Is this a logical way to setup 2x LM317 in a driver?

[Benm]

I dont see why either chip would not work. Their function is equal to the lm317, but with a lower forward drop or higher current capability.

 

[Leodahsan]

I'm using lm337 with heatsinking.. works perfecly.

I'm not really into electronics, someone could help with this guy question:
Why not use the Current regulator to drive a pass transistor. That way you can pick a transistor that will handle whatever current your supply can handle and your load might demand?

maybe with a voltage regulator configuration that could be doable?

 

[rhd]

To be honest, I don't see how that would be a benefit (even though I don't really understand his suggestion anyway).

But if you're already using a current regulator, and just adding another component, I'm having trouble seeing how there could be a benefit in terms of voltage drop, or efficiency. And obviously there's an additional level of cost / complexity.

 

[Leodahsan]

Using a LM1117 (1A max) you can use a 2n3055 and give the diode 15amps. lower voltage drop, right?

 

[anselm]

Of course that would be doable. Use the lm317 to drive a transistor, for example a MOSFET.
You are just adding another stage in regulation, using the transistor as an "amplifier" instead of
driving the diode directly off the lm317.
There are many, many ways to drive a laser diode.
The lm317 apparently is one of the most simple ways.

Until recently, the 1.5A limit was enough to drive any diode of visible wavelength.
Sure, powerful and current hungry IR diodes have been around for a while, but
not many of us use them.

Using a transistor in this way might be interesting to power arrays of diodes.

 

[rhd]

Can I test whether I understand this concept correctly? (if I do - this sounds neat!)

Basically, you would be using the LM317 to set the level of current, but not actually driving the diode THROUGH the LM317.

Instead, you would use it to control a transistor. The transistor would get a parallel feed from the same power source. This way, the only voltage drop affecting the diode's circuit, would be the voltage drop of the transistor.

Is that at all correct?

 

[Cyparagon]

Has any one hare tryed a LM 350 or a LM1117 I think they may not work as a current regulator !

Why do you say that?

About driving a transistor with a regulator: Why do it in 2 or more chips when you can do it in 1?

 

[Leodahsan]

^for near no voltage drop... or very lower in 1117 case. Here in my country, the only regulators being selled are the 317 and 337. Never asked 1117.

 

[123splat]

Hopefully to clear up the issue for some: The idea of the 'Pass Transistor' hooked up in Common base mode (i think) is based upon adaptation of old non-IC PS design to a period of time when IC voltage requlators (used as current sources) could only pass much LESS THAN 1 Ampere. the Idea is that you use a, say, 150mA capable regulator to drive a High Current Power Transistor (2N3055 is an excellent example for this application) to get the current source up to Multiple Ampere output levels. For further design considerations, try looking up the data sheet for one of those older IC regulators (like a 4195 or uA723) on someplace like alldatasheet.com
As you might be able to tell, my EE skills are somewhat antiquated.... May not be the best solutions, but they got us to the Moon. Thanks for your putting up with the old fool...

 

[Flaminpyro]

For one reason only, to stir up brain cells
it seems to have worked

Why do you say that?

About driving a transistor with a regulator: Why do it in 2 or more chips when you can do it in 1?

 

[jib77]

Has any one here tryed a LM 350 or a LM1117 I think they may not work as a current regulator !

LM1117 works just fine

http://laserpointerforums.com/f39/fs-lm1117-based-linear-driver-1-3a-fixed-60213.html#post851547

The LD1085V is like a 3A version of the LM1117: LD1085V STMicroelectronics Low Dropout (LDO) Regulators

 

[rhd]

I know how much I frown upon purely answer-seeking behavior on forums.... so maybe if I'm upfront about it, it won't be as bad ?!?

Would someone be willing to spoon-feed me an example of how a low-current push button could be used to trigger a typical LM317 DDL driver?

I have this vague sense that using an NPN transistor should allow a low current circuit loop to turn on/off a higher current LM317 driver circuit.

 

[Leodahsan]

^ what you mean? something like a relay to get triggered by 2v 10mA and activates the 4.5v 1.5A when you want?

 

[rhd]

Exactly.

Though it would probably be preferable if it could be triggered by ~8v 10mA (ie, the 2x li-ion voltage that I would be feeding the LM317 with)

 

[anselm]

Oh come on, now your just being lazy.:na:

Example Transistor Circuit with LEDs - Electronics
Transistor Switches
Transistor Circuits
This one's a bit more technical with equashuhns 'n stuff:
Using the Transistor as a Switch

You're welcome.


EDIT:
much easier though, if you're using the LM317, is simply placing the switch in series with the
current adjusting resistors. Only very little current flows between the adjust and output pin anyways, so most switches will handle that OK.

 

[rhd]

I know I was Sorry, and thanks.

I guess what confused me, was the notion of the transistor AMPLIFYING the base current.

I don't really want that, because I want the LM317 to set the current. I sort of just want the transistor to act like an IC relay. IE, it would be awesome if the transistor could just do it's thing as a switch, and not have any interaction with the LM317 / main laser driver (apart from turning it on and off).

But I really don't get transistors!