LED DC boost circuit

[Johnny147]

Hello,
I just discovered these dc step up boards rated :
3-5 v in
3-15 v out
1.2 a max

I'd like to use them to drive 6~8v diodes at 100~200mA from a single 4.2v li-on cell.

Do you think that tiny pot can be precise enough to set 100ma ?
Or is there major risk to fry the diode because of the pot 4-16v being too coarse ?

May-be I should try with a resistor first ? What value ?

Thanks !

Greetings

Board DD 06 ajsb

 

[FireMyLaser]

That is a constant voltage source. You need constant current.

 

[Johnny147]

Current will spike when switching On-Off ?

In my actual understanding, constant voltage would make constant current.
Like cranking up the voltage until desired current flows.

 

[FireMyLaser]

Laser diodes (and LEDs) becomes more conductive as they heat up, which mean that current draw will increase.
Is an unstable feedback loop with can destroy the diode. Even if you were to keep the temperature of the diode absolutely fixed, a change in the voltage to the diode by just a few millivolts will cause a huge change in current. You may get away with setting the constant voltage to around 8-9V and use a resistor to limit the current, but this is considered poor practice when driving lasers. Do it at your own risk.

 

[Buffo]

There are drivers available that can power laser diodes with high forward voltage requirements from lower voltage DC sources. These drivers include a boost circuit to raise the voltage to the proper range. I know DTR's shop sells them, but I'm sure you can find other suppliers too.

If you don't want to use a driver like that, you could use a standard driver (or even build your own using an LM-317 configured as a constant-current driver) and still use the DC-DC converter in your original post to boost the single-cell LiPo voltage up to the proper range for your diode.

But yeah, there is a *huge* difference between a constant voltage driver and a constant current driver. Constant current is what you need. Constant voltage will make you a sad panda, as it will destroy your diode in short order.

Adam

 

[Johnny147]

Now I understand. Thanks ! I will use them with LM-317 as constant current driver.
You saved me from frying precious diodes

 

[Johnny147]

Is this simple design okay ?
Won't there be too much current inside the 0.15W potentiometer ? Because a 4w-pot is very large

Thanks!
(I'm waiting for my components)

 

[FireMyLaser]

10 ohm will give you 125mA max, not 150mA. And if you know that you want a fixed set current, then you don't need the pot (which would be fine at this current anyway). 125mA will likely suit your diode fine. Also, the LM317 will require about 4V to regulate constant current, so the boost need to be set to at least 10V if your diode really is 6.0V. You'll know if LM317 is regulating if you measure 1.25V between adjust and out pins. Lastly, it is good practice to put a 100nF-10μF capacitor across the pins of the laser diode.

 

[Encap]

LM338 for the few penny's more is more robust than LM317 and can handle up to 5A --good if you ever want to use the drive with a higher output diode .

Maybe this will help--good detail of adjustabile constant current source made with LM317

 

[FireMyLaser]

Both are inefficient linear regulators, and I really don't see the practicality to use either of these over 1A in a pointer as they make so much waste heat. So don't bother getting LM338 unless you really need it.

 

[Encap]

Both are inefficient linear regulators, and I really don't see the practicality to use either of these over 1A in a pointer as they make so much waste heat. So don't bother getting LM338 unless you really need it.

They both are bad waste heat wise, true, also the LM350 3A

 

[FireMyLaser]

They are essentially the same, but with different max amperage. But at a practical limit of ~1A, that makes LM338 and LM350 obsolete for our purposes. You may still use them, but I wouldn't pay more for them.

 

[Johnny147]

Thanks ! I'll add a cap and use 125mA for low power cyan/green 6v diodes.

Now if I want to drive a red 3v diode at 700mA
R = 1.25/I -> 1.78=1.25/0,7
Then I use 1.8 ohm, and the boost at 8v ?
Big 5w resistor is needed ?

3v / 1.8 ohm =1.66A -> x 3v = 5W

I think I fryied a small pot doing this in the past

 

[FireMyLaser]

Resistor power is calculated 1.25v x I, so 0.875W, round up to 1W. In my experience any 1/4W pot has been fine up to 1A as long as it is used with the calculated series resistor (but it is pushing it). A pot by itself will be in trouble.

Again, a pot is not necessary if you don't intend to have the current adjustable for a specific reason. Best is just calculate what resistor you need and just use that.

 

[Cyparagon]

150mA is just so low... If it were me, I'd probably use the constant voltage boost plus a simple current limiting resistor to prevent thermal runaway. 3.3-10 ohm 1/4-watt would be fine.

 

[FireMyLaser]

I told early on that this is poor practice and risky. He may fry the diode at setup if something is unaccounted for.