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Topic: Noob transistor question(s) (Read 2728 times) previous topic - next topic


I have a few questions to clear up the things I read.. and truly understand the specs when I see a transistor datasheet..etc.

1.) what is difference between NPN vs PNP?  outside of things I turned up during searches (contruction..etc)
How do you know to use one over the other? When looking.. some only list voltage values.. where others list current specs..etc..  some both.

2.) I understand you can use a transsitor as a 'trigger' to power other devices the Arduino couldnt normally form the battery pack directly.. using the Arduino pin as a 'trigger' to close the 'gate' to allow the juice to flow....

but does the transistor 'regulate' that at all?

(Im wondering what these 5v & 6v ratings on transistors I see is for?  2N222A/2222/4401...etc.)

or does only worry about the vF voltage drop across the transistor?  (or not at all?)

I have a motor controlled/toggled by an Arduino pin set-up like this:

I cobbled it together from schematics elsewhere..etc  and it works..

but now I have upgraded to a different motor.. that seems to be able to take 6v+ (down to 3 even to move)..

and I started thinking how much voltage I was giving it in this set-up.... and how to properly (without getting too technical) test it?

do I just (when I get home) throw a multimeter across the two leads going to the battery? and turn the motor on using the Arduino and check the output?

Wondering if using a different transistor might be better?

I have a few available to me currently:



I see specs like:




Collector/Base Voltage (same as Vcbo?)
Collector/Emitter Voltage (same as Vceo?)
Emitter/Base Voltage (same as Vebo?)

but not sure how they APPLY to the circuit deisgn..

for example I see this:

Vcbo = 75v
Vceo = 40v
Vebo = 6v

what does the 6v stad for?  (or 75/40 for that matter?)

max input from the battery source it can take is one of them.....yes?.  Is that the COLLECTOR/BASE voltage?

**(gen. electronics section..   be nice!)  :)


You have asked a lot of questions, and reading on Wikipedia will probably help you most.

NPN and PNP are both junction transistors.  They refer to the types of semiconductor used.  Your schematic shows a NPN transistor, and you appear to be using it correctly. 
A positive voltage applied to the base relative to the emitter will allow a current to flow from the (positive) collector to the emitter.
(This statement is a simplification).

For a PNP transistor, the statement above reads:
A negative voltage applied to the base relative to the emitter will allow a current to flow from the (negative) collector to the emitter.

I.e. they switch and control current in a different way.  Your NPN is what is know as a "high side switch".  You could have a different schematic and use a PNP but to control it with your Arduino is would have to be a "low side switch".  Depending on what you want to control and the characteristics drives what type to use.  Both types can have very different ratings.


Does it regulate?
Well, they can, but it depends on your application and you probably don't want to.

Transistors in digital or logic mode are driven fully on or are fully off.  In an ideal world they would not get hot, because when off their power output (as heat) is V times 0.  When on, an ideal transistor would have a power output (as heat) of I times 0.
In reality, a fully off transistor does indeed dissipate zero watts.
But when on the Saturation Voltage dropped Collector to Emitter (Vce) is 0.2V or so.  So the power dissipated is I times 0.2.  That is
what the voltage Vce matters, especially for high power transistors (though it does not vary all that much).  (By the way, in this case,
I is limited by the load or supply, not the transistor, so too big a motor lets too much current through and the transistor blows.)

You can also run a transistor in a mode where is is only partly on.  In that case the power output is calculated in the same way.
And will be bigger, which is fine with a big enough transistor.  Which is why you have to know the ratings of it.  Also controlling it
exactly in such a mode is harder.

The specs refer to maximum voltages and current in specified conditions.  For example, Vcbo is voltage allowed between collector and base before the semiconductor breaks down permanently.  You should choose a transistor where these ratings will not be exceeded under any condition of your circuit.  Vebo is probably the biggest reverse voltage you should put between emitter and base - in your circuit that is going to be zero.  Vceo is important in that is the most the transistor will stand before it breaks down - in your case the 7V supply should be no problem.  The current specs are the ones to watch more. Dissipation is the amount of heat the transistor can throw away, again under certain conditions, and changes if it is one that you can heatsink - it will be Vce times Ic  (Collector Current).

I hope this helps, a bit.


This explains how to use transistors.

Check the current drawn by the motor under load.

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