Linear current regulator help please!

[Attemp2Learn]

Hi all, im new here and just a hobbyist trying to learn.

I have been playing with visible laser diodes for the past 2 years now, and am finding myself needing higher currents.

What I need is a constant current source capable of 0 amps (or close to it) up to no more than 1.2A. adjustable through the larger type pots used as volume knobs.. which can provide a SMOOTH sweep through the range of the pot.

The voltages can vary, but in order for the laser to fire I need at least 4.5v, or the laser will see just the current and produce no light.

I have made several circuits with the notorious lm317 / 350 etc, but all the heat seems to trap in the pot. This is an issue for me as I also require a circuit that takes the load away from the pot adjusting the current.


I also tried switching with a 555 and irf510 setup, but could only get the laser to operate under control from .5A to 1A. I am almost there but have hit a wall in two regards.

I have been able to make an adjustable 'driver' that allows control from 0-300mA and in this sense anything over 300mA jumps to the full output as the pot fails. Ive tried combos of resistors etc and am stuck.

I have to avoid any pulsed type regulation as the laser diodes involved do not like spikes and anomalies.

Anything pulse based directly affects the low end output, and would really like to go with a linear current regulator.

The load to be used with this is a visible laser diode requiring minimum of 4.5v 200mA threshold, and 1.2A max current.

Any help would be greatly appreciated, and thank you in advance.


Forgot to mention, I am still somewhat of a n00b, so please keep this in mind

 

[MRCecil]

What I need is a constant current source capable of 0 amps (or close to it) up to no more than 1.2A. adjustable through the larger type pots used as volume knobs.. which can provide a SMOOTH sweep through the range of the pot.

Are you using the pot as a preset for the max current before you trigger the diode, or are you cranking the pot to get the trigger? I am a little unclear on this. Could you post schematics of how exactly your circuit is laid out? That would be most helpful.

 

[Attemp2Learn]

Thanks for the quick response...

No, I am trying to set this up so I can adjust the power on the fly. I have not drawn anything up in regard to schematics, just been trying everything on my breadboard.

I have tried a lamp dimmer circuit from the old radioshack 'mini engineers handbook, 555..) but again these type circuits dont work on the low end and can prove fatal to rather expensive LD's.

The circuit tried with the irf510 was in this link > Engineer's Mini-Notebook - 555 Timer IC Circuits
Scroll down to the project labeled 'Power fet lamp dimmer'

At this point im up to try anything that will achieve the desired results..

 

[MRCecil]

OK, I'll take a look at it tomorrow and get back to you, or someone else might in the interum. It's beer thirty right now...

 

[Attemp2Learn]

Thats cool, ill look at some more threads in the meantime just in case my original search missed something.

Just to be clear though, im not looking to improve or necessarily use the circuit in that link.. If someone knows how to get a kitchen sink to work in my application, im open to it.

Power consumption, heat, and size are not an issue. This will be a controller for my bench.

ALL ideas welcome

 

[ronv]

current regulato

I think this might work, but have some other reviews. I made some assumptions that you had a 12 volt supply.

 

[Attemp2Learn]

LOL, your assumption would be correct. 12v is the common voltage I use, should have mentioned that.

Now seeing as im a n00b with circuits, and have to LQQK very long and hard to understand, where does the pot go in that schem?

Also it says 3.96v @ 1.2A. I'm thinking that this will not deliver the 4.5v required? or is there something I may change to up the voltage a bit?

The problem with laser diodes is that you can supply all the current needed, BUT, if the voltage is even shy by 1/2 volt, the diode will get hot (as it eats the current) and not lase, ie no light.

5v would be the safest voltage and that's under load of course. As I said before, the voltage wandering is not an issue, that is as long as it doesn't drop lower than 4.5v, 5v being a safe bet

The LM358 is actually quite common in laser assemblies that I have got from china. They are commonly used to provide TTL blanking in laser projectors which controls the on/off sequence when running more than one color.

So there is a good chance that this would work, just have to get the output voltage a tad higher, and figure out where the pot would be wired in.. as embarrassing as that is, I cant see where it hooks into the circuit

Thanks for the input though, I have a good feeling about this one.

 

[ronv]

current regulator

Maybe we should go through the circuit. To kind of get an idea it is like your regulator circuits with an amplifier for the pot. Lets start with the .5 ohm resistor. It converts the current thru the diode, 2N3055, the .5 ohm to ground. The voltage across the resistor is .5volt/amp (E=IR). The voltage across the resistor is measured and amplified 6.6 times by the LM358 at the top right. So now we have the sense resistor .5ohm x 1.2amps x 6.6 = 3.96 volts. This is where the notation came from. The circuit doesn't know what the voltage across the diode is, only the current thru it. Now down to the next amplifier (358). The 100K is now your pot. It is used to set a voltage to compare to the amplified voltage across the sense resistor. You can set this voltage anywhere between 0 and about 4 volts based on the zener diode reference and the voltage divider of 27K and 100K. This translates to 0 to 1.2 amps. The rest of the circuit is a voltage regulator. If the current thru the sense resistor is less than what you have set with the pot the output of the amplifier goes positve turning on the transistors harder until the voltage (current) is the the same as your set point. Let me know if this is to many ic's. Maybe we can come up with an easier way.
As a side note: Does your laser have a photo diode? This may be a better way to control it. Spec. Sheet? Is the 12 volts regulated?

 

[Attemp2Learn]

Thanks for the run down on this circuit!! It is explanations like this that will most certainly aid in my learning process

In regard to the multiple IC's, I would prefer using a single IC if possible. If it is too much trouble though, don't worry about it.

This is the spec sheet for the new LD that is requiring such current... > https://www.electro-tech-online.com/custompdfs/2010/06/NDB7352-E.pdf

It does not have any optical feedback, and is more of a brute than anything. The stability of output is more dependent on operating temp, and the wavelength wandering by a few nm +/-
is not of much concern for me.

My biggest concern is the ability to control the current to it, as smoothly as possible. I am also trying to avoid using multi-turn pots, making the adjustments in a single turn.

Also, as the LD heats up, its resistance changes. So with heat comes less resistance. This is where the current regulation is a must. Example, if I was to run this LD with only resistors to control the amount of current it sees, as the LD warms up, the current reaching it will increase. This isnt too much of an issue as I am running these withing the safe upper limits at max. I have tested these and noted that COD occurs at approx 1.8A and near instant death @ 2A.

With 1.2A being the upper limit, a little wandering wont hurt.

 

[ronv]

Laser Driver

There is a simple one at this link under constant current supply for high power laser. It uses an off the shelf regulator and a votage reference. Should work.

Sam's Laser FAQ - Diode Laser Power Supplies

 

[MRCecil]

I took a look at the Mims/Radio Shack booklet you provided, and your experiments prove it will not work, so enough said.

I quickly put together a simulation of what I thought you were asking for within the limits of the diode. I saw later that you provided the datasheet and the sim does appear to fit within the operating parameters.

In the attachments, are the schematic of the sim and graphs of the Iout and Vout; the lower is the voltage across the load (diode) and the upper is the current through the load.

Note that R1 is 1.2 ohms to set the current limit at ~1.0A. To change that limit use this formula:
R1=1.25V/I where I is the desired current limit. Remember that you must select that resistor with the proper power rating based upon the greatest duty cycle you will be using. To be absolutely safe, a 5W non-inductive wire-wound would be the choice if the current limit was 1A in all cases; 1A being the columnating spec in the datasheet.

Also note that R3, 5 ohms, is the load simulating the diode. Further note that the base drive to Q1 is up to you given the lack of information of the driving circuit provided. The schematic provides a general principle only, and is set at a 50% duty cycle of 1hz, BUT IS definitely variable within limits.

Before I forget, be sure to include C1, the 1uf cap, which prevents V & I overshoot, to protect the diode. I would use a 16V 1uf tantalum with a 12V supply.

I hope this helps. If I have left something out or you have further question, let me know.

Good Luck!
Merv

 

[Attemp2Learn]

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[MRCecil]

Originally Posted by Attemp2Learn
I tried posting this in thread, but it wont allow, something about expired toke.. I will try again in a bit.


For @#$% sake!, I typed a long post and get a forum error... Guess this forum type doesnt save to clipboard..

Thanks for both replies, I am leaning toward the 317 based one, as I have all the parts already for that board. Also if I etch some boards
for individual use with lab type laser mounts, I can avoid costly variacs...

Two questions though, is there a substitute for the Q1 in that schem? for the DH4511? Im still learning the types of parts used and suitable alternatives...
I tried searching it and saw something listing a 7805, would this work in lieu?

Also, as embarrassing as it is, where does the pot go in this one? All the lettering in that drawing is throwing me off a bit, but got most of it.

An expired token only means you have timed out on the current time allowed for a page. If in doubt, copy the post before hitting reply. If the error message comes up, hit renew on your browser and paste the message when appropriate.

The DH4511 can be replaced by the more common MJ2955, the PNP compliment of the NPN 2N3055, OR others capable of the current requirements. I used that device in the sim because I don't have the MJ2955 in my library yet. In either case, I would recommend using the TO-220 case for ease of construction if you choose that.

About the pot, you have not provided much information regarding the methods you have tried to employ, with only references to problems with pots. That is why I noted in my post that the triggering of the pass transistor (DH4511) was up to you. I can help you with that, but I need more information about what you are trying to accomplish, such as duty cycle, Ton, Toff, etc, and any range of those parameters you have in mind. It is hard to breath in a vacuum...LOL!

The "lettering", to which you refer, are the different parameters for the type of simulations I ran and those for the supplies and the switch. They can be confusing for one not familiar with LTSpice, so don't let that bother you at all.

I think that covers the points of your PM. In the future, could you please use the forum to reply. I know you were confused this time by the token notice.

Sorry I was late in my response today as promised, but I was having too good a time on the Thompson River here in the Montana Rockie's western slope, and got back to camp late. Gonna hate going home in October!

One last thing...be sure that you always use the proper eye protection when operating a lazer of that power. The sight of a sunrise or sunset are too beautiful to never be seen again.

Cheers,
Merv

 

[Attemp2Learn]

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[Attemp2Learn]

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[Attemp2Learn]

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