# circuit/ component to increase starting current/ voltage slowly?



## joniverson (Jul 31, 2007)

Say I have a laser powered by a simple battery/ resistor circuit. The voltage/ current to the laser is acceptable due to the resistor, but I want the start-up voltage/ current to increase slowly- like over a couple of seconds- instead of instant -on. What type of circuit/ component would I add?

Thanks,
Jon


----------



## dr_lava (Jul 31, 2007)

it's not feasilbe to have that slow turn-on with a simple r-c circuit. You can do it with a few more parts:



From sam's laser faq:



> By contrast, this circuit can only *decrease* the current from the 1.25V/R value, but it easily handles a 10:1 range (or even much more) and the voltage across the sense resistor is never more than 1.25V, allowing low supply voltage (e.g. 5 V) and keeping the dissipation low.
> 
> ```
> In +-------+ Out  R1 .25 ohms
> ...


Put a large cap from adj to ground and this should slow the turn-on rate appreciably.


----------



## VaThInK (Aug 2, 2007)

dr_lava: Is that circuit tested and known to work? Anybody else tested it to back this up? I've tried to built it in the past and it didn't work and I gave up after my third attempt. I wasn't sure what is the anode end of the voltage reference supposed to connect to :thinking:.

This is where I lost it:


> The current sink, I, which powers the floating 1.23 V reference, is not critical and may be a simple current mirror (sorry to see the TL011 gone!), or even a resistor to ground or any available negative voltage, depending upon the desired current-source voltage-compliance range.


----------



## joniverson (Aug 2, 2007)

Me, too. I'm not sure what that "I" is supposed to be or how it is adjusted. And, once this is determined, I would assume it is to be adjusted as described for the Simple Diode Power Supply at the same site:
*Simple Laser Diode Power Supply*


Best circuit I've found: 


```
In +-------+ Out     R1
  (+) o-----+---| LM317 |--------/\/\-----+-----+------o LD anode
            |   +-------+      18 ohms*   |     |
        C1 _|_ +    | Adjust              |    _|_ C2    __|__
      22uF ---      +---------------------+    --- 1uF   _\_/_
            |  =                                |          |
            |                                   |
  (-) o-----+-----------------------------------+------o LD cathode
```

Note: Resistor value depends on your specific laser diode current requirements. Discussion below assumes a laser diode with a 72 to 100 mA drive range.
Power is 5.5 to 9 VDC. I use a 9 volt battery. 
Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic transistor type case) and the LM317T (TO-220 7805-type case), the pins are, left to right, Adjust-Output-Input. 
For the resistor, I use a small carbon 10 ohm in series with a precision 10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms. 
On initial power on, use three garden variety diodes stacked in series instead of the laser diode. Put a current meter in series with the diode stack and adjust the precision resistor for 50-60 mA. Disconnect power and replace the diode stack with the laser diode. Connect up power again, still watching on the current meter. The diode will probably initially glow dimly. I use a diode that lases at about 72 mA, and has a max rating of 100 mA. I use about 85 mA for normal ops. 
Turn up the current, never exceeding your diode's max limit. The dim glow will increase in intensity, but at some point, a distinctive step in intensity will occur. Your diode is lasing. Remove the current meter as desired. Enjoy!



I'm not sure of the advantage really over the simpler circuit above. 




Jon


----------



## VaThInK (Aug 3, 2007)

joniverson said:


> Me, too. I'm not sure what that "I" is supposed to be or how it is adjusted. And, once this is determined, I would assume it is to be adjusted as described for the Simple Diode Power Supply at the same site:
> *Simple Laser Diode Power Supply*
> 
> 
> ...



The simpler circuit you've mentioned requires a higher wattage resistor for it to be able to withstand higher current compared to the circuit in question. I personally would like to built a current adjustable PSU to drive LD that is capable of producing 2A or more. In this case the simpler circuit would require a HUGE wire wound resistor and a potentiometer cannot be used instead; hence not variable and not versatile. That's the only difference I can think of.


----------



## Gazoo (Aug 3, 2007)

VaThInK said:


> The simpler circuit you've mentioned requires a higher wattage resistor for it to be able to withstand higher current compared to the circuit in question. I personally would like to built a current adjustable PSU to drive LD that is capable of producing 2A or more. In this case the simpler circuit would require a HUGE wire wound resistor and a potentiometer cannot be used instead; hence not variable and not versatile. That's the only difference I can think of.



VaThink,
You can add a transistor to the circuit to handle higher currents. Take a look at the LM317T data sheet. Or search on google for circuits using the LM317. There is tons of information out there using it.


----------



## VaThInK (Aug 4, 2007)

Gazoo said:


> VaThink,
> You can add a transistor to the circuit to handle higher currents. Take a look at the LM317T data sheet. Or search on google for circuits using the LM317. There is tons of information out there using it.



Thanks Gazoo for your tip. I'm actually aware of those circuits. However, I've never seen one that is current adjustable. All the circuits I've seen are only voltage adjustable instead of current or both. Do you have any links for current adjustable ones?

Cheers.


----------



## Gazoo (Aug 5, 2007)

VaThink,
I keep thinking of using the LMT317T as a voltage regulator, it is stuck in my head so I do apologize. I am experimenting with using it as a current regulator. But my question would be this. If we use it as a voltage regulator, would it not in fact be limiting current as well? If we pump more voltage into the diode, we are also pumping more current into it. 

As far as using it as a current regulator, the following link might be of some help. 

http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm

It doesn't appear as though all of the current is going through the resistor. For your application, I think you could get by with a 5 watt resistor and pot. Of course you would also need the higher current version of the LM317. 

Unfortunately, I don't have a diode yet to experiment with. I am waiting for my diodes from SenKat's GB. So I don't have anything that will draw enough of a load from the regulator to test it as I would like to. I am using plain ol' led's and they only draw a few milliamps. My open can diode is mounted in a flashlight in the memedith module, and I don't want to risk killing it. I hope to have the diodes tomorrow and if they perform as well as expected I will be getting a few more.


----------



## Corona (Aug 6, 2007)

You could also build a current regulator around an opamp, voltage reference, and pass transistor. Basically you sense the voltage across (current through) a low value resistor in series with the laser diode, compare that to the reference (divided down to some reasonably low voltage to limit the sense resistor "burden" and dissipation) and adjust the base drive to the pass transistor to affect current control. By "you" I mean "the op amp circuit".

It's a straightforward comparator circuit, the opamp inputs are (or can be) a very high impedance and by adding a capacitor to the voltage reference input, a turn-on slope is provided.

The circuit must be configured such that the sense resistor is in the positive leg (since the DVD diode is common cathode; i.e. the cathode is connected to the device package which is usually grounded to the flashlight body).

This has advantages over other circuits where the sense resistor must drop 1.25V (or more) as the sense voltage is determined by your needs and can be very low (tens of millivolts is a good region to work in with cheap opamps). And the ramp-up is easily implemented using very small capacitances.

If this sounds of interest, I can draw something up and post it - but it's OpAmps 101 and isn't hard to figure out.


----------



## VaThInK (Aug 8, 2007)

Gazoo: I don't think a voltage regulator circuit would act as a current regulator as well. You need both circuits to drive LDs. Thanks for the link mate. Sam's has that circuit as well on his FAQ page though and it also needs big resistor if higher current is desired.

Corona: I'm interested. It would be great if you could post a schematic of it. It's too technical for me as I don't have any background in electronics. It would be hell of a lot easier to understand if I see it visually. I'm sure many of us would benefit from this as well.

Thanks mate.


----------



## Gazoo (Aug 8, 2007)

VaThInK said:


> Gazoo: I don't think a voltage regulator circuit would act as a current regulator as well. You need both circuits to drive LDs. Thanks for the link mate. Sam's has that circuit as well on his FAQ page though and it also needs big resistor if higher current is desired.
> 
> Corona: I'm interested. It would be great if you could post a schematic of it. It's too technical for me as I don't have any background in electronics. It would be hell of a lot easier to understand if I see it visually. I'm sure many of us would benefit from this as well.
> 
> Thanks mate.



Yes, I am finally getting it through my thick skull current regulation is a must with these diodes..lol. Corona, when you can get around to it I would like to see the circuit too. I know op amps can be used for many things. The only experience I ever had with them was in computers many years ago. Thanks.


----------

