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Using Arduino => General Electronics => Topic started by: NCC1966 on Jul 06, 2016, 11:17 pm

Title: What resistor value at the transistor base?
Post by: NCC1966 on Jul 06, 2016, 11:17 pm
I am going to use a 2N2222 transistor to light up an array of LEDs. I should hook no more than 15-20 LEDs in each transistor.

What's the correct the value of the resistor that goes between the transistor base and the Arduino pin?

Thanks!

 :)
Title: Re: What resistor value at the transistor base?
Post by: larryd on Jul 06, 2016, 11:19 pm
Draw a diagram showing what you are proposing.


.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 06, 2016, 11:46 pm
I am going to use a 2N2222 transistor to light up an array of LEDs. I should hook no more than 15-20 LEDs in each transistor.

What's the correct the value of the resistor that goes between the transistor base and the Arduino pin?

Determine the DC voltage (Vemitter) that you are expecting at the emitter (for however many LEDS you plan to use, and knowing the operating current of the LEDs).

Then know decide on what voltage you're going to apply (on the source side of the base resistor)... eg, Vinput.

Look up the specs for 2N2222. If the continuous base current information says don't go over 20 mA (or whatever the value will be), then Vinput = Ibase*Rbase + 0.6 + Vemitter.

So Rbase = (Vinput - Vemitter - 0.6)/Ibase; where Ibase could be 20 mA.
Title: Re: What resistor value at the transistor base?
Post by: Wawa on Jul 07, 2016, 12:14 am
The question can't be answered correctly if the collector current is unknown.
Are these LEDs all going to be in parallel, or in series, or a combination of that.
In general, for small transistors like the 2N2222, the base current should be >= 1/20 of the collector current.
There is usually a saturation graph in the datasheet.
Leo..
Title: Re: What resistor value at the transistor base?
Post by: MarkT on Jul 07, 2016, 12:25 am
Assuming 20 LEDs each using 20mA, then that's 400mA load, so you want 20mA minimum for the base,
but the Arduino outputs are abs. max. of 40mA, so lets say 25mA base current, so at 5V logic supply
thats about 4.2/0.025 = 170 ohms or so, so 150 is the closest standard value (output pins have a few dozen
ohms internal resistance anyway, so 150 is better than 180).
Title: Re: What resistor value at the transistor base?
Post by: Wawa on Jul 07, 2016, 01:06 am
OP also mentions "in each transistor".
How many transistors.
At those base currents, you also have to watch total MCU current.
Leo..
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 07, 2016, 02:12 am
OP also mentions "in each transistor".
How many transistors.
At those base currents, you also have to watch total MCU current.
Leo..
Not sure. Without enough output current capacity on each arduino output pin, probably better to just use 1 arduino output for 1 transistor.
Title: Re: What resistor value at the transistor base?
Post by: Wawa on Jul 07, 2016, 02:47 am
Not what I meant.
"Array" was mentioned.
At 25mA per pin, you can only use 6 pins.
http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations
Leo..
Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 07, 2016, 07:12 pm
My simple diagram:

(http://i.imgur.com/D7hz69I.jpg)

I found the following formula:

Rb = Vr x hfe / Ic

Ic - 0.2 (sum current of all leds)
Hfe - 40 (worst case scenario Ic = 500 mA, V = 10V)
Vr - 5 (Voltage across resistor)

Rb = 5 x 40 / 0.2 =  1K

Is this correct?

So if it would be 5 LEDs the resistor would be 2K; if it would be 20 LEDs resistor would be 500R and so on?

PS: It doesn't matter the voltage used to feed the LEDs (12V)? So, it could be 12V or 12000V?
Title: Re: What resistor value at the transistor base?
Post by: CrossRoads on Jul 07, 2016, 07:45 pm
LED resistor: (Vs - Vf - Vce)/.02A = resistor, Vs = Vsource, Vf = forward voltage of LED, Vce = voltage drop across transistor collector to emitter, similar to a diode drop
Example (12V -2.5V - 0.7V) / .02A = 440 ohm

Base resistor: (Vs - Vbe)/.02A = resistor, Vs = Arduino voltage. Vbe = voltage across  base to emitter, similar to a  diode drop
Example (5V - 0.7V)/.02A = 215 ohm  20mA base current will turn it on pretty well.
N-Channel MOSFT would be better, will run much cooler than an NPN.
AOI514 from Digikey.com will work well. Logic level so Arduino can drive directly, very low Rds so it will have very little voltage drop across and stay cool, vs NPN where the 0.7V is fixed and power dissipated can rise a lot with current flow.
http://www.digikey.com/product-search/en?keywords=aoi514 (http://www.digikey.com/product-search/en?keywords=aoi514)
Title: Re: What resistor value at the transistor base?
Post by: CrossRoads on Jul 07, 2016, 07:46 pm
12000V, would be hard to find a transistor that can switch that.
Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 07, 2016, 08:29 pm
I got it, thanks!!!!!

 :D

So Vbe and Vce is the dropped (dissipated) voltage and in the case of NPN transistors it is kind of a "constant" valuing 0.7 right?

Unfortunately I browsed the online stores where I am used to buy parts and they don't have the AOI514 so I will have to stay with the 2N2222.

I don't think it will be a problem though because probably the circuit won't stay on for time enough to get hot. It is going to be used for light a spaceship miniature so I will turn it on for only a few minutes while I show up for friends or for my own enjoyment. I think I will be safe!

Thanks again!

 :smiley-mr-green:

EDIT: I saw that you used .02A as for divisor of the formula for the base transistor. Should I increase this value depending on how many LEDs I am going to use? For instances, if are 10 LEDs so it would be 0.2 intead 0.02A?
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 07, 2016, 08:42 pm
So Vbe and Vce is the dropped (dissipated) voltage and in the case of NPN transistors it is kind of a "constant" valuing 0.7 right?
In your circuit, with the diode/resistor pairs on the collector side, Vbe will be approximately 0.6 Volt (or could assume 0.7 Volt too). Reason...... Ve is grounded at 0 V. The 0.6 V is Vbe(saturation) from manufacturing specs guide for the 2N2222. It's just minimum value assumption for Vbe..... assuming 0.7 V is fine too. It is linked to the 'diode' drop of the transistor's own 'pn junction'.

Vce is the voltage (Vc - Ve).

Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 07, 2016, 08:51 pm
In your circuit, with the diode/resistor pairs on the collector side, Vbe will be approximately 0.6 Volt (or could assume 0.7 Volt too). Reason...... Ve is grounded at 0 V. The 0.6 V is Vbe(saturation) from manufacturing specs guide for the 2N2222. It's just minimum value assumption for Vbe..... assuming 0.7 V is fine too. It is linked to the 'diode' drop of the transistor's own 'pn junction'.

Vce is the voltage (Vc - Ve).


What is Vc and Ve? Voltage-collector and Voltage-emitter?
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 07, 2016, 09:33 pm
What is Vc and Ve? Voltage-collector and Voltage-emitter?
Absolutely correct. If you have chosen current limiting resistors Rd for the diodes, then Vc can be found from the sum of voltages in the collector to emitter branch section......needs to sum to 12V.

12V = Ve + Vce + V_Rd + Vdiode

Vce is (Vc-Ve)

12 = Ve + (Vc-Ve) + V_Rd + Vdiode

Ve is grounded (0V) in this case. But the Ve in the above equation cancels with the -Ve anyway.

So 12 = Vc + V_Rd + Vdiode

So Vc = 12 - V_Rd - Vdiode

We usually know Vdiode. And we also usually know (by design) V_Rd, since we design the current through the diode (which also goes through Rd); V_Rd = I_diode * Rd.

Could also start from the collector..... with voltage Vc.

Vc + V_Rd + Vdiode must sum to 12 Volt.

So again, we get Vc = 12 - V_Rd - Vdiode
Title: Re: What resistor value at the transistor base?
Post by: polymorph on Jul 07, 2016, 11:21 pm
Actually, the 2N2222 datasheet specifies that Ib = Ic/10 when used as a switch.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 07, 2016, 11:40 pm
Actually, the 2N2222 datasheet specifies that Ib = Ic/10 when used as a switch.
That condition is just part of their own testing condition right? Ib doesn't have to be around Ic/10 when used as a switch.
Title: Re: What resistor value at the transistor base?
Post by: Wawa on Jul 08, 2016, 12:13 am
Hfe is not used for switching.
For switching applications it is important to get the lowest possible Vce.
That could be less than 0.1volt for this transistor.
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.
Look at the P2N222A datasheet from On Semiconductors.
Figure 4 shows a saturation graph with a range of base/collector currents.
Leo..
Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 08, 2016, 04:11 pm
OK, I got the idea now!

There is so many different parameters in the datasheets and so many different formulas to be used that the solution is to make a bunch of calculations (that will result in a bunch of different resistor values) and then try with each one of them to see what is the correct. The good choice will be one that will work and causing less heat. Of course a couple transistors will burn in the process. I got it now!

LoL!

 :smiley-yell:

My hope was to avoid such empiricism.

 :P
Title: Re: What resistor value at the transistor base?
Post by: pwillard on Jul 08, 2016, 06:57 pm
The empiricism with "switching" exists because some fail to make it obvious that a "full on" saturated transistor is not going to be  "more on" than "full on".  You can certainly TRY to drive a transistor MORE ON...  but after a point... you will gain nothing but more current onto the base pin than is needed.

Much of the information in the data sheet is far more useful when operating the transistor in analog/linear mode as a controlled amplifier.
Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 08, 2016, 07:02 pm
Sorry for the sarcasm, but it's really very confusing for some is a newby (like me)!

 :)

For this base resistor calculations for instances, I have had several different responses with several different values for the resistor!

What really scares me is that we are talking about a single coupling to light a few LEDs. I can't imagine how hard is to design something more elaborated!

 :smiley-eek:
 
But getting to the point now... since the resistor to be used at the transistor base is very arguable and can be subject to several variables I imagine that the solution for my case will be to pick a bunch of different resistors and start with the bigger value I have (say 100K for the sake of transistor safety). Probably it won't work and the LEDs won't light. Then I go replacing it by smaller values and testing until it suddenly work (LEDs lighted) and then it will be the right one for the circuit. Does it sound like a good plan?

Thanks!
Title: Re: What resistor value at the transistor base?
Post by: MarkT on Jul 08, 2016, 07:12 pm
For completeness think about this point.  In normal, linear, amplication using a BJT the base-collector
junction is reverse biased (for NPN that means collector more +ve than the base), so that there is
an electric field across this junction pulling electrons directly across from the base.  Most electrons
injected into the base region are immediately pulled to the collector by this field, leaving a tiny fraction
to leave by the base electrode.

When a transistor saturates the base-collector junction is forward-baised, the the electrons in the
base are not attracted to the collector at all, they are repelled. They get there solely by thermal diffusion,
which is why the effective gain is far far less.   It only works because the degree of doping is highly different,
emitter >> base >> collector (typically emitter is 10000 times more heavily doped than the collector, that's
why the emitter is not the same as the collector).

Large old designs of power BJT's have a saturation gain of 3 to 5(!), small signal transistors about 10 to 30,
some of the more modern devices manage 50 ('superbeta' devices).
Title: Re: What resistor value at the transistor base?
Post by: pwillard on Jul 08, 2016, 07:42 pm
This is why using the Hfe value from the datasheet is misleading.  You actually want worst cast (lowest gain) value.

Why I don't even do the math any more is simple.

1) I use a small subset of NPN small signal transistors from my parts bin that I have had for years (2N4400, 2N3904, 2N2222).

2) With 5V logic and a typical load  current (from 30 to say like 200ma), I have used a 1000 Ohm resistor for the base resistor and it has served me well with all three transistors.  Why 1000 ohms?  Because I also have a bunch of those too.

Am I treating this like exact science?  I could... but I don't.  I don't need to be so pedantic.


Now... when moving to 3.3V, does this rule of thumb still apply?  Nope.   You will need to determine the proper base current  using 3.3V in your equations.  But once figured... you can pretty much use any off the shelf small signal transistor of the type I mentioned above... keeping in mind you don't want to exceed max collector current.

In my case... I just reach for a 680 Ohm resistor for the base... and bam done.  Works the way I want (Most of the time).




Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 08, 2016, 11:36 pm
Cool, so 1K is a good point to start with!   8)
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 08, 2016, 11:45 pm
Cool, so 1K is a good point to start with!   8)
Not necessarily. It depends on the situation. For example..... if you have a 5V source, and the voltage difference between the source and the transistor base is say 4.4 V (after taking account of a diode voltage drop), then a 1 kilo-Ohm resistor would give you maybe 4.4 milliAmp of base current. But just say you need 20 milliAmp base current....... a 1 kilo-Ohm resistor won't allow it. The resistor would need to be smaller... like 220 Ohm.

If you want to use a 1 kilo-Ohm resistor, then the voltage source would need to output a voltage much higher than 5V if 20 milliAmp base current is needed........ but since you're trying to drive the transistor with maybe 5V from an arduino output..... then better to stick with 5V (arduino digital pin output) and a small resistor.... like 220 Ohm.
Title: Re: What resistor value at the transistor base?
Post by: Wawa on Jul 08, 2016, 11:49 pm
Next problem.
One LED on a 12volt supply.
Very inefficient. You waste 80% of the battery.
Several LEDs can be connected in series with one current limiting resistor.

Common red/green LEDs have a Vf (forward, or working voltage) of 1.8-2.2volt.
Up to five of these LEDs can be connected in series on your 12volt supply.
If you just connect two in series, you already halve battery consumption.
Blue/white/power LEDs have a Vf of ~3.3volt. Only three of these can be connected in series.
You ofcourse have to re-calculate the current limiting resistor.
That could take another 10 posts.
Leo..
Title: Re: What resistor value at the transistor base?
Post by: pwillard on Jul 09, 2016, 01:38 pm
Quote
Not necessarily. It depends on the situation. For example..... if you have a 5V source, and the voltage difference between the source and the transistor base is say 4.4 V (after taking account of a diode voltage drop), then a 1 kilo-Ohm resistor would give you maybe 4.4 milliAmp of base current. But just say you need 20 milliAmp base current....... a 1 kilo-Ohm resistor won't allow it. The resistor would need to be smaller... like 220 Ohm.

If you want to use a 1 kilo-Ohm resistor, then the voltage source would need to output a voltage much higher than 5V if 20 milliAmp base current is needed........ but since you're trying to drive the transistor with maybe 5V from an arduino output..... then better to stick with 5V (arduino digital pin output) and a small resistor.... like 220 Ohm.
<sigh> This answer is misleading and I'm not sure anything I can say will help.  5mA at the base is WAY more than is actually need to "turn on" in a 5V scenario as it is.  The 220 Ohm recommendation is overkill and not warranted unless you have an extremely low gain, high power BJT and if that was the case... you would be best served by adding an additional driver transistor ahead of it.

But, this issue of base resistor seems to forever a point of debate.  I'll just keep doing what I have successfully done for 40 years.
Title: Re: What resistor value at the transistor base?
Post by: NCC1966 on Jul 09, 2016, 05:06 pm
Not necessarily. It depends on the situation. For example..... if you have a 5V source, and the voltage difference between the source and the transistor base is say 4.4 V (after taking account of a diode voltage drop), then a 1 kilo-Ohm resistor would give you maybe 4.4 milliAmp of base current. But just say you need 20 milliAmp base current....... a 1 kilo-Ohm resistor won't allow it. The resistor would need to be smaller... like 220 Ohm.

If you want to use a 1 kilo-Ohm resistor, then the voltage source would need to output a voltage much higher than 5V if 20 milliAmp base current is needed........ but since you're trying to drive the transistor with maybe 5V from an arduino output..... then better to stick with 5V (arduino digital pin output) and a small resistor.... like 220 Ohm.
Well, so 1K is indeed a good start and you just confirmed it!

 :D

The point is that I couldn't find any definitive answer to my question, but a lot of "it depends"! LOL!

 :smiley-yell:

So, my idea is to START with a 1K resistor and see what happens. If it doesn't work I replace it with a smaller value. Did it work? Nope... then replace with a smaller value. Did it work? Yes!!!! Keep with it!

From my point of view this what I got from this thread... that there is not a technical reliable way of determine a resistor value for a transistor base. It's just empiric since the opinions are SO controversial!

 :o

Please don't get me wrong. I just though (in my ignorance) that electronics was a more exact science!

 :)

I appreciate all the responses because it was thanks of them that I got the point. I consider my question as answered.

Keep the good work friends!
Title: Re: What resistor value at the transistor base?
Post by: Wawa on Jul 09, 2016, 11:58 pm
Bla, bla, bla.
Just use a 1k base resistor (small transistor, 5volt Arduino) if collector currents are lower than 100mA.
Relay, some LEDs etc.
Use a lower value base resistor if collector currents are above 100mA. Then it's wise to calculate the value.
Use ~1:20 for small transistors, ~1:10 for power transistors, and ~1:250 for darlingtons.
Leo..
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 02:06 am
Quote
Determine the DC voltage (Vemitter) that you are expecting at the emitter (for however many LEDS you plan to use, and knowing the operating current of the LEDs).

Then know decide on what voltage you're going to apply (on the source side of the base resistor)... eg, Vinput.

Look up the specs for 2N2222. If the continuous base current information says don't go over 20 mA (or whatever the value will be), then Vinput = Ibase*Rbase + 0.6 + Vemitter.

So Rbase = (Vinput - Vemitter - 0.6)/Ibase; where Ibase could be 20 mA.
There is no such parameter as Vemitter.
see datasheet (http://global.oup.com/us/companion.websites/fdscontent/uscompanion/us/pdf/microcircuits/students/bjt/2N2222_philip.pdf)

Do you see a parameter called "Vemitter"

Common small signal transistor parameters are:
VEBO for the open collector configuration used by the OP
VCEO

(no Vemitter) Did you just make that up ?
It actually doesn't make any sense since the emitter is connected to GND so "Vemitter" (if such a parameter were to exist) would be 0 V all the time.
How could it not if it's grounded ?
I can only surmise you meant VEB (base to emitter voltage)

Quote
Sorry for the sarcasm, but it's really very confusing for some is a newby (like me)!
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.
For leds, if you wanted 20 mA per led through each transistor,

0.020 A/50 = 0.000400A  (200 uA).
 You could get that with 5V - 1.3 V = 9250

Anything less than 9.1 k would probably work.
However , for most applications , using the 2n2222, driven from a 5V arduino signal, the most common value of base resistor used is 1 k ohm . As pointed out by Pwillard, this is more than enough.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 02:48 am
(no Vemitter)
My meaning of Vemitter means 'voltage at the emitter', obviously.
Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 03:24 am
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.
Title: Re: What resistor value at the transistor base?
Post by: alnath on Jul 10, 2016, 09:27 am
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 ;-) ) :  

(http://zupimages.net/up/16/27/lra5.png)

Ib=Ic/10



Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 09:52 am
You're right Alnath. It should be Ib= Ic/10.
Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 10:15 am
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?"
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 10:19 am
How much do the LEDs draw ?
Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 10:25 am
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.
My bad. :(
(But at least I showed the method.)

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

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.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 10:35 am
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.

One of my previous posts in this thread already mentioned Vemitter is zero Volt. I wrote that before raschemmel asked about 'Vemitter'.

I wrote "Reason...... Ve is grounded at 0 V". I meant 'emitter' is grounded.... 0V. Page 1 of this thread.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 10:37 am
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 (http://www.radio-electronics.com/info/circuits/transistor/circuit-configurations.php)
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 ?
(http://forum.arduino.cc/index.php?action=dlattach;topic=411284.0;attach=173271)

It's 0V , is it not ?
Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 10:51 am
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.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on 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...)


Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 11:01 am
Ω Ω Ω Ω Ω Ω

(I'm glad that only took me 30 years to learn that...)
:D
We're never too old too learn.
    or
Better late than never.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 11:03 am
It's more like an OPEN COLLECTOR than COMMON EMITTER.
Transistor Configurations (http://www.radio-electronics.com/info/circuits/transistor/circuit-configurations.php)
Learn your circuit configurations before coming to argue with me mate.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 11:05 am
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 ?
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 11:09 am
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.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 11:11 am
Quote
My meaning of Vemitter means 'voltage at the emitter', obviously.
You still haven't realized there is no such parameter ? Can you find that on a datasheet ?(no)
As with everything else you post on this forum, you just made that up because you have no formal training in electronics.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 11:13 am
You still haven't realized there is no such parameter ? Can you find that on a datasheet ?(no)
As with everything else you post on this forum, you just made that up because you have no formal training in electronics.
You're the one that assumed that 'Vemitter' comes from a data sheet. You just conjured this 'data sheet' thing in your own mind. Was already mentioned to you (by me) that I meant that 'Vemitter' just means voltage at the emitter. So just accept it, and move along.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 11:17 am

Quote
I meant that 'Vemitter' just means voltage at the emitter. So just accept it, and move along.
The problem is that the voltage on the emitter is 0V because it is connected to ground so what is the point of saying Vemitter when you can just say GND ?
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 11:19 am
The problem is that the voltage on the emitter is 0V because it is connected to ground so what is the point of saying Vemitter when you can just say GND ?
To ensure that Ve (or Vemitter) is taken into account if somebody connects the load on the emitter side (emitter follower configuration). Watch it with your deliberate snubs raschemmel.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 11:24 am
Quote
To ensure that Ve (or Vemitter) into account if somebody connects the load on the emitter side (emitter follower configuration).
Can you not see where the load goes in the OP's schematic ?

Do you see anyone connecting a load between the emitter and ground ?
Is anyone else concerned about that possibility in this thread ?
What configuration do YOU call this ?
(http://forum.arduino.cc/index.php?action=dlattach;topic=411284.0;attach=173273)

BTW,
Can anyone explain why the ground zigzags from the emitter to the ground symbol ?

What's that all about ?
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 11:31 am
Can you not see where the load goes in the OP's schematic ?
Let me enlighten you. The OP did not post a schematic at first. The OP posted a schematic at some stage after their first post. So it makes sense to use Vemitter.... to cover bases.

Quote
What configuration do YOU call this ?
It's a common emitter configuration. But you called it an open collector configuration. I don't mind what you call it. You can call it whatever you like.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 11:36 am
Quote
Let me enlighten you. The OP did not post a schematic at first. The OP posted a schematic at some stage after their first post. So it makes sense to use Vemitter.... to cover bases.  
With no formal electronics training I doubt you will be enlightening anyone.

Try doing a "sanity" check on your posts before posting. See if you can find  anyone else who is saying something similar because you are clearly just pulling stuff out of the air ( making it up as you go along).

 We like to represent professional electronics opinion on this forum not amatuer conjecture.

If you are not an electronics person by trade then maybe you should research your information a little better, maybe with Google. ( like don't make up parameters that don't even exist).

Your contributions are appreciated if they are based on fact and not conjecture.

There are no "bases" to cover . This application has always been a low side switch is not going to be changing.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 11:53 am
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.
Agreed. A great rule of thumb you mentioned OldSteve. The Ic/Ib = 10 one
Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 11:58 am
Agreed. A great rule of thumb you mentioned OldSteve. The Ic/Ib = 10 one
At least I got that right. :D

(Actually, I think it was the OP's schematic that had me thinking there were only 10 LEDs / 200mA collector current. I should have read back from the first post. :( )
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 12:01 pm
At least I got that right. :D

Haha! You were spot on. You had it right mate. Thanks for showing that.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 12:04 pm
In the OP he approximates, saying " maybe 15 - 20 LEDs per transistor"
Title: Re: What resistor value at the transistor base?
Post by: OldSteve on Jul 10, 2016, 12:15 pm
In the OP he approximates, saying " maybe 15 - 20 LEDs per transistor"
Yeah, as I've since seen when I revisited it. I read that post a couple of days ago, then today I picked up at a later point. Saw the schematic and another reference to 10 LEDs and went with it. As I said, my bad.

Not to worry, all's well that ends well. :)

It's been a moot argument anyway really, since the current would be far lower if the LEDs are set up in series strings. Strings of 2-3 LEDs in series would work well and lower the collector current to about 1/3 to 1/2 the present value.
(Then the resistor calculations/discussions would have to start all over. That might be fun. :D )
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 12:15 pm
In the OP he approximates, saying " maybe 15 - 20 LEDs per transistor"
But soon after.... we said...consider 1 arduino digital output per transistor.
Title: Re: What resistor value at the transistor base?
Post by: raschemmel on Jul 10, 2016, 12:18 pm
Then why use transistors ?
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 12:25 pm
Then why use transistors ?
To switch one set of 10 LEDs. Otherwise.... just use 1 digital output and a suitable switch circuit (MOSFET) that handles more current to cater for the number of LEDs that the OP needs to turn on. Whatever you like.
Title: Re: What resistor value at the transistor base?
Post by: Southpark on Jul 10, 2016, 12:49 pm
don't call voltage swing "duty cycle"
I didn't. Show us where I called 'voltage swing' a 'duty cycle'. You either conjured that in your own mind, or you just made up some false statement for some reason.
Title: Re: What resistor value at the transistor base?
Post by: mrburnette on Jul 10, 2016, 06:29 pm
Ye Gads!

All of this dialog is boring... we live in the 21st century and can certainly do better because we have better free tools. (http://www.falstad.com/circuit/)  Paul Falstad had done a great job of making these kind of questions non-forum for newbies and even intermediate knowledge Arduino'ites.

Visual and hands-on learning will drive home the interrelationships of complex subjects. 

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