Idea: easily adjustable current for a small build

[Pontiacg5]

If I read that thread correctly, he used the flashlights boost driver to get the voltage high enough to use a linear driver.

I agree, I think we should wait for some more experience to shed some light. I know I read somewhere that it was a bad idea though...

 

[Lumify]

So.... putting a second pot with the driver is bad? Btw, when you say I could put the 2nd pot "in series with" the driver, does that mean [battery]--[2nd pot]--[driver]--[diode] or does it mean [battery]--[driver]--[2nd pot]--[diode]?

@Benm all I know of the circuitry of the microdrive is what I've gathered from this: http://hacylon.case.edu/ebay/laser_diode/Micro Boost Drive V1 manual.pdf

A pot is basically an adjustable resistor, so would a pot be better for the job just a resistor? And would I slaughter a poor defenseless diode in the process? BTW, there's no way I'm implementing this into my new BDR-205 build without testing it on a clunker first, or at least making sure it's stable. :undecided:

 

[Pontiacg5]

The way that is being suggested is a resistor in parallel with the pot already on the driver, that way if the pot's resistance is turned down too much the resistor will take over and keep the diode at its safest maximum level.

Not 100% sure yet, but putting a pot between the diode and driver in series (meaning driver---pot---diode) might kill the diode.

I don't know the beat way to go about it, but I think that putting a resistor in parallel with the pot is the beat idea.

 

[aXit]

If it was a boost driver (the constant current kind) it would just up it's output voltage to maintain the constant current, putting a resistor in series would have no effect; except for going out of the possible output range. You'd have to ask drlava about the function of his particular driver. As for a linear, if it was a voltage source, it would work in decreasing the current in the series circuit, so it would reduce the brightness; if it was a constant current, again, it would compensate by raising its voltage to maintain a constant current. If that voltage exceeded the max output of say, a LM317 driver, (~2.25V below input) then it would start dimming due to the reason above.

As far as a pot damaging a diode, TTL has nothing to do with it at all. The diodes we use can easily deal with pulses in the 10s of kHz; the issue is the cap that is in the DDL circuit, if the wiper lifts off, the cap can be charged to a higher voltage, and when the wiper makes contact again, let all that current through the diode, possibly killing it. A real driver with modulation wouldn't have any output capacitors.

Of course this depends on the location of the capacitor itself, if it was in between the pot and the diode, there wouldn't be any problem with the pot.

Keep in mind, this is just as a possibility, placing a pot in series with something is far from an efficient or effective way of lowering power to a device.

 

[bobhaha]

Awesome... thanks for clearing that up.

btw what I meant to say was, in TTL modulation, the diode undergoes a series of off/on pulses.. similar to what I thought Pontiacg5 was talking about. But I understand what you mean now.

So would that mean you can still do it as long as you have a cap in series with the second pot? If so isn't that an easier fix then what was mentioned above? I would guess so.

 

[Benm]

If i'm looking at the datasheet correctly, it already has a limited adjustable range, which you can preset using solder bridges.

Most logical would seem to desolder the pot off the board, and wire a larger pot of equal value in its place, such that you can make it accessible in the host.

You may be able to extend or decrease the control range by tinkering with the resistors on the board or adding one in series or paralel with the pot, but i'd have to see a real schematic to give a definite answer on that. It might be possible to reverse engineer the schematic from the actual driver, but i can't do it from just the pictures in the pdf.

A real driver with modulation wouldn't have any output capacitors.

Not usually indeed - but with the LM317 you don't have much of a choice since the output cap is more or less required for stable operation. Drivers that have modulation usually have opamp based current sources that are stable without output caps, and fast enough to keep current within limits even when reconnecting the diode when its on. They would be a bit bigger then this little driver though

 

[Lumify]

There's no limit to what I could do with a few resistors, a dummy load, and a multimeter. I have the last 2, but I don't know what kind of resistors I would need. I'll probably try messing around with the MicroBoost drive once I get my hands on some resistors. If a pot is just an adjustable resistor, then couldn't I just put a second pot in parallel with the original pot? That way I could just set the first pot, and frequently adjust the new one.

Are you guys sure that putting a resistor parallel to the pot would limit the current to whichever has the highest resistance?
battery--<==(line 1: driver; line 2: pot)==>--diode
Wouldn't the current prefer the path of least resistance, and thus go through the path that would result in the highest current? I think I would have to put a second pot in series with the driver like this: battery--driver--2nd pot--diode. With that setup, the current after the driver would be the max (ignoring inefficiencies), and I could trim that max current down with the second pot.

It may just be easier (and more efficient) to get a different driver. Flexdrives have a similar limited adjustable range set by solder bridges. http://hacylon.case.edu/ebay/laser_diode/MicroFlexDrive_V5_manual.pdf In that manual, it shows two preset ranges I might use through different solder bridge combinations. I'd probably use either the 100-312 mA preset or the 108-412 mA preset. The first I could keep at max all the time without worrying about it, and the second I would use the max sparingly. It would be ideal to have one with a max of about 350-380 mA. Does anyone know of or sell any like that?

Also, for my build, I'm using a BDR-205 diode. Does anyone have any idea what the output power is at extremely low currents?

 

[rkcstr]

Here's an Excel sheet I made for my driver, but should work for any linear driver or really any circuit that sets current based on a reference voltage across a resistor. You can play around with values of resistors and pots to see the current ranges they'll give you.

http://sites.google.com/site/rkcstr/Home/potoutput.xls?attredirects=0&d=1

You can modify the reference voltage up top, but it should be the same for LM317, 1117 or other linear type ICs.

The pot and R2 are in series while R1 is assumed in parallel with the pot, so if you're just doing a resistor and pot, use R2, not R1. Just leave R1 blank.

There are two calculations to compare different settings, you don't have to enter values for both, though. The graph will show the output current over the total percentage of pot turn travel. The end points of the graphs (min/max current) are shown and the maximum power dissipated by the part is calculated below each.

 

[bloompyle]

I'm not entirely sure, but touching the case of the pot to the case of the laser would short the circuit out wouldn't it? The laser wouldn't work at all if any metal of the screwdriver was touching the case.

At least I'm pretty sure it wouldn't, can anybody fill me in who knows for sure?

You could get one of those rubber lip things, like for instance those double sided door wind blocker things, these. The things you slide under the door with thin foam cylinders on each side, that keep heat inside your house, and wind out. You could get something like that made out of rubber, that would have little lips on each side to keep it in place, and that would go around the hole and would keep the screwdriver from touching the case and short ciruiting everything.

 

[Pontiacg5]

How about a high-low switch. I can tell you how to do that! One side would be, say 50mW the other side would be 100mW. That would be really easy, you could even get a switch with more positions and get more power settings.

 

[Lumify]

I've actually seen one like that. SKYlaser Power Adjustable Laser Review - SlipperyBrick.com For my build, it would probably be more like 50mW and 500mw though. Yay 12x blu-ray! The most I would really want would be like 4 positions. I'd probably set them to 10mW, 50mW, 200mW, 500mW. I wonder if I could do maybe 2 switches, and those 2 switches would determine the power that it is switched to. The 4 combinations would be: Off/Off, Off/On, On/Off, On/On. I'm not sure how I would actually physically do this, but possibly you would know how to, Pontiac. Or maybe having 2 switches is sort of pushing it. I'd still love to do a 50mW/500mW thing with this. How would I do that?

I haven't thrown out the knob/max current idea, I'm just looking at all of the options.

 

[Pontiacg5]

Well, you could get a 3 position switch. You could have 10-200-500 that way.

What you would do, or what I would do, it might not be the beat way, is...

set the pot on the driver for the current that gives you 500mW. Measure the resistance of the pot and get a resistor of the same size.
Reset the pot for the current that gives you 200mW, once again measure the pots resistance and get a resistor to match that resistance.
Do the same for 10 mW...


Then, you need to remove the pot from the board, and solder a lead where the power comes to the pot, it might be one of two places. A pot has 3 pins, one is the wiper, and that is the pin that changes resistance as you rotate the pot, the other two are the inputs, the resistance between those two pins are the max resistance that the pot has. (something like that, it makes sense and works in my head!) I'm betting that only two of the three leads of the pot are actually used, so you need to find the one that is not the wiper (usually the wiper is the middle pin) and solder the lead on there.


Now, you have a lead soldered where the pot used to connect. Now you need to solder a lead where the wiper of the pot used to be. After that, you just do this...

When the switch is in the middle position, the 200mW resistor will be used, and you will have 200mW and the same for the other two positions! Some switches are wired a little different, but you should be able to figure it out!

One issue though, Diodes have a threshold current. A 12X diode might not lase at 10mW, the current might be too low. I haven't searched or checked if it will or not, but I know diodes do have a threshold current

Take this post with a grain of salt, I'm no master electrician, some info might be a little wrong!

 

[Benm]

The above approach would work if the driver used ony one half of the pot, i.e. connects only to the wiper and one of its ends. Some drivers connect to the wiper and both ends of the pot, in which case you need sets of 2 resistors for each power setting.

As for the threshold current: All laser diodes have that, so you need to adjust current to actual power output. It also depends on temperature a bit, so its going to be very tricky to get 10 mW out of a 12X bluray diode reliably.

 

[Pontiacg5]

Well, if the driver board uses both ends of the pot then you only need a resistor that is the same rating as the pot, and you would only need one.

Right? I don't think its possible to use both sides of the pot at the same time is it? The resistance would just add up to the max of the pot. Correct me if I'm wrong!

 

[Benm]

Its perfectly possible, i do it all the time with drivers. One end goes to ground, the other to a reference, and the wiper allows you to set the current. You would need 2 fixed value resistors in the proper proportions to replace the pot in such a case, which is still feasible using a switch.

 

[Lumify]

I have no idea how diode drivers work at all, so I'm just going to sit here until you two figure this out... Also, the more I look at the size of this driver, the more I wonder how I'm going to do all of this.
If the power runs through both of the pins of the pot, then is the power divided by current? So if the total current before the pot is for example 300mA, then would it be 150mA from each pin?

I just found a video of someone that did this! YouTube - Variable blu-ray laser control They used the first idea, with some sort of knob. The switch thing sounds great, but I think I'd rather go with a knob. It would be sort of easy to do this, wouldn't it be?

To set a max current that I could adjust a pot up to, I would put a resistor in series after the driver. To find the size of the resistor I'd need, I'd simply set the pot to the max output current that I would like to have for the diode, then measure the resistance. We'll call that resistance A. Then I'd measure the driver's minimum resistance, and that would be resistance B. A-B=resistor

That's what I think I'm going to do, but I'd love to see how the other approach would work.