Connection to this laser diode QL78J6SA - Page 1
Forum section: electronics / beginners

author: wajihalam
author_rank: Newbie
timestamp: March 01, 2018, 10:22:38 pm
content: Hi,I was trying to lit up this laser diode. The laser diode model is QL78J6SA. The specification for the connection is attached. I am not sure which end I should connect positive voltage and to which end ground. Also, what is the use of PD here?Any input to this question is appreciated.Thank you again.

author: Benta
author_rank: Super Contributor
timestamp: March 01, 2018, 10:34:28 pm
content: I strongly suggest you read up on laser diode drive electronics.To get a laser diode running, it's necessary to monitor the optical output. This is why the PD (=photodiode) is integrated. Your drive circuit needs to read the PD and adjust drive to the LD (=laser diode) accordingly.You'll probably need cooling as well.This is not just a red LED.

author: wajihalam
author_rank: Newbie
timestamp: March 01, 2018, 10:49:40 pm
content: what is pin 2 in this particular case? QL78J6SA

author: wajihalam
author_rank: Newbie
timestamp: March 02, 2018, 12:21:54 am
content: To add to my last post, I believe that this laser diode utilizes a common anode configuration. That means, pin 1 is connected to negative (ground), pin 2 is connected to positive supply, pin 3 is connected to ground again.Please correct me if I am wrong.

author: amyk
author_rank: Super Contributor
timestamp: March 02, 2018, 12:24:37 pm
content: Did you not see the informative reply that Benta posted almost 2 hours before you?

author: Benta
author_rank: Super Contributor
timestamp: March 02, 2018, 04:18:41 pm
content: wajihalam, you might want to follow this thread:https://www.eevblog.com/forum/projects/precision-constant-optical-power-laser-driver/

author: wajihalam
author_rank: Newbie
timestamp: March 03, 2018, 06:52:36 am
content: I know about constant supply that is needed for the laser diode to work. However, I am still confused about pin 1, 2 and 3 connection. As shown in the attachment for QL78J6SA, Pin 1 is laser diode cathode, and pin 3 is laser diode anode. However, I am not sure where to connect which pin. Do you know of any circuit diagram?

author: amyk
author_rank: Super Contributor
timestamp: March 03, 2018, 05:04:31 pm
content: You should probably start with something simpler first, like an LED.Also,https://www.google.com/search?q=how+to+read+a+schematic

author: Buriedcode
author_rank: Super Contributor
timestamp: March 03, 2018, 05:42:39 pm
content: I think the reason people are telling you to be cautious is, if you are unable to distinguish the anode and cathode from that datasheet/schematic, then there is a high risk of you damaging the laser diode (they are very delicate, and easily killed with static, over voltage, over current, over temperature.. harsh language etc..) and it also shows a lack of understanding and as such, possibly unaware of the real dangers to your eyes.Please dont' take offense to this, I am not trying to put you off. Just that you should be aware of the requirements of driving laser diodes - it isn't just a case of a "constant current supply", you'll need circuit protection. And to stop you wasting your money by destroying a laser diode (I've destroyed about 3 whilst testing a circuit before) to read up, and test on red LED's first. LED's are similar - they are diodes and as such exhibit much of the same characteristics but do not cost as much, and cannot blind (assuming you're dealing with 5mm ones).In regard to your question, the schematic shows the laser diode and photodiode in three configurations labelled A, B and C. It clearly states the pin numbers of the laser diodes anode and cathode with "LD" - laser diode, and "PD" = photodiode - used for feedback for the driver to achieve a relatively constant optical output power (laser diodes optical output depends on temperature, and so varies greatly during operation).You ask about ground, that is a relative term, but I'm pretty sure you mean cathode, or negative terminal. Many constant current drivers use low-side current sensing, so the negative terminal of the laser diode isn't actually at "ground" potential, best to use the terms "anode" and "cathode".edit: apologies to other posters for effectively echoing what they said. Wasn't sure if the OP understood or just skipped over the replies so I thought I'd reiterate.

author: wajihalam
author_rank: Newbie
timestamp: March 04, 2018, 12:30:08 am
content: Thank you all for your reply. I would ignore comments from amyk as it is no where close to the answers that I am looking for. I would respectfully ask you to post some relevant comment if possible. Please don't get offended.To Benta and Buriedcode, Thank you again for taking your time to give such an extensive reply. Those were really helpful. I am just new to laser diode and I was confused with the schematic that is why I posted the question. If the diode in schematic is common anode or common cathode. It was a simple question which needed a simple answer. Before I posted the question, I had already read about the circuitry required to operate the laser diode. However, I was not able to be sure about this particular diode with 3 pins. I am still reading about it.Thank you all for your help again.