TIP 120 Transistor datasheet help reading

Thread Starter


Joined Feb 14, 2016
I'm trying to figure out max voltages, but the datasheet for the TIP120 appears to have 3 differnet max voltages, which appear to be across Collector-Base. Collector-Emitter, and Emitter-Base.

I thought I was using a voltage regulator at first and hooked a 9v + up to the base, ground to the collector, and an led to the emitter then back to ground. This made the transistor get very hot.

So I looked up the TIP120 to try and figure out what voltages it can take and discovered it was a transitor not a regulator, but still trying to find out what is an acceptable voltage to put on the Base to activate the transistor, I don't understand why the datasheet want's to combine it with the collector or the emitter.

I would think it would be Base = x volts, while collector goes to ground, and emitter goes to the ground terminal of the load circuit. So Collector_base / Collector-Emitter and Emitter-Base makes no sense.



Joined Feb 24, 2006
All transistor want to have about 0.7V across the base emitter junction to turn them on. In this on condition, the collector will be a few tenths of a volt above the emitter and a large current will flow from the collector to the emitter. So you need a resistor to limit the current into the base, and you need a resistor to limit the current through the collector to the emitter. That should get you started. Check the ebook associated with this site for a more complete description with pictures.

Transistor voltages are specified as being the maximum voltage that can be applied across the two (2) pins mentioned, with the 3rd pin open-circuited ie: not connected. The nomenclature is "V" followed by sub-script containing three (3) capital letters: the 1st and 2nd letters specify the terminals to which the voltage is applied, and the 3rd letter is "O" for "open circuit". Capital letters denotes DC values (of current or voltages), whereas lower-case letters would denote the signal (non-DC) component only.
In the example of the TIP120 datasheet, the ratings are:
VCBO = 60V
VCEO = 60V

What is interesting in the case of this device is that VCBO is same as VCEO. Usually VCBO is higher than VCEO. The reason for this is quite simple: this is a darlington-pair, the package contains two (2) transistors connected in the famous darlington configuration. Furthermore, there are internal resistors place across the B-E junction, so that means that when this device is tested for VCBO and VCEO, the base is not really open circuited.

You will need to forward bias the package B and E terminals to about 1.2V to start to turn on this device. The current gain will be very high (over than 1,000), however due to the 8kOhm resistor across BE of Q1, there will have to be more than about (0.6V / 8k) = 75uA flowing into the base terminal to start to turn this device on, so take that into account when designing the base circuit.

Hope this helps.
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Joined Aug 1, 2013
It sounds like you are a long distance from understanding how transistors operate. A transistor is an electronic component, not a complete device. You don't just hook up a power source to two of its terminals. I suggest you surf the web for some basic transistor operation tutorials. I think this site has some e-books on basic electronics. Maybe one of the moderators can provide a link.


Thread Starter


Joined Feb 14, 2016
I am new to electronics, however I have used transistors before. I just didn't understand how to read the datasheet as it pertained to the two leads. FabioBarone answered my question exactly.