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Topic: Confused about DataSheet, need help find a cure to a problem. (Read 11717 times) previous topic - next topic

crullier

Hello everyone,

I have some questions about interpreting the information found of a standards 2N222. I have received tons of help from some good people and understand the principle of how BJTs work.

The problem I am trying to solve is why a transistor seems to get hot when it its actuating an electromagnet rates at 1A IF such electromagnet stays on for more than 2-4 seconds.

Needless to say I have been doing much reading and watching videos in efforts to better understand what I am looking at when looking at the datasheet of a standard PNP 2N222 and I have some questions.

Let me start by saying that I bought this transistor at radio shack and I cannot find a data sheet for a 2N222. therefore I am using this as starting point or point of reference.

https://www.fairchildsemi.com/datasheets/PN/PN2222A.pdf


I know that there needs to be a current between the Base and Emitter to turn on the transistor (Hfe).
I also know that there is a current multiplier of across the collector and Emitter.

By questions are:

I am but confused with some information found the data sheet. (please correct me where I am wrong)

First under Absolute Max Rating. There is Vebo with a max of 6.0 v.  This tells me that the base should not be supplied with a voltage near or exceeding 5v. A High from an arduino through a resistor should be okay.

Now, when I look at the ON Characteristics, is shows:

a min. of 0.6v and a  ax of 1.2V in one instance and a max 2V with no min on the other. I find this confusing.
Does this take into account the 0.7v or in this case the 0.5v capacitance value?




Last, how does one properly interpret the Current Gain conditions to achieve the multiplier on the chart?




In essence I would like to to be able to identify how much current it takes to turn on a transistor, and how much voltage it needs to work. additionally I would like to know how much current can flow though it before it gets hot and breaks. All in efforts to asses and understand the cause of my problem.





Here is my circuit.




based on this,  12v-0.6v / 1000 = I  11.4 mA. from base to emitter... that is all I know...


KarolisL

Formula is not so simple:

Take a look at this:

http://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/

and 1A when in short periods, stady state. I cant copy what is written above but it says that its good for switching currents up to 500mA

Grumpy_Mike

Quote
I would like to know how much current can flow though it before it gets hot and breaks.
Read
http://www.thebox.myzen.co.uk/Tutorial/Power.html

and
http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html

Quote
In essence I would like to to be able to identify how much current it takes to turn on a transistor
It depends on the gain or Hfe. This changes with the collector / emitter current.
So in order to answer your question you need to know what you want to switch. If the current is half an amp you will see from the data sheet the Hfe is 40. So you need to put one fortieth of half an amp to turn the transistor on. Which is 12.5mA.

screwpilot

a 2n2222 especially a plastic cased one CANNOT drive a current of 1amp
transistors are intended to work with a max amperage well below it's max rating (like 200mA for a 2n2222)
you need a bigger transistor like a TIP122

there must be a chart that tells what's the hfe at various collector currents (it's basically a curve)
this means that there's some sort of feedback walking back (but you don't need to take into account this for your application)

the veb is the REVERSE max voltage that can be applied to the base in sespect to the emitter (this is a spec you don't need to take in account usually)

when you put a resistor on the base you are supplying a current not a voltage (we say :) ) the max v"BE" that is NOT a reverse voltage is max 1.2V but you will drive it at 0.7V for not destroying it (vBE vEB) this way the "base emitter" will work exactly like a diode

screwpilot

you also need a spike suppressor diode across the electromagnet (that points at +Vcc)
you need a good diode a fast\powerfull one, the simpler can be an 1n4007 (not 4004 or 4001)

MeSat

The data sheet states the 2N2222 will drive 1A.  What is the current needs for the electromagnet?  Measure the voltage across the collector and emitter and see how much voltage is across it.  If there is a voltage drop, it will heat the transistor more.

I do agree, you need to go with a larger transistor for steady state usage at 1A.

Gains are typical and they vary between transistors.  If your electromagnet doesn't have a current limit to 1A, you will be trying to put more current through the transistor.  Measure the current connected straight to the 12V supply to confirm the current drain.


screwpilot

i know that the metal cased 2n2222 can drive 800mA MAX
and again you MUST use a bigger transistor

crullier

Thank you for the responses.

Let me answer some of the questions presented:

Quote
Gains are typical and they vary between transistors.  If your electromagnet doesn't have a current limit to 1A, you will be trying to put more current through the transistor.  Measure the current connected straight to the 12V supply to confirm the current drain.
Do I did a quick measurement and I got the result below (0.80 and tapered down to 0.77) The electromagnet started to get warm but that was over 10 seconds or more and it would not happen under normal operations. As you can see the meter in the PSU is a little different than my multimeter but its pretty close. (need to get a better multimeter at some point?)




@Mike,

Quote
It depends on the gain or Hfe. This changes with the collector / emitter current.
So in order to answer your question you need to know what you want to switch. If the current is half an amp you will see from the data sheet the Hfe is 40. So you need to put one fortieth of half an amp to turn the transistor on. Which is 12.5mA.
Your response make very much sense. So if I want to drive an electromagnets that draws 500mA I need a resistor 1/40*500 = 12.5mA  which means I will need a resistor of 0.912 ohms.  (based on the formula I = V-Vbe/Rb)

So I can use a 1K resistor and I should be able to drive a 500mA electromagnet.

(I hope on the right track)

Now my question is: in our example we are using the condition Ic = 500mA, Vce=10V.  Where or how does the Vce=10V come into play? What is that trying to tell me.  In my circuit, I would be driving the electromagnet with 12v between CE because that is what I need for that part. Also, It is interesting the the datesheet mentions a max of 1A, but the highest hFE they show is 500mA (congruent with what was mentioned earlier I guess)


I also realized that I forgot to attach the Absolute Max Rating image of the datasheet but in there Vebo is 6.0V. What is this and how does it play into the circuit? We are not putting voltage through the base are we? Does this mean that I cannot put more than 6v at the base? (meaning I would need a voltage divider etc if I wanted to?)










Grumpy_Mike

You should always aim to have more base current than you need from the calculations so I would not go to 1K but to the next one down 910R or 820R.

The 10V in the data sheet is what they measured it at, it might make a slight difference in the gain but not very significant.

As they only tell you the Hfe at 500mA then it is wise to use the transistor at that rating. Note it drops as the current increases, at a higher current it will drop even more and throw you base current calculations out.

As mentioned before this base voltage rating is a reverse voltage rating when the emitter is higher than the base. In the forward direction you can not get more than about 0.7V across a base because it is a diode. Do not confuse this with the voltage you put on the other end of the base resistor.

crullier

#9
Jan 26, 2015, 11:00 pm Last Edit: Jan 26, 2015, 11:03 pm by crullier Reason: added link
Thank you guys for the help.

Quote
As mentioned before this base voltage rating is a reverse voltage rating when the emitter is higher than the base...
I see what you mean. Thank you for the clarification. I should have also note then it says "Emitter-Base" and not Base-Emitter. I thought it was confusing that the use the same characteristic in two places, but it make sense - 6.0V is the MAX.

I wanted to add that I came a cross a very informative video this morning that went over many of the things you guys shared but one thing that called my attention was that it is important to make sure the transistor is ALL the way on or ALL the way off. If you linger, then this is where things start to heat up and there is a voltage across the C and E. This explains and reinforces the why I want to have the correct base current to drive the collector.  Do you guys agree?

http://youtu.be/UlDz0vQUTlc

Grumpy_Mike

Any voltage across the collector emitter multiplied by the current through it is the power it will dissipate. With a transistor as a switch the product of these two is minimised. If you operate in the linear zone it is not. It is not crime for a transistor to get hot, it is natural, the trick is not to let it get too hot.

In practice it is often thermal considerations that limit the maximum current you can use a transistor at.

screwpilot

a small transistor like the 2n2222 will surely not close the circuit perfectly
switch it on end measure the voltage across the C and E, you will find that it's far from beeing 0V
the more the current the more this potential will be and the more the power it dissipates will be

you cannot reduce this voltage gap by simply adding more current on the base, you are already overstressing it

raschemmel

FYI, the "all the way on/off" mode is called SATURATION MODE. Read the datasheet specs for this mode.
Your datasheet doesn't specify which package. The plastic package doesn't dissipate as much as the metal
can.
2n2222 TO-18 PACKAGE


You can parallel two (or more) of these for more current but usually fets are used for parallel device mode.

Grumpy_Mike

No you can't parallel up transistors to switch more current unless the transistors are on the same die, it just dosen't work. Better people than me have tried and failed to do this.

screwpilot

you can easily parallel two or more bjts as long as they are identical and you put a low ohmage hi pow resistor per collector (like 1ohm 1\2W in your case)
i've seen a lot of schematics with paralleled bjts

but (again) use a more powerfull transistor and a spike suppressor diode

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