NPN/MosFet  Min. voltage drop Curr. Regulator

I've wrote earlier that I want MosFet regulation. It's about the most efficient /simple liner current regulator because of less battery-space and waste heat hassles.

The circuit waste only about .5v!  with voltage determined by the type and ambient temperature on the NPN transistor.
Parts:
(some connections noted after the component)
1. Tiny 100k Ohms (gate, B+)
2. 2N2222 (Em.-2.2- Grnd,Base-2.2-source, col.-Gate )
3. 1/2W 2~2.8 Ohms for current range of .25 ~.180 Amp respectivly
4. Small TO-5 etc.. Low voltage, Vmos, Pmos, Hexfet (any flavor of powerNMOS)
RShack IRF510 MOS for example, not ideal at .5 Ohms d-s res.
5. 1n400x Reversal Protection silicon diode (across the Laser)
6. 4.7uF 6.3v Electrolitic  (across Laser)
7. small .1 disk or Poly. Capacitor (across Laser)
8. Laser diode  (a-B+,C-Drain)
Ideal power 2X AAA or AA NiMH Batteries (Grnd,B+)

Might need a Cap across gate and Grnd. for noise rejection?
I'll be back with DATA latter.
Credits/Cheers/Pictures, etc..  to:
http://instructables.com/id/EK2XAPS11GEWOF2YSD

I havn't built it yet. But it's now here for your convenience

DGM

I took a look - some warning bells went off in my head. This could sort of be considered a low drop out regulator. If you build it, put it on a scope and test it thoroughly with a dummy load befor trusting your LDs to it.

"- Q2 is used as a variable resistor. Q2 starts out turned on by R1."
---> Q2 starting out "on" can be bad for the LD (pulse)!

"- Q1 is used as an over-current sensing switch, and R3 is the "sense resistor" or "set resistor" that triggers Q1 when too much current is flowing."
---> Perhaps, but you would have to calibrate it for your power source. Temperture change will cause the regulation to drift.

"- The main current flow is through the LED's, through Q2, and through R3. When too much current flows through R3, Q1 will start to turn on, which starts turning off Q2. Turning off Q2 reduces the current through the LED's and R3. So we've created a "feedback loop", which continuously tracks the current and keeps it exactly at the set point at all times."
----> This will not be constant across all voltage input levels. You want to plan for the life of your power source - if it's batteries, from full charge to almost dead. 2xNiMH will not cut it (2xAlkaline is not an ideal choice either) - don't forget that there ia a voltage drop across the MOSFET and the bias resistor R3.

If you try this, best wishes and I look forward to seeing your data. Cheers,

Paul

I have used a similar configuration to limit the peak current from a power supply I built, but in this case it was mainly to protect the power supply from being damaged if a short circuit occured in whatever it was connected to. I wouldn't use it as a precision current source for a laser diode, because as chimo said, it will drift with temperature. Perhaps this could actually be beneficial - as the laser also heats up the current will increase, helping to maintain constant optical power? ;D

If R1 is sufficiently large, then the RC time constant (where C is the mosfet gate capacitance) will slowly turn the output on, so a large peak at startup should be avoided. If you decide to add additional capacitance (or even if you don't), verify that the output is stable before using a laser diode on it. The feedback loop coupled with a poorly placed capacitor could cause the output to oscillate.

Thanks guys,
Good analysis. [smiley=thumbsup.gif]

I like the advantage of Gate capac. for a LD soft start.
But that's why we put a Cap. across the LD.

Now I'm thinking of going really low-drop by swapping out(substituting) the NPN for SMD fet with resistive voltage divider. ;D ;D ;D
I'm not too concerned about 2 batts. being too little voltage. My goal is to make smallest
burner; a
"TINY TITAN LASERROD"
With two push-buttons, configured for 3 power levels [smiley=2vrolijk_08.gif]
Built in a 1/2 inch copper pipe, AA powered. Heatsinking assured.

We can always fall back to lithium Batts.. or a 3rd batt. option.
I don't care if current falls off with depleating batts. [smiley=happy.gif]

But my favorite power circuit is Digital Controlled Triple Charge Switching.
It's a Telsa power shuttle circuit. SHHHHHH [smiley=lipsrsealed.gif]
Goggle KEYWORD: Bedini battery something.....

Toasted [smiley=beer.gif]

This is a fairly typical constant-current source, usually the mosfet is just a bipolar transistor... using a low-RDSon type mosfet whill shave a few 100 mV off the minimal required drop at best. For the mosfet, you could use something like a BUZ11, for the transistor any high-hfe small signal transistor (e.g. bc547) works fine.

Regulation in these things is far from perfect though, output current still varies with both supply voltage and the actual drop over the mosfet and sensing resistor (mostly when its under 1 volt).

UPDATE:

I only had one fet that could turn on under 2 volts and results were acceptable. ;D

Got ZXMN3A06DN8TA mosfets e-ordered that have a better 1 volt gate-on spec...

Cheers [smiley=beer.gif]