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DIY Homemade laser diode driver

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Hemlock_Mike

Hemlock_Mike

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I found another question about the capacitor. NO - The capacitor does not charge slowly -- It is a greedy low impedance component used here to immediately eat a voltage spike from a "restrictive" power source like the LM317. However, It can be very unforgiving about "dumping its load" if unchecked in the circuit.
Mike
 





styropyro

styropyro

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I know how the capacitors prevent a voltage spike, but are they gonna be more hassle with them thank without them? I don't want the capacitor to discharge all of it's load into my diode.
 
Hemlock_Mike

Hemlock_Mike

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Styro --
That's why you solder the capacitor to the diode. So it can't take a dump on your diodes face ;D What we're saying here is stay clear of JCL's (jap clip leads) found at the rat shack. I blew a GOOD LD because of that mistake.

Mike
 
Gazoo

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I have found out the hard way by not using a capacitor. ;D Daedal's circuit will provide the ultimate protection for the diode given it is very easy to work with, and the parts are easily to get.
 
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yuip

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Hemlock Mike said:
DDL is doing his best to explain a really basic circuit which has been around for years.  He's telling you that the 4 ohm resistor sets the "MAXIMUM" current from the LM317 and the 10 ohm is the variable part which reduces the current to a lower level.  HINT: Start low.
NOW:  He is working with a specific LD (diode) which has its own parameters so YMMV with what you connect this to.  Each diode has its own forward voltage drop and the "effective" ohms (V/I) is not linear.  You have to be careful at the top end.
A bunch of us, SenKat, DDL, Gazoo and myself among many others, have found this out the hard way   :(    You will be in good company if you cook an LD yourself  :eek:

Mike
Click to expand...

Yeah, and he's doing a pretty good job of explaining things too!  :)

Thus far I've been lucky and only fried 2 diodes myself, and I am well aware of how much it sucks.

And thanks for clearing up my misunderstanding about the capacitors. The only true experience I've had with them is from a disposable camera modified into a ~400 volt sparking device.  :p Those do take a good while to charge up to full power. Then again, they have some big resistors on the board to limit the current...
 
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Xenodius

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Excellent work DDL! Great guide-- I was tired already, but trying to comprehend everything you said and envision it in my mind seriously just gave me a headache... trigonometry and God are the only other things that have ever done that! ;D ;D ;D

But okay... Little confusion about the appropriate resistor. You linked a 4 ohm, 1/4 watter from Radio Shack, but a 3.9 ohm, 1/2 watter from Digikey..? Assuming the one from radio shack is appropriate ATM because of your diagram!


Again, honorificabilitudinitatibus job! (Thats ain't jibba jabba by the by!)
 
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yuip

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Actually, the RadioShack link is for a pack of 500 resistors, of varying resistance.

One of the few things I know about electronics is that the wattage rating of a resistor is the amount of heat it can handle. So it doesn't really matter if you use say, a 1/2 watt resistor when a guide or kit calls for a 1/4 resistor of the same resistance.

The difference between 4.0 and 3.9 ohms is pretty neglible, but AFAIK there is no 4.0 ohm resistor, only 3.9 ohm and then 4.3 ohm.

Anyway, you can make exactly 4 ohms of resistance by placing two 2 ohm resistors in series if you want. Daedal said that a 4 ohm resistor would limit the maximum current at about 312.5mA.

I myself am worried about what wattage resistor is appropriate. Daedal used a 1/2 watt 10 ohm resistor so I assume that 1/2 watt resistors are the way to go.

So yeah, I guess stick with a 3.9 ohm resistor, and as long as you don't have the pot turned to max all the time your diode should live a good while. :)

Hope that helps.
 
Gazoo

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yuip said:
Actually, the RadioShack link is for a pack of 500 resistors, of varying resistance.

One of the few things I know about electronics is that the wattage rating of a resistor is the amount of heat it can handle. So it doesn't really matter if you use say, a 1/2 watt resistor when a guide or kit calls for a 1/4 resistor of the same resistance.

The difference between 4.0 and 3.9 ohms is pretty neglible, but AFAIK there is no 4.0 ohm resistor, only 3.9 ohm and then 4.3 ohm.

Anyway, you can make exactly 4 ohms of resistance by placing two 2 ohm resistors in series if you want. Daedal said that a 4 ohm resistor would limit the maximum current at about 312.5mA.

I myself am worried about what wattage resistor is appropriate. Daedal used a 1/2 watt 10 ohm resistor so I assume that 1/2 watt resistors are the way to go.

So yeah, I guess stick with a 3.9 ohm resistor, and as long as you don't have the pot turned to max all the time your diode should live a good while. :)

Hope that helps.
Click to expand...

In the circuit the wattage of the resistor does not matter as the current flowing through the resistors is very small.

When the trim pot in this circuit is set to 0 ohms, the output voltage will be 5 volts! This will supply too much current and kill the diode, which is why you want to start with the trim pot set at it's full 100 ohm value.

The formula for the LM317T is voltage out will be equal to 1.25 X ( 1+ R1/R2 ).

Since daedel is using a 6 volt battery, and the diode is causing a half volt voltage drop makeing the battery a 5.5 volts, You won't see 5 volts out of this circuit unless you use a 9 volt battery. As this circuit is designed, you might see 3 volts out at the max. The LM317T is very inefficient...this is the only drawback of using it. If you want to seriously overpower the diode, then use a 9 volt battery, and not those little square ones.

Added: The dropout voltage of the LM317T is 2 volts. This means that the voltage input must be at least 2 volts higher then the voltage output. Given we are actually using a 5.5 volt battery on account of the silicon diode dropping a half a volt,, The most we can expect to get out of this circuit is around 3 volts. This is why it would be much better to use a 9 volt or higher power source. Keep in mind the more voltage that is applied to the LM317T, the warmer it will be. If it gets hot, there are various very easy ways to provide heat sinking for it.

There are regulators out there that are more efficient and have a voltage drop of around 1 volt. And they are wired the exact same way. But they are more expensive and Radio Shack doesn't carry them.
 
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yuip

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Eeek, okay. I forgot that the pot was controling the voltage. Thanks for clearing things up. :)

I think I'll just stop providing any electronics advice, and just ask questions before someone makes a mistake!  :p ;D
 
Gazoo

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Hello yuip,
The most helpful formulas are good ol' ohms. law. Whenever I need to calculate current, voltage, resistance or watts, I refer to the following webpage:

http://www.the12volt.com/ohm/ohmslaw.asp

It provides an ohm's law pie chart as well as various calculators.
 
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yuip

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Hey Gazoo,

Thanks! I will definitely have to take a good look at that page every now and then. :p Good thing I just bookmarked it, thanks again for the link!
 
Gazoo

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Everything to make this driver is here. I ordered this kit:

http://cgi.ebay.com/LM317T-T0-220-V...5152564QQihZ017QQcategoryZ50912QQcmdZViewItem

This kit includes everything we need to make our own driver, and at a very reasonable cost. The only thing I don't like are the trimmer pots that are included with it. I will probably end up getting a few of the pots like daedal is using. Also it only includes 1 circuit board. But a similar circuit board can be found at Radio Shack. For the price it is decent :)

There is also an SMD version of the kit:

http://cgi.ebay.com/LM317M-SMT-Volt...2582019QQihZ017QQcategoryZ36331QQcmdZViewItem

I am staying away from SMD for now.
 
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yuip

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Hey Gazoo,

Nice find. That kit has a good bit of stuff included. But that kinda sucks that it only has 1 board when you could make several circuits from that kit.

The surface mount kit is cool too, but I don't think I could solder such tiny components together just yet. Though that is very handy for the people wanting to build it into a pointer case or a flashlight.

Thanks a lot!
 
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