Testing DDL Circuit with multimeter

[viroy]

Pardon me :
I have a question about testing DDL's circuit with a meter to get exact values...
I also have a question about the pot.

The pot adjusts current, voltage or both? (The way I understand it is, the pot adjusts the mA and the Voltage stays the same) if so... how do you achieve an exact voltage?

I have an old digitek ds830b multimeter that I cannot find a picture of anywhere. It says 500v, 200ma max... looks like DC only.
What do I set this to so that I can read the MW and MA going to the LD? (I have 3 settings on the meter that say mA. One is alone and just says '200 ma', the other 2 settings are together saying '1.5v' and '9v' and has 'mA' written behind them).
For voltage I'm assuming to use the '20' setting? Or should I use the '2000m' setting?

Sorry, I'm a bit of a noob.
I appreciate any help!

P.S. - I am using a bit smaller of a capacitor than the 35v 47uf shown on DDL's schematic, mine says 16v 47uf... Should I get a 35v one?
Im going by DDL's schematic from: laserpointerforums.com/forums/YaBB.pl?num=1185701612

 

[chido]

First, I'm sorry I didn't see your post in my thread.  :-[

Ok then, the pot will only adjust the current going to the diode, when using DDL's driver you only have to worry about the current, the diode will only take the voltage it needs so you don't have to worry about it.
Your capacitor will do just fine.
Check this thread for testing your driver before connecting your diode to it and set your multimeter to the 2000m voltage setting.

 

[viroy]

Thank you for the quick response Chido
That thread helped alot! i do however have some more questions now lol

1) I would like to learn how to do the math and determine the values needed on my own rather than relying on pre-made schematic values.
i.e. modify a DDL driver to run a 1w 24v diode off a 120v powersource (which is on the way, its a 808nm i will use for cutting), or one to work specifically with a 30ma ?v bluray diode off a small battery (which i aready have working from the first DDL driver i built), or even some random DVD diode i pull.

2) I see people using different value resistors after the lm317 to get different ma output ranges. What is the math i need to know so that i can figure out what ohm resistors to use to achieve a desired result? do i need to know about different chips than the lm317?

I am confused about voltage pull from different LD's, in particular the blurays. I have read threads that say you need to use a 9v source, and others that say one of the 3.6v LI batterys is fine. I noticed the ma rating on the LI battery is like 500ma or more... either way the Ma is obviosuly enough... but is that really enough voltage? better question yet...
3) is there a way i can test diodes to tell exactly what voltage they need?
Say someone just hands you a diode, no specs, dont even know what wavelength... can you determine the voltage needed? im assuming the only way to determine max current threshold is to raise the level until it dies?
:-/

 

[viroy]

wow i see blazelasers is using a 3.6v LI battery for their 300mw lasers
batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2763

 

[chido]

Thank you for the quick response Chido
That thread helped alot! i do however have some more questions now lol

1) I would like to learn how to do the math and determine the values needed on my own rather than relying on pre-made schematic values.
i.e. modify a DDL driver to run a 1w 24v diode off a 120v powersource (which is on the way, its a 808nm i will use for cutting), or one to work specifically with a 30ma ?v bluray diode off a small battery (which i aready have working from the first DDL driver i built), or even some random DVD diode i pull.

2) I see people using different value resistors after the lm317 to get different ma output ranges. What is the math i need to know so that i can figure out what ohm resistors to use to achieve a desired result? do i need to know about different chips than the lm317?

I am confused about voltage pull from different LD's, in particular the blurays. I have read threads that say you need to use a 9v source, and others that say one of the 3.6v LI batterys is fine. I noticed the ma rating on the LI battery is like 500ma or more... either way the Ma is obviosuly enough... but is that really enough voltage? better question yet...
3) is there a way i can test diodes to tell exactly what voltage they need?
Say someone just hands you a diode, no specs, dont even know what wavelength... can you determine the voltage needed? im assuming the only way to determine max current threshold is to raise the level until it dies?
:-/

1. The max input voltage the LM317 can handle is 37v so there's no way of modifying it to hancle 120v.

2. Gazoo's canned response:

The LM317 is no mystery and very easy to work with. The following calculations always apply since it uses 1.25 volts for its reference voltage, and ohm[ch8217]s laws don't lie.

To calculate the resistor needed for a given current, take 1.25 and divide it by the current. So say you want to drive a SenKat diode with 250mA. 1.25 divided by .250 = a 5 ohm resistor.

Another way you could do this is to take 1.25 and divide it by the resistance. 1.25 divided by 5 = .250.

Next you will want to calculate the wattage of the resister needed. We know 1/2 watt resistors are common for use with the regulator. But to figure it out, simply take the 1.25 and multiply it times the current. 1.25 times .250 =.3125 watts.

The rule of thumb for the voltage going into the regulator is it should be 3 volts more than the voltage going to the diode. A SenKat diode running at 250ma's will have about 3 volts across it. Therefore a minimum if 6 volts is needed.
I recommend 6 NIMH batteries or 2 RCR123's for use with Daedal's driver.

This is why you need at least 8 volts to run the blu-ray. You will find when you have it hooked up, the voltage across it will be appx. 5 volts. But since it will only draw ~38mA, you can run it with a 9 volt battery.

3. When using DDL's driver the only thing you have to worry about is the current, the diode will take whatever voltage it needs to work. If you get a diode with absolutely no specs, then there's no way of knowing how much current you can put into it. Unless you have a laser power meter, then you can test how much current you're putting into it with a DMM and see the output by using the power meter.

 

[viroy]

hmm, how did you determine that the 16v cap is ok to use?
i know what all the general parts do, i just dont know the math to determine any of this myself

 

[chido]

Your LD will use approx. 3v, the 16v cap can hold 5 times the voltage your LD will be using. It'll be fine.
Also, Gazoo said he used 16v capacitors and everything went ok.
AND look at the link you typed on your first post, scroll down and you'll see a schematic rog8811 posted, he shows a 16v capacitor.

 

[viroy]

Im gonna have to buy you lunch sometime if u come to WI Chido [smiley=beer.gif]
Im almost there, just need clarification on a few other things please.

Next you will want to calculate the wattage of the resister needed. We know 1/2 watt resistors are common for use with the regulator. But to figure it out, simply take the 1.25 and multiply it times the current. 1.25 times .250 =.3125 watts.
So i always want to go above, but closest to the wattage? so for this i would use a 1/2 watt resistor and if it were something like .715 watts i would use a 1 watt resistor correct?

This is why you need at least 8 volts to run the blu-ray. You will find when you have it hooked up, the voltage across it will be appx. 5 volts. But since it will only draw ~38mA, you can run it with a 9 volt battery.
So i would need 31.25 ohms of resistance at .05 amps behind the lm317 assuming 40ma draw?
how does the potentiometer affect this? how would i know to use a 50ohm or 100ohm pot?
i dont understand what you mean by since it only draws 38ma, i can use a 9v. whats the difference between the 8v and 9v in relation to the draw?

it really seems like im missing something.... like for example it doesnt seem right that regardless of input, if i use a 5 ohm 1/2 watt resistor after the lm317, i would get 250ma. is that just an inherent characteristic of the lm317?
so it doesnt matter if i used a source of 6v 3000ma or 24v 50000ma, it would result identically down to 250ma?

last but not least... If the lm317 uses 1.5v, why am i aiming for 3v over??? what happend to the other 1.5v?

Sorry for all my confusion, i dont know where else to turn to except trying to find a professor at the local university that might have some spare time to answer these types of Q's.
You guys are invaluable to me! [smiley=dankk2.gif]

 

[chido]

So i always want to go above, but closest to the wattage? so for this i would use a 1/2 watt resistor and if it were something like .715 watts i would use a 1 watt resistor correct?
Sure, but you don't necessarily need to get close to the wattage, i.e. if you were driving a diode at 300mA you'd be getting .375 watts, you could use a 1 watt resistor if you want, you don't have to use a 1/2 watt resistor.

So i would need 31.25 ohms of resistance at .05 amps behind the lm317 assuming 40ma draw?
how does the potentiometer affect this? how would i know to use a 50ohm or 100ohm pot?
i dont understand what you mean by since it only draws 38ma, i can use a 9v. whats the difference between the 8v and 9v in relation to the draw?
Yes you're right about the 31.25 ohms, but where did you get .05 amps from?
The pontentiometer puts out a certain amount of current at whatever resistance you set it at. The resistor is there to limit the current going to the diode in case you accidentally set the pot to the least resistance, i.e. let's say you want to drive your diode at 230mA, you set the pot to 5.4 ohms of resistance and use a 5ohm resistor to limit the current to 250mA in case you accidentally set your pot to 0 ohms.
What Gazoo meant by the 38mA 9v thing was this. A red laser diode usually needs an average of 3v to laze but it needs high a current compared to a bluray which usually needs an average of 38mA to work and about 5v. The LM317 always keeps 3v to itself, so when using a bluray diode you need an input of at least 8v to keep the regulator from dropping out. With reds you'd need NIMH, lithium batteries, alkalines or any other kind of batteries with a high mAh rating to keep the red diode running for a long time, but since blurays only use an average of 38mA you can get away by using a simple 9v battery.

it really seems like im missing something.... like for example it doesnt seem right that regardless of input, if i use a 5 ohm 1/2 watt resistor after the lm317, i would get 250ma. is that just an inherent characteristic of the lm317?
so it doesnt matter if i used a source of 6v 3000ma or 24v 50000ma, it would result identically down to 250ma?
That's what the resistor and pot are for, they limit the current going to the laser diode. You can not use a source that exceeds 37v or 1.5A, otherwise the LM317 won't work. It doesn't matter if you put in 1A into the LM317, it will just let all that current go straight to the LD and kill it unless you use the pot, the resistor, or both.

last but not least... If the lm317 uses 1.5v, why am i aiming for 3v over??? what happend to the other 1.5v?
The LM317 doesn't use 1.5v, it uses 3v. Like I said before, for red LDs you'll need at least 3v extra. The LM317 keeps 3v, your laser diode will keep another 3v, so you need at least a 6v input to run the circuit. Use 6 NIMH cells, or 2 3.6v rechargeable CR123 batteries to give a total of 7.2v input, if you use the exact 6v, as soon as the batteries get drained the circuit will start to drop out and it won't send the the total amount of current you want to drive your LD at, so your laser will just keep getting weaker and weaker every time you use it.

I hope this helps.

 

[viroy]

Yes you're right about the 31.25 ohms, but where did you get .05 amps from?
Heres how i came up with .05 assuming a 40ma draw

Next you will want to calculate the wattage of the resister needed. We know 1/2 watt resistors are common for use with the regulator. But to figure it out, simply take the 1.25 and multiply it times the current. 1.25 times .250 =.3125 watts.
1.25 x .040 = .05

doh, so i actually meant watts... not amps

So would this resistor work? or is that too many watts? i couldnt find a smaller one
30 ohm 3 watt Sprague Blue Jacket $2.00

 

[chido]

Ok ;D Any more questions????

 

[viroy]

oh... how can blazelasers use a single 3.6v battery to power a 300mw laser?
seems to me thats not enough voltage... are they stepping it up?

 

[chido]

They can power them because they're using a totally different driver, I don't know if it boosts voltage, I have no idea how their driver works. The 3.6v lithium battery has enough juice to power a laser diode and I bet they're using a really efficient driver. The LM317 isn't really that effecient since it needs 3v to work, but there are tons of other drivers out there that are way more efficient than the DDL circuit. One example is the AMC7135 driver which can be powered with a single 3.6v lithium battery.

 

[viroy]

Ah cool, i get it.
So that would be good for reds or greens but not for higher power diodes such as bluray because it requires more voltage, correct?

Well i think im out of questions on this subject! [smiley=cheesy.gif]
Chido, you rock man, thanks so much!... is there anything i can do for you?
im a network engineer so if you have any computer/networking questions i can help.
A little bit of sad irony... im a descendant of george ohm.
[smiley=dankk2.gif]

 

[chido]

So that would be good for reds or greens but not for higher power diodes such as bluray because it requires more voltage, correct?

Yup, you're correct, although you can still use a regulator to boost the voltage, so it's possible to drive a bluray with a single 3.6v battery.

Well i think im out of questions on this subject! [smiley=cheesy.gif]
Chido, you rock man, thanks so much!... is there anything i can do for you?
im a network engineer so if you have any computer/networking questions i can help.
A little bit of sad irony... im a descendant of george ohm.
[smiley=dankk2.gif]

You're welcome, I'll pm you in case I have any questions, but I'm not that bad with computers so I think I'll live.

Are you really his descendant???

 

[viroy]

Yes, on my fathers side.
Our family came from Germany in 1909 (have the original sponsor papers too! almost 100 years old!)
There are actually alot of Ohm's living around here in south central WI
Big German/norwegian/polish area population here, i can get lutefisk and limburger less than 2 miles from my home ;D
Limburger is good stuff but not a fan of the lutefisk