Minimag Laser QUestion

amkdeath · Nov 14, 2007

So Im gonna do a 808nm 200mW IR laser from eBay, in a minimag. a tut I read said just hook up the diode to the minimag and ur done, so I was relieved.

Today I had spare time and no life and decided to hook up a DMM.

+lead from minimag--------- + on DMM

- lead from minimag---------- - on DMM

I got 3.something AMPS!!!! HOLY SHIZER!!! that will fry a diode for sure.....
I hooked up a 10ohm in series with the DMM:

+ lead minimag------ ___R____------- + on DMM

- lead minimag -------------------------- - on DMM

I got 280mA ish and a hot resistor.

will 280 fry the diode? will I fry my resistor? is there any other way to fix this?

Thanks:

amk

Dirtbag · Nov 15, 2007

good question, i would like to know this too for my first laser project.

rog8811 · Nov 15, 2007

I don't know about IR LD's as such as I prefer to see my laser output. BUT the same applies to any build you are contemplating....... A proper drive circuit will make a successful build more likely and more satisftying.
I know there is a lot to read to get the information but it is well worth it.....there can be few things worse than seeing your CD burning laser looking like the power on light of a CD player instead.......

You may be lucky with a resistor for a while but again and again threads appear saying I killed my diode what did I do wrong. Shortcuts in this hobby cost money and heartache.

Regards rog8811

knimrod · Nov 15, 2007

You're lucky you didn't blow the fuse on your meter hooking it up directly to the batteries in current measuring mode.. With no load hooked up, and you hooked directly to the batteries, it was like a dead short. An Amp meter is designed to be placed in series with a load for current measurement, not be the load.

With a 10 ohm resistor as a load, it's no surprise you measured 280mA. Ohms law (E=IR) figures like this: 3/10=0.3 ie: 3 volts / 10 ohms = 300mA. The resistor got hot because the power (I2R) figures as so: .280^2 x 10 = .784 Watts.

There is a thread around here somewhere that tells you exactly how to calculate for a current limiting resistor.. You'll have to search for it.

amkdeath · Nov 15, 2007

knimrod said:
There is a thread around here somewhere that tells you exactly how to calculate for a current limiting resistor.. You'll have to search for it.

Ok I think I found a solution. If one hooks up 12ohms resistance, one gets 250mA current in that setup (3/12=.25). The easiest way to do this is to get a 68ohm resistor and a 15 ohm resistor from radioshack, and according to ohms law, when the 68 and the 15 are hooked up in parallel, you get 12.289 ohms of resistance, which gives you 244mA's of current, which is perfect for a diode.

This is how to hook up resistors in parallel for those who dont know:

|---__68 ohm R____|
--| |------
|--___15 ohm R____|

Plus, this will fit quite well in a minimag, for behind the reflector is a lot of space (for 2 resistors)

This is not regulated (im guessing) and will not stop spikes (im guessing) but it still works (I know).

I still recommend DDL's circuit, If you can fit it into a minimag (highly unlikely) (well, you guys may be better at making circuits and compacting them than I am, but for the minimag, im going with the two resistors).

regards,
amk

a_pyro_is · Nov 15, 2007

amkdeath, I must point out that you've forgotten to subtract the forward Voltage drop of the diode. But I must again refer to my post here
Driving a diode this way will never yield you anywhere near as much power OR lifespan as will an inexpensive and easy to build driver will.

If you have any trouble, PM me and I'll do whatever I can to get you going with a pretty new driver.

amkdeath · Nov 15, 2007

a_pyro_is said:
amkdeath, I must point out that you've forgotten to subtract the forward Voltage drop of the diode. But I must again refer to my post here
Driving a diode this way will never yield you anywhere near as much power OR lifespan as will an inexpensive and easy to build driver will.

If you have any trouble, PM me and I'll do whatever I can to get you going with a pretty new driver.

Thank you for the link, yet I know how to build a driver. The thing is, the driver would NEVER fit in a minimag, and im broke right now cant afford drocies and stuff.

The driver I built will do good in my other laser, which involves senkat diode and radioshack enclosure.

For this one though, I want it to be limited. is there any way to calculate initial voltage drop of diode? When I used DMM with minimag with minimag bulb inside, I got a good 2.8 volts, running on like 3 amps. without the bulb 3.0 volts on 3 amps. So, what do I have to do so that I DONT have to fit a driver in? I donno I guess I have room for a couple more resistors, mabe a cap and a recitifer, but no space for an LM317T.

Rhith · Nov 15, 2007

Building your own driver to fit in a standard flashlight will be a bit difficult unless it already has a driver in it. I managed to build one for my Dorcy Metal gear but it was still somewhat of a hefty size.

Gazoo · Nov 15, 2007

At the bare minimum, using a 2.2 ohm 1/2 watt resistor in series with the diode will give you around 160ma's using alkaline batteries. You will still need to solder a capacitor across the diode...I am using 10uf 16 volt capacitors for portables.

Keep in mind if you decide to use nimh batteries the power will drop considerably. If you decide to use E2 lithiums the power will increase considerably.

If I were going to use a minimag, I would use an AMC7135, a protected li-ion battery and a dummy cell.

a_pyro_is · Nov 15, 2007

A TO-220 LM317 can be cut down to a very small size.
First grind off most of the tab on top leaving a little bump you can solder to, and trim up the plastic case a bit.
Next solder one lead of eash of two 10 Ohm resistors to the little bump you left.
Now solder the other lead of each resistor as well as a few inches of wire to the Adj pin.
Solder another color wire to the Vin pin.

I've not vet finished, but I believe I can fit the 317, two 10 Ohm resistors, a reverse polarity protection diode, a .1uF metal-film cap, and a 47uf Tantalum cap all inside the Aixiz module.

I'll post pics if I get the time to make it work/fit. But I'll prob end up using the AMC7135 for most apps from now on. It's just too easy.
I also want to track down some N size NiMH cells. I wonder if a minimag could be made to hold 4 N cells, and a 7135? ;D

*Edit*
Gazoo, do you find that the 7135 drops out at 3.4V? That's been my problem, I'd love to use a 3.7 Li or 3 cell NiMH for 3.6, but it leaves a little too much power in the battery.

Gazoo · Nov 15, 2007

Mine drops out at 3.2 volts. If you are going to use 4 cells, you will need to find a way to heat sink it as I believe it will get too hot especially if it is enclosed. Of course the alternative would be to use a resistor between the batteries and the 7135 to cut down the voltage a bit..

Another thing you could try would be to solder the + connection on the other side of the polarity protection diode. I know it will run with lower voltages...at least it did for me, but below 3.2 volts the current will begin to drop.

a_pyro_is · Nov 15, 2007

3.2 V sounds OK, maybe my traces are putting too much resistance in the circuit. I'll keep playing with it.

amkdeath, sorry for the mini threadjack. :-[ I hope it's providing you with some info you might be able to use.

amkdeath · Nov 15, 2007

a_pyro_is said:
3.2 V sounds OK, maybe my traces are putting too much resistance in the circuit. I'll keep playing with it.

amkdeath, sorry for the mini threadjack. :-[ I hope it's providing you with some info you might be able to use.

Its quite alright, I dont mind, in fact I learn things from these posts.
If you get the aixiz module and the driver to work plz post pics.

Ill see into the LM317T modification, I would set that up like a normal driver right? or can I leave out the recitifer and some other parts so I end up with somehting smaller? plz help. If I get it small enough, I could totally remove the reflector on the minimag leaving room for small driver, but then I would need a push button switch on the bottom of the flashlight, as its the reflector that does the on off things in the minimag. No reflector = On forever.
Can someone show me how to custom mod the tailcap of the minimag into a push switch? I dont want to buy premade switches.

BTW, you can still see 808nm right?

thanks,

amk

a_pyro_is · Nov 15, 2007

I've seen a momentary tailcap mod on a minimag built by
removing the spring
drilling a hole in the tail cap ~1/4"
gluing the spring onto the head of a nylon bolt
gluing a metal washer followed by a neoprene washer onto the shank of the nylon bolt
soldering a wire between the metal washer and the spring
drilling two sets of holes transversely through the tailcap about 1/3 the diameter away from the outside edge ~1/16"
placing the shank of the bolt through the hole in the tailcap from the inside
placing two lengths of ~1/16" Al rod through the two sets of holes drilled transversely
soldering the Al rods in place.

Now the circuit is broken until you press on the shank of the nylon bolt which raises the metal washer to touch the Al rods completing the circuit.

BTW 808nm is barely visible

amkdeath · Nov 16, 2007

a_pyro_is said:
I've seen a momentary tailcap mod on a minimag built by
removing the spring
drilling a hole in the tail cap ~1/4"
gluing the spring onto the head of a nylon bolt
gluing a metal washer followed by a neoprene washer onto the shank of the nylon bolt
soldering a wire between the metal washer and the spring
drilling two sets of holes transversely through the tailcap about 1/3 the diameter away from the outside edge ~1/16"
placing the shank of the bolt through the hole in the tailcap from the inside
placing two lengths of ~1/16" Al rod through the two sets of holes drilled transversely
soldering the Al rods in place.

Now the circuit is broken until you press on the shank of the nylon bolt which raises the metal washer to touch the Al rods completing the circuit.

BTW 808nm is barely visible

sorry for n00b question, searched on google found nothing:
what is:
The shank of a screw?
al rods?

looks good.

amkdeath · Nov 16, 2007

I was bored so I am posting pics of my radioshack enclosed driver, to be driver for a SenKat diode (when I get it)

Minimag Laser QUestion

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