Understanding the simplicity of Daedal's circuit

[Gazoo]

Re: Understanding the simplicity of Daedal's circu

Hey buddy, I understand. I hope you didn't read my post before I edited it...you know...the part where I said I was applying all those voltages directly to the LD.. ... ;D

 

[Daedal]

Re: Understanding the simplicity of Daedal's circu

;D

We all do at one time or the other

--DDL

 

[o_DEATH_ANG3L_o]

Re: Understanding the simplicity of Daedal's circu

Okay okay I think Daedal just clarified some things along with Gazoo. So basically this circuit allows input voltage up to 30 volts. So depending on the load given at the output, it only "gets" the necessary voltage required by the load, and then the rest is taken by the LM317T which is why a higher voltage input would result in a hotter chip. Am I right? Well I have been doing some further testing with cheap 5 mW lasers from Wal-Mart. I hooked these laser pointers up to Daedal's circuit with an input of 9.75 volts. I read the voltage that the pointer was receiving and it was only 5.25 or so. With the rheostat at maximum resistance I was getting 30 mA of current on the laser pointer. Now at least resistance I was getting around 115 mA. I do of course realize that these laser pointers have a 100 ohm resistor in their circuits. But I also I have had them apart and the LD is a flat chip. I did test this LD directly with Daedal's circuit. The results, I got 6.50 volts going to the LD and at max pot resistance, still 30 mA. Of course it was the same 115 mA at least resistance. So I now know that a GB LD or any other DVD burner LD would be safe using my 9.75 volt PSU along with Daedal's wonderful LD driver circuit. . So if you read this lengthly post I thank you and welcome any criticism. Meaning let me know if I am wrong in anything I just said.(Well actually typed.)

P.S.- The picture is showing two 5 mW $2.88 Wal-Mart laser pointers.

 

[chimo]

Re: Understanding the simplicity of Daedal's circu

In current source mode the LM317 essentially acts as a variable resistor.

It will vary its "resistance" until the current that flows through it is equal to the design current (1.25V drop across the sense resistor).

If this is not possible due to the source voltage not being high enough to meet the minimum drop-out requirements, it will fall out of regulation and not provide the designed current. In this case, the actual voltage measured across the sense resistor will be less than 1.25V.

In current source mode, because the LM317 acts as a variable resistor, it is desirable to ensure the source voltage will meet the minimum drop-out voltage for the range of inputs (i.e. fresh battery to nearly depleted ones).

For example,
Let's say the input voltage is 12 volts, the load voltage is 3 volts at 250mA.
The sense resistor would have to be 5 ohms and it would have a voltage drop of 1.25V.
So we have 3 volts across the load and 1.25 volts across the resistor.
10 - 3 - 1.25 = 5.75V That's whould be the voltage across the LM317.
That poor little LM317 is sinking 1.44Watts (5.75V x 0.25A).
The load is only getting 0.75Watts. The sense resistor is consuming 0.3125Watts. The power source is providing 2.5Watts and only 3/4 of a watt is getting to the load. The circuit is 30% efficient! If this is squeezed into an enclosed space like a flashlight head, then some of that excess heat is going to flow towards the LD.

I hope this helps clear things up for folks.

It is true that LD's act just like any other diode. They cause a voltage drop in the circuit.

Perhaps you are right about the diode not taking any of the extra voltage, and I would assume and I verified that your measurements are correct.

I'll have to figure out eventually where that extra voltage is going... I'm too sleepy to think now...  ;D

--DDL

 

[laserrod]

Re: Understanding the simplicity of Daedal's circu

Great reconsiliation of the circuit man

 

[Benm]

Re: Understanding the simplicity of Daedal's circu

tomokadesigns.com/laser1.jpg
tomokadesigns.com/laser2.jpg

That's a good piece of soldering work!

Daedal's circuit works well, simple but effective, thank you for developing it for this purpose. I just wish the voltage drop wasn't 1.25V, kinda blows the idea of 4 NiMH cells.

Running on 4 NiMH's is definitely possible, but not using the LM317 (at least not remotely within specs). I've posted a current source elsewhere in this board that'll easily drive a LD at 200 mA from 4 volts input, but it will be hard to construct -that- small.
See http://www.laserpointerforums.com/forums/YaBB.pl?num=1189529869