Learning electronics: Release the Magic Blue Smoke

The Something Awful Forums > Discussion > Hobbies, Crafts, & Houses > Learning electronics: Release the Magic Blue Smoke

Jayls5: Mar 5, 2007; by angerbeet

mtwieg recommended I post here.

I build lasers, and I need help making a circuit for a laser diode.

I've built circuits that drop voltage down, but now I want one that steps voltages up. This is for the better battery options... like a single AAA 1.5V source, etc.

Now, I have no idea how to go about making a reliable circuit that does this. I need it to put out 4.5-5.5V (5V preferably) and 35-45mA current to this diode. If it's possible to vary these precisely with a resistor swap (or pot), let me know.

Is there a way to adjust current/voltage output on these?

If I'm missing any pivotal information, let me know as well.

# ¿ Jan 28, 2008 02:33

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# ¿ May 3, 2024 16:21

Jayls5: Mar 5, 2007; by angerbeet

mtwieg posted:

What you want is called a Boost converter (sometimes called a step up regulator), which can take voltages a low as 0.5 volts and turn it into something higher. They're pretty efficient (usually over 80%, sometimes as high as 95%), but they're slightly tricky to use. Also, you won't find them in your local radio shack, so you'll most likely have to order them online.

Generally the voltage is set the same way as an lm317: a resistor divider from output to ground, with the divided voltage being held at some reference voltage.

Right, I've researched it before. I've read the article you posted and a few others. I still haven't been able to find a specific circuit design of the type I need. That's my main issue. Lots of LED flashlights use these step up regulators, but they put out way too much current for my application. That's my dilemma

# ¿ Jan 28, 2008 03:35

Jayls5: Mar 5, 2007; by angerbeet

mtwieg posted:

Your circuit would look something like this:

Your output could be adjusted via any of the three resistors. The exact design and component values will depend on your boost converter (datasheets typically tell you exactly what you need to know).

When you say they give to much current out, I think you're misunderstanding what these things do. They regulate voltage, not current (though most will limit the current output). You control current by voltage and the load impedance at that voltage.

Well, like I said, I'm not that great at this stuff yet. I have a few questions...

Will something (such as a laser diode) only pull a specific current at a certain voltage? Is that why a steady voltage source implicitly acts as a current regulator? I was trying to tell my friend this originally if that's the case.

Now, I am concerned about the properties of a diode itself. It is my understanding that they get "runaway" resistance. As temperature increases, resistance decreases on the diode... further increasing the current and heat to it as a result. This quickly results in diode death. Now, on these boost converters, the circuit will keep this from happening?

Part of the picture in your link is cut off on both sides, do you have the rest of it? I really appreciate the help.

# ¿ Jan 28, 2008 18:40

Jayls5: Mar 5, 2007; by angerbeet

mtwieg posted:

Diodes aren't characterized by resistance. They have I vs V characteristics which generally look like this:

Usually you will operate it at around the "turn on" voltage, where it starts looking like a low impedance. Just putting a resistor in series with it of appropriate size will usually work fine (as in the schematic I posted). As for current regulation, it most likely isn't necessary. I highly doubt the properties of the diode will change enough to significantly change its power output.

Do you have a datasheet for the diode?

I think this is the same one I have:
http://www.photonics.uk.com/products/sanyo_violet_laser_diodes/dl-3146-151.pdf

# ¿ Jan 29, 2008 06:23

Jayls5: Mar 5, 2007; by angerbeet

mtwieg posted:

Everything seems fine, then (though parts of the pdf won't display). The circuit I showed should be fine. The best way to regulate current would be with the series resistor. I doubt the diode's heat will have enough positive feedback to cause it to fail.

If you really want to, you could use a circuit that makes a current source out of your voltage source. A simple one would be as follows:

A normal red LED provides a stable voltage reference of around 1.5 volts (this may vary from LED to LED, but the point is it remains pretty much constant regardless of the load on the circuit). The current through the laser diode will be (practically) equal to the current through R2. The voltage across R2 will be the Voltage of the red LED minus the base emitter voltage of the transistor (0.6 volts). To sum it up, the laser LED current is equal to (Vd - Vbe)/R2, or 0.8/R2. So you can easily adjust R2 to change current (just make sure it can't go to zero).

You will still need to boost the voltage, since at the very minimum the LED needs 3 or four volts. Supplying this with five volts from a boost converter should be fine.

The one downside of this is that it requires some extra current in order to get that 1.5V across the red LED. There are more efficient ways to do it, but they're fairly complex and aren't worth the trouble.

Also, did you ever simple consider using a higher voltage batter?

I forgot to check this thread for a while.

I have been using a higher voltage battery from the start (9V). I was merely interested in shrinking the size of the device. 9V batteries are too large for an aesthetically pleasing laser pointer, and they're far more expensive... $2.50 a piece.

The blu-ray laser takes a bit more voltage to lase. It needs 5-5.5v just to run. I'm not sure exactly what I'm going to do for a cheap practical driver that uses 1.5V AAA just yet, but I definitely appreciate the input.

# ¿ Feb 8, 2008 04:06

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