# constant collector current whatever the load

Hi, I want to use TIP122 transistor. This is the datasheet http://www.adafruit.com/datasheets/TIP120.pdf

It is written that hfe is 1000 at Vce = 3 volt (test conditions). In my case, I put 12 volts on Vce. If I used resistors to make base current equal to 0.25 mA, Is this guarantee that Ic will still at 0.25 A even if the load increased or decreased ?

If not, How can I guarantee constant current on the collector ?

Transistors behave only approximately as current sources or sinks. If you want a reasonably constant current source you need an active element. There are many, many possibilities (for example using a voltage regulator as a current source) so Google is your friend.

Ok. Thank you.

I have searched for constant current source in google and I found LM317 ic which seems to be useful to get constant current source.

Though the TIP122 would have been a (roughly) constant current sink - does it
matter whether source or sink?

In practice the variation of current gain between devices means that you would
never program the collector current just by setting the base current - some sort
of feedback circuit would be needed to tame the current value. Often an opamp
is used with a transistor or darlington in the output as emitter-follower to boost
the current (effectively converting a low current opamp to a high current opamp).

IF you have a resistor from emitter to GND and feed the base with constant voltage you get an constant current generator.
Current= voltage at base - 1,2 V (it’s darlington) /R emitter.
Well, it Will varies with temperatur, what ex of transistor and the Vce.

Use left pictures
The zener apply constant voltage to base
IF you Google “constant current generator” and select pictures you have lots of circuits to test
Pelle

Here is the common 2 transistor method of making a constant/current limit circuit.

http://www.merghart.com/p/27/Two-Transistor-laser-current-source

It is temperature dependent but it is not too sensitive to transistor beta. Replace Q2 with the TIP122 for your circuit.

The compliance voltage e.g., the voltage where is starts current regulating, will be about 2.5V with the TIP122. It will be about 0.7V lower with a non-darlington general purpose NPN.

Here is an explanation of compliance voltage and some warnings about what not to do. There is also a nice op-amp circuit that will give you precise current control.

http://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/04/03/current-sources-and-sinks-understanding-compliance-range.aspx