Old 05-02-2010, 02:49 PM #1
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Default plz help !!!

ok i setup the driver for my red laser diode. i don't want to burn my laser diode so i connected the driver circuit to the dummy load circuit as explained.


the laser diode works on 2.6 volt and 35 ma.i measured at the diodes as explained and the only value i get is 0.87 v , it doesn't change and at the 1 ohm resistance 0mv, well it starts at 6mv but decreases fast to zero.

my voltage supply is ld voltage +3 v = 5.6 but i tried with 9 v and still nothing


plz help me


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Old 05-02-2010, 02:52 PM #2
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Default Re: plz help !!!

Need a drawing of how you connected the driver to the test load and
where you connected your DMM and on what scale of the DMM...

We don't know if your test load or driver is built correctly or if your DMM
is on the right scale or if your testing at the correct points...

Too many variables to give you an answer..



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Old 05-02-2010, 02:52 PM #3
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Default Re: plz help !!!

i would post a picture of the bream board but it looks hazy. anyway, i am 100 % sure it is connected correctly. it is nothing about the design
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Old 05-02-2010, 02:55 PM #4
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Default Re: plz help !!!

OK... if you say so...
Now you need to answer the other questions...

Also...which driver circuit are you using...???


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Old 05-02-2010, 02:56 PM #5
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Default Re: plz help !!!

the design below is the one from It can be done - Laser driver

believe me i connected every thing like it is drawn in the picture
Attached Thumbnails
plz help !!!-ddl-circuit-test01.jpg  
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Old 05-02-2010, 03:02 PM #6
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Default Re: plz help !!!

the one i attached but i added another 10 ohm to get a total of 5 ohm and i am using a 100 ohm pot but as i said i need 35- 45 ma for the circuit so thats 1.25/35-45ma and 2.3-26 volts operating voltage
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Old 05-02-2010, 03:03 PM #7
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Default Re: plz help !!!

You will need at least 7 volts for testing... batteries have droop.
You still have not answered what scale on your DMM you are using

Are you holding the probes across the 1 Ohm Resistor...and have you
checked that your resistor is indeed 1 Ohm with your DMM...
and are you sure the Diodes are in the right direction...

A clear photo would still be easier...

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Old 05-02-2010, 03:03 PM #8
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Default Re: plz help !!!

i measured tat the vout and adj and i got 1.25v but when i connect across the 1 ohm resistance i get zero and across the 4 diodes i get max 0.87 v and voltage supply is 9v
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Old 05-02-2010, 03:09 PM #9
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Default Re: plz help !!!

yes it is 1 ohm and i am sure it is across the 1 ohm. i am not what u mean by what scale if u mean mv or V i used mv when i measure across the 1 ohm and volts across the diodes.

about the direction of the diode, is the little gray circle the positive or negative?
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Old 05-02-2010, 03:21 PM #10
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Default Re: plz help !!!

Quote:
Originally Posted by harryd View Post
the one i attached but i added another 10 ohm to get a total of 5 ohm and i am using a 100 ohm pot but as i said i need 35- 45 ma for the circuit so thats 1.25/35-45ma and 2.3-26 volts operating voltage
1.25/45mA = ~27 Ohms not 5 Ohms...

1.25/5 Ohms = 250mA...

You still have not answered the questions I asked..
Answering questions that you think I would like to know
does not help me or you to figure this thing out...

If you wired up the parts to the correct connection points...
and your electronic parts are in good working order...
and your electronic part values are correct....
and your batteries are fully charged a fresh....
and your DMM is functioning properly....
and your DMM is or the correct scale...

your Laser Driver circuit will output the current your set
it to and you can measure that current (1mV=1mA) across
the 1 Ohm resistor with your DMM...

If not.... there is a problem with....

1) your circuit hookup
2) your parts or their values
3) your batteries
4) your DMM or it's scale setting
5) a combinatoin of any or all of the above..

[EDIT]
BTW.... using the [EDIT] button at the bottom of your posts will
stop you from Double Posting when not required...
It is considered bad Forum etiquette...

The Band of the Rectifier Diode (the Cathode) connects towards the
Negative (ground) of the Driver..


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Last edited by lasersbee; 05-02-2010 at 03:27 PM.
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Old 05-02-2010, 04:26 PM #11
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Default Re: plz help !!!

ok thanks a lot man, it is working now. however, i still have a problem when i measure across the diodes it gives 2.9 v and the operating voltage is 2.3 and max of 2.6. how can i reduce this 2.9 to like 2.3 or 2.5?
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Old 05-02-2010, 06:19 PM #12
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Default Re: plz help !!!

anyone plz?
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Old 05-02-2010, 06:54 PM #13
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Default Re: plz help !!!

If you get the correct current reading across the one ohm resistor then don`t worry about voltage across the test load diodes. You are building a current regulator here. A current regulator will let enough voltage pass to the load that the load draws the set current of the regulator. A test load is different from an LD. For this type of driver (linear) you don`t need a test load, you can stick an ammeter across the output and read the current directly. You need a test load for a boost driver, these burn out if tested without a proper load. If your driver is giving the correct reading across the one ohm resistor, don`t forget to discharge the output cap before connecting the LD. If you don`t short this cap, the LD will be blown by the residual charge as you connect it. Many LD`s have died this way ...

PS : This is the review section, this topic might have got noticed more if placed in the appropriate part of the forum ...
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Last edited by billg519; 05-02-2010 at 06:57 PM.
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Old 05-02-2010, 07:06 PM #14
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Default Re: plz help !!!

Quote:
Originally Posted by billg519 View Post
If you get the correct current reading across the one ohm resistor then don`t worry about voltage across the test load diodes. You are building a current regulator here. A current regulator will let enough voltage pass to the load that the load draws the set current of the regulator. A test load is different from an LD. For this type of driver (linear) you don`t need a test load, you can stick an ammeter across the output and read the current directly. You need a test load for a boost driver, these burn out if tested without a proper load. If your driver is giving the correct reading across the one ohm resistor, don`t forget to discharge the output cap before connecting the LD. If you don`t short this cap, the LD will be blown by the residual charge as you connect it. Many LD`s have died this way ...

PS : This is the review section, this topic might have got noticed more if placed in the appropriate part of the forum ...
I agree with you 100% but it will not hurt to use a Test Load..
That just adds to the confusion of New Members...IMO

If all the New Members were told to use tests loads for ALL their
Laser Driver adjustments there would be less confusion to
whether the LD Driver they are going to use actually needs a Test
Load...

I can just see a Noob adjusting a Boost driver without a Test Load
and blowing it because he misunderstood..

@ harryd.... like Bill said...

just check the current output of your driver... the Driver will furnish
the proper voltage to the Laser Diode at the adjusted current.
The Laser Diodes internal running resistance will regulate the
voltage at the current you adjust.


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Last edited by lasersbee; 05-02-2010 at 07:10 PM.
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Old 05-02-2010, 09:45 PM #15
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Default Re: plz help !!!

i know i ask a lot of stupid questions but i only have one laser diode and i have to submit the project in less than a week and i live in egypt ( they dont sell laser diodes here). i am pretty much screwed if the laser diode burns.

3 questions :
1- some people told me that this laser diode i have wont work on that driver because it needs a negative loop or something like that can u verify that plz... here is the datasheet of the laser diode DL-3147-165 pdf, DL-3147-165 description, DL-3147-165 datasheets, DL-3147-165 view ::: ALLDATASHEET :::

2- about the discharge thing, i just connect both ends( +ve and -ve to be connected to the laser diode) of the circuit together to discharge?

3- what is the relation between current and power?
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Old 05-02-2010, 10:47 PM #16
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Default Re: plz help !!!

According to your Data Sheet.... the Max output is 7mW at a Max
Current of 50mA... you will use pins #1 and #2 only...

If you build the LM317 driver and between the Adjust pin and output pin
of the LM317 you put only a 27 Ohm resistor is series with your 100 Ohm trimpot...

1) with the trimpot at Maximum resistance (the lowest current setting) the total
resistance will be ~127 Ohms..

1.25/127 Ohms = ~10mA

2) with the trimpot at minimum resistance (the highest current setting) the total
resistance will be ~27 Ohms..

1.25/27 Ohms = ~46mA

This is well in the range of your Laser Diode... Test it with your Test Load
across the 1 Ohm resistor with your DMM set to 200mV full scale.

Don't forget... your Laser Diode will only start to Lase at ~25mA as stated
in the Data Sheet..

Yes.... to discharge the output cap of your Laser Driver Just touch the wires
that connect to your Test Load together with NO POWER to the Driver.
That will discharge the capacitor... I would do this 3 times to be sure it is
discharged.


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