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### Topic: What resistor value at the transistor base? (Read 7266 times)previous topic - next topic

#### Southpark

#30
##### Jul 10, 2016, 02:48 amLast Edit: Jul 10, 2016, 03:05 am by Southpark
(no Vemitter)
My meaning of Vemitter means 'voltage at the emitter', obviously.

#### OldSteve

#31
##### Jul 10, 2016, 03:24 amLast Edit: Jul 10, 2016, 03:24 am by OldSteve
I'm amazed that this thread has gone for so long. As has been mentioned a number of times now, the correct way to drive the transistor into saturation for switching is to provide Ic/10 or at the very least, Ic/20 to the base.
For Ic = 200mA, you need 10mA to 20mA into the base to fully turn the transistor on, resulting in the lowest Vce(sat).

Driving with 5V, about 4.4V is across the base resistor, so it's value should be between 4.4/0.02 and 4.4/0.01. (220Ω and 440Ω.)

There's no need for any other fancy calculations.
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#### alnath

#32
##### Jul 10, 2016, 09:27 amLast Edit: Jul 10, 2016, 09:31 am by alnath Reason: image
My meaning of Vemitter means 'voltage at the emitter', obviously.
As Raschemmel said, emitter is grounded, then "Vemitter" is 0V

Quote
Why don't you just do what everyone else does (most people who know anyway) and divide the desired collector current by the typical DC current gain (100) to calculate the required base current.
I have to disagree about that. We are in the switching mode, the gain is irrelevant here and this part of the datasheet gives the answer (which has been given several times in this thread ;-) ) :

Ib=Ic/10

#### raschemmel

#33
##### Jul 10, 2016, 09:52 amLast Edit: Jul 10, 2016, 09:54 am by raschemmel
You're right Alnath. It should be Ib= Ic/10.

#### OldSteve

#34
##### Jul 10, 2016, 10:15 amLast Edit: Jul 10, 2016, 10:19 am by OldSteve
I'm surprised that Vbe(sat) is so high. (Didn't read the datasheet earlier.)
So if it's about 1.5V at Ic = 200mA that leaves 3.5V across the base resistor. 3.5/0.02 = 175Ω. A 180Ω or 220Ω resistor would do the job nicely. Ib = 19.4mA if Rb is 180Ω. With 220Ω, the base current would be 15.9mA, Ic/12.6. Still quite acceptable.
(All assuming a collector current of 200mA.)

I just added this since the original question was "What resistor value at the transistor base?", not "How much current should be 'force-fed' into the transistor base?"
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#### raschemmel

#35
##### Jul 10, 2016, 10:19 am
How much do the LEDs draw ?

#### OldSteve

#36
##### Jul 10, 2016, 10:25 amLast Edit: Jul 10, 2016, 10:33 am by OldSteve
How much do the LEDs draw ?
Good point. I picked up on a reference to 200mA along the way.

So it's 400mA.

40mA into the base. Vbe nearer to 2V. 3V across the base resistor. 3/0.04 = 75Ω.
(But at least I showed the method.)

And a 1K resistor definitely wouldn't cut it.

Edit: But, of course, an Arduino can't supply that current directly. It either needs to be pulled down to 20mA to 30mA, or a driver transistor used.

Personally, I'd throw the (ancient) 2N2222 in the bin and use a MOSFET, or at the very least, set the LEDs up in series strings.
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#### Southpark

#37
##### Jul 10, 2016, 10:35 amLast Edit: Jul 10, 2016, 11:00 am by Southpark
As Raschemmel said, emitter is grounded, then "Vemitter" is 0V
For the common emitter configuration .... which is what the OP is using...and which is the usual configuration for connecting a load...the voltage at the emitter is zero Volt. For an emitter follower configuration, the emitter voltage will not be zero Volt.

I wrote "Reason...... Ve is grounded at 0 V". I meant 'emitter' is grounded.... 0V. Page 1 of this thread.

#### raschemmel

#38
##### Jul 10, 2016, 10:37 amLast Edit: Jul 10, 2016, 10:59 am by raschemmel
Wawa recommends IB = C/30 which would be 400/30 = 13.3 mA base current.
RB = (5V-1.5V)/0.0133 A = 263 (260) ohm.

(OldSteve, where are you getting the ohm symbol from ?)

Quote
For the common emitter configuration .... which is what the OP is using...and which is the usual configuration for connecting a load...the voltage at the emitter is zero Volt. For an emitter follower configuration, the emitter voltage will not be zero Volt.
It's more like an OPEN COLLECTOR than COMMON EMITTER.
Transistor Configurations
The load is NOT across the collector and emitter as in common emitter.
The load is BETWEEN Vcc and the collector as in an OPEN COLLECTOR configuration so the emitter is grounded and the voltage on the emitter is 0V.

What is the voltage on the emitter ?

It's 0V , is it not ?

#### OldSteve

#39
##### Jul 10, 2016, 10:51 amLast Edit: Jul 10, 2016, 10:59 am by OldSteve
Wawa recommends IB = C/30 which would be 400/30 = 13.3 mA base current.
RB = (5V-1.5V)/0.0133 A = 263 (260) ohm.
No, he said the same as me - Ic/10 to Ic/20 :-
Quote from: Wawa
In general, for small transistors like the 2N2222, the base current should be >= 1/20 of the collector current.
Quote from: Wawa
1:10 is used if you want to switch close to the transistor's maximum collector current.
For lower currents, 1:20 should be enough.
Quote from: Wawa
Use ~1:20 for small transistors, ~1:10 for power transistors, and ~1:250 for darlingtons.
I think that in this application, at 400mA, it doesn't really qualify as "small signal", so between 1:10 and 1:20 IMHO

Quote from: raschemmel
(OldSteve, where are you getting the ohm symbol from ?)
That's another of my little secrets.

Just between you and me, from the Windows "Character Map". (A standard Windows accessory.)

Edit: And it is most definitely a 'common-emitter' configuration. The load can be either between the collector and ground or the collector and Vcc.
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

#### raschemmel

#40
##### Jul 10, 2016, 10:57 am
Quote
Just between you and me, from the Windows "Character Map". (A standard Windows accessory.)
Ω Ω Ω Ω Ω Ω

(I'm glad that only took me 30 years to learn that...)

#### OldSteve

#41
##### Jul 10, 2016, 11:01 am
Ω Ω Ω Ω Ω Ω

(I'm glad that only took me 30 years to learn that...)

We're never too old too learn.
or
Better late than never.
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

#### Southpark

#42
##### Jul 10, 2016, 11:03 amLast Edit: Jul 10, 2016, 11:07 am by Southpark
It's more like an OPEN COLLECTOR than COMMON EMITTER.
Transistor Configurations
Learn your circuit configurations before coming to argue with me mate.

#### raschemmel

#43
##### Jul 10, 2016, 11:05 amLast Edit: Jul 10, 2016, 11:10 am by raschemmel
Quote
Learn your circuit configurations before coming to argue with me mate.
If you know your configurations then why are you referring to a Vemitter when it's 0V ?

#### Southpark

#44
##### Jul 10, 2016, 11:09 amLast Edit: Jul 10, 2016, 11:09 am by Southpark
If you know your configurations then why are you referring to a Vemitter when it's 0V ?
That's because on page 1 of this thread, I already mentioned Vemitter is zero Volt. You just didn't realise it, that's all.

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