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VCEsat collector-emitter saturation voltage
IC = 10 mA; IB = 0.5 mA - 90 200 mV
IC = 100 mA; IB = 5 mA - 200 400 mV <- this is what you need
VBEsat base-emitter saturation voltage
IC = 10 mA; IB = 0.5 mA - 700 - mV
IC = 100 mA; IB = 5 mA - 900 - mV <- this is the one
...the transistor is non-linear and you really need to look in the curves
Is my reading correct in that applying a 15mA base current will allow 150mA through the transistor while applying 50mA
to the base will allow the full 500mA to flow through the transistor?
Is my reading correct in that applying a 15mA base current will allow 150mA through the transistor while applying 50mA to the base will allow the full 500mA to flow through the transistor?
It seems there are two ways of working out the right base current. One would be to fully saturate the transformer, and the other would be to saturate it enough for the load I'm trying to switch.
What is the downside of fully saturating the transformer instead of calculating the base current? I'm guessing it wouldn't be as efficient?
Looking at the datasheet, I'm lost as to which figures I should be using to calculate both the Vce(sat) (as per my previous post) and the hFE.
For example, the hFE has 5 different values. How do I determine which one to pick?
They all have a Vce of 10V and varying Ic values.
The load I am trying to control is a 12 volt relay which has a 16.7mA rating @ 12V.
It seems there are two ways of working out the right base current. One would be to fully saturate the transformer (transistor),
and the other would be to saturate it enough for the load I'm trying to switch.
What is the downside of fully saturating the transformer instead of calculating the base current? I'm guessing it wouldn't be as efficient?
For example, the hFE has 5 different values. How do I determine which one to pick?
They all have a Vce of 10V and varying Ic values. The load I am trying to control is a 12 volt relay which has a 16.7mA rating @ 12V.
Firstly, thank you to everyone who has contributed to this thread.
I'm having trouble with these calculations, and I'm obviously missing something - but I can't see it.
It has been said that a 1K resistor at 5V will saturate the transistor. According to ohms law, I=V/R, I=5V/1000ohms, I=0.05A or 5mA - nice and low and perfect for an Arduino to drive.
But according to the datasheet, for the transistor needs 50mA to be fully saturated which just doesn't seem right not to mention the Arduino wouldn't cope with that.
What have I done wrong?
Is my reading correct in that applying a 15mA base current will allow 150mA through the transistor while applying 50mA to the base will allow the full 500mA to flow through the transistor?
The reason that most use 1K is that you are free to use any load up to the 800mA rating of the 2N2222, without worrying that it will be out of saturation.
-other portion of text-
OK, so I'll try a calculation...
My load is a 12V relay that draws 16.7mA and say I want to drive it from a 5V trigger.
Looking at the hFE table, I presume I would base it on the 10mA hFE of 75.
So, 16.7mA / 75 = 0.22mA.
If I triple this for some buffer - it gives me 0.66mA.
So, with ohms law, R=V/I, R=5V/0.00066A = ~7K5.
Am I good so far?
I know I have to select a preferred resistor value also, but I found this website handy for that:
http://www.electronics2000.co.uk/data/itemsmr/res_val.php
I'm assuming a 5% standard 1/4W resistor will be fine. and 7K5 is a standard E24 value. How does this sound?
Can I also point out where you do ohms law to find the base resistor, I think you have to use 5v-0.7v instead of just 5v, can somebody else also confirm this?
Is this how electrical engineers design circuits with transistor switches in them, is this the proper method to calculate the values or does this one have some downsides to it?
Is this how electrical engineers design circuits with transistor switches in them, is this the proper method to calculate the values or does this one have some downsides to it?
Is this how electrical engineers design circuits with transistor switches in them, is this the proper method to calculate the values or does this one have some downsides to it?
For basic saturated switching, yes, this is the way it's done.
Although from the postings in this thread, you've seen it can be done actually 3 different ways
1) using just the test values from datasheets, i.e. Vce(sat) usually shown at a few different test currents
2) using the charts (find the collector current you need, scan across the chart to get the base current needed)
3) using the Hfe(min) or Hfe(typ) values. Some people use the typical values, others prefer the minimum values.
then adding 2x to 3x more base current than actually calculated, "just to be safe" and to cover all the variability in manufacturing or sourcing equivalent parts from different manufacturers.
As for the ohms law question, calculating the base resistor using ( 5V - 0.7V ) is actually more correct, but it usually doesn't matter, if your base voltage is high enough, since you will always multiply the calculated base current by 2x or 3x anyways. So using 5V or (5V - 0.7V ) is not an issue. Give it a try, and convince yourself that in the saturated switch case, it doesn't matter, especially after you just take the base current calculated x2 or x3 anyways.
It does start to matter when you have much lower base voltages. So if your base voltage was 1.0V, then 1.0V is much different than (1.0V - 0.7)=0.3V. And if you wanted 15ma base current, you can see the difference here in the calculations:
R=E/I = 1.0 / 15ma = 66 ohms, or
R=E/I = 0.3 / 15ma = 20 ohms
So if you ignored the Vbe drop of 0.7V and used the first equation, and put a 66 ohm resistor there, using a 1.0V base voltage, you would have an actual base current of I=E/R = (1.0 - 0.7) / 66 = 4.5ma and the switch would probably not turn on enough to supply the needed collector current. So at lower base voltages you can't take such liberties. At higher base voltages there is usually enough wiggle room that it can be safely ignored.
The other time when you shouldn't ignore the Vbe drop is in amplifier design, where you are not making a simple current switch, and are actually trying to use the transistor in it's forward biased active region.