Constant current source to power a VCSEL - Page 1
Forum section: electronics / beginners

author: Bigman
author_rank: Regular Contributor
timestamp: December 06, 2014, 11:31:40 am
content: Hi everybody,I am a hobbiest more provicient in mechanics but with an "beginner" interested in electronics. At the moment, I am trying to design a circuit to power a VCSEL with a Li-Battery. To do so, I decided to go with a "Ube-"Constant-Current circuit. My first attempt was just with two transistors as you can see from the attachment cc_2xNPN_not_temp_stable.png. Since this one is not temperature stabilized, I made a 2nd one, but I exchanged one of the transistors by a TLV431 (see cc_TLV431.png).My target for both circuits is to have a constant current of 10mA flowing throught the VCSEL (if you are interested I can also show you "my" math regarding the resistors, which I hope to have received by my choices of resistors).But what I am wondering and would like to ask for support:When I turn the circuit on/off, I could imagine a have to protect the VCSEL from voltage-spikes. That's the reason why i have added in both circuits the capacitor C1 ... but I have no glue, what capacity-size for such a C1 capacitor would be appropriate???For all tips, hint, recommendations, I would be very grateful!Cheers

author: awallin
author_rank: Frequent Contributor
timestamp: December 06, 2014, 03:05:14 pm
content: op-amp + FET is the usual laser diode driver circuit.See Libbrecht & Hallhttp://www.its.caltech.edu/~atomic/publist/diodecontroller.pdfor a more modern one by Erickson:http://www.physics.byu.edu/faculty/durfee/Publications/Erickson08au.pdfit's a good idea to short-circuit the diode when not in use, to protect against ESD.If you don't need to modulate the current at a high frequency something like a "lasorb" transient voltage protector might also be a good ideahttp://www.lasorb.com/

author: Bigman
author_rank: Regular Contributor
timestamp: December 06, 2014, 04:15:42 pm
content: Quote from: awallin on December 06, 2014, 03:05:14 pmThank you for your reply. Those two circuits are looking damn complicated and require a lot more components compared to my original "basic" version(s). Well, on a first glance, my impression is those two circuits you suggested are also intended to modulate the current ... but I just need a permanent, constant current of 10mA (no modulation at all).

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2014, 02:11:55 pm
content: What's the forward voltage of the laser?The first circuit will work at a lower voltage, about 0.7V plus the saturation voltage and the laser diode drop.The second circuit needs 1.24V in addition to the saturation voltage and the laser diode drop but has superior current regulation.

author: Bigman
author_rank: Regular Contributor
timestamp: December 08, 2014, 05:14:25 pm
content: Quote from: Hero999 on December 08, 2014, 02:11:55 pmI am working with a VCSEL and I assume a voltage drop at that VCSEL of 2.2V (according to datahseet).Regarding the saturation voltage at the transistor. Well, at the very beginning I had no glue which value to take and I guessed it around 0.7V. But then, I came across follwoing diagram out of the transistor's datasheet (see picture attached; Source: Datasheet von Toshiba, Transistor 2SC3138). I admit to have strong doubts if I really can use this, because for Ic=10mA, Uce(sat) would be below 0.2V (!?).However, the first circuit with 2 transistor would work either way:Voltage drop at ...... VCSEL: 2.2V... Uce(sat) transistor: 0.2V (or 0.7V)... Resistor R2: 0.7V (because of the other transistor)Sum = 3.1V (or 3.6V) <-- both is fine for one Li-BatteryFor the 2nd circuit with the TLV431:Voltage drop at ...... VCSEL: 2.2V... Uce(sat) transistor: 0.2V (or 0.7V)... Resistor R2: 1.286V (because of TLV431)Sum = 3.686V (or 4.186) <-- 3.686V still looks acceptable for one Li-Battery, but 4.186V unfortunately not