 'not quite emitter follower' by adding that small collector resistor ? "
Then i noticed its single-mistake tolerance .
For example , a conventional arrangement like Dave showed would have a base pulldown resistor too, to assure cutoff .
But, inserting the transistor backwards in that one would exceed its reverse Vbe rating.
Sooo,,,
I surmised that when this author was a grad student supervising undergrad lab classes, he saw every beginner's mistake that can be made.
Nothing hurts a beginner's confidence like wrecking parts. So he designed into this beginner's lab exercise what fault tolerance he could.

But that was just a guess. I could be totally wrong.

 

[Jony130]

If you do not make any mistake in wiring. This circuit will work even if you replace R1 and R2 with a short/jumper. Because you have a load/resistor on the emitter side.

 

[psparky]

Why do you even need the transistor?

Wouldn't a switch with a resistor and LED accomplish the same thing?

Sorry, I was never great at transistors.

 

[davenn]

For example , a conventional arrangement like Dave showed would have a base pulldown resistor too, to assure cutoff .

yup good point, Jim ... have to admit to neglecting the second resistor at times

Why do you even need the transistor?

Wouldn't a switch with a resistor and LED accomplish the same thing?

Sorry, I was never great at transistors.

Because the circuit is an instruction on how to use the transistor as a switch
This becomes important when the lesson is taken further and the base current is being supplied directly from a source
other than the main supply rail as shown ... for example the output of a uprocessor, TTL or CMOS logic or even a 555 timer
all of which may not have the source current capacity to switch an external device directly ... say a relay

cheers
Dave

 

[psparky]

Interestingly enough, I am very good at op amps and bode plots and things like that. Much weaker at transistors.
I had the same exact teacher for transistors...that I had for the op-amps. Electronics I and II.
It's just that the teacher held his expectations much lower the first semester (I didn't learn a lot because I wasn't overly challenged...easier tests)
Then for electronics II all hell broke loose and the expectations seemed nearly impossible (I learned a ton about op amps and bode plots!...toughest tests possible!)

Goes to show people will rise to the bar that you hold them at.
Food for thought!

When students take the FE test, they generally stink at the 10 or so difficult transistors they put on the test...I am not alone~!