Opinions on this circuit design please

Hi all,

I want to use a high power laser diode modulated at 40-some kHz. to send “tight beam” IR control signals (like a TV remote control code, for example).

I know there are commercial “TTL” laser controller boards available, but I’ve found none that can switch at 40 kHz or above.

So (please see attached circuit concept) I drew up a little high side switch intended to be run by an Arduino output pin. The switch turns on or off the power to the LM-338 chip (a 5 amp adjustable regulator similar to the LM-317).

The LM-338 is wired as a CONSTANT CURRENT source and the resistor across the output and control pins controls the current. The actual component values are yet TBD… but whatever they are will not change how the circuit works.

So, I would appreciate any comments on the circuit design, can it be improved, etc…

Thanks!

(click img for full size)
laser_ctrl.jpg

I think that this type of circuit could be easier. Change R1 to 1k, and calculate R2 for the laser current you want (~0.65volt drops across R2). The mosfet must be a logic level type, TO-220, with heatsink because it's used in it's linear region. T1 can be any small signal NPN. Leo..

Looks reasonable.

Suggest you use MOS FETs, at least for the series pass.

Wawa: I think that this type of circuit could be easier. Change R1 to 1k, and calculate R2 for the laser current you want (~0.65volt drops across R2). The mosfet must be a logic level type, TO-220, with heatsink because it's used in it's linear region. T1 can be any small signal NPN. Leo..

I see how that one works... but somehow I don't feel "good" about it. If the gate voltage control transistor fails, the mosfet would dump as much current as possible into the laser.

I've used 3 terminal regulators for years (decades actually) and they are bulletproof.

larryd: Looks reasonable.

Suggest you use MOS FETs, at least for the series pass.

The series pass is just a switch. Not arguing... but why is a mosfet better than a BJT for this use? It will always either be saturated or off, so the Vce-sat is all I have to worry about (and that's quite low). Will a mosfet be better here (current will be around 4.5 amperes or so).

I have never switched a regulator that fast.

For a comparison:

IRL5602 has a on resistance of ~.06 Ω at 4.5 amps = .3v Vds 1.3 Watts

A BJT has a Vce saturation voltage as high as 1 to 2 volts. 4.5 to 9 Watts

Of course, lower the saturation, the less heat.

Both will work though.

Linear regulators are not expecting to be switched on and off at that high frequency. A normal application as a power supply usually wants the output voltage to rise at a predictable, slow, speed instead of coming on hard like a switching transistor.

I would be surprised if you got a good waveform of controlled current like that.

It's not like you get any efficiency benefit like a switchmode power supply. It's still a linear supply so it's pretty much the same power loss as using a resistor.

If I was constrained to use the LM338 then I'd use it in its conventional voltage supply mode and then switch the LED through an appropriate current-limiting resistor. Use a capacitor in the normal way to help the regulator cover the current surges when the transistor switches on.

If I wasn't constrained to use that regulator then I'd look at constant-current LED drivers that are intended for PWM applications. You may find it difficult to find datasheets for that frequency: 40KHz is much higher than most PWM frequencies.

krupski:
I want to use a high power laser

Please remember: don’t look in laser with remaining eye.

wvmarle: Please remember: don't look in laser with remaining eye.

I actually have a tiny burned spot in my left eye from years ago.

I was in a lab at the university where there is a 25 watt CO2 laser (infrared) and although I was wearing the safety goggles, somehow a tiny part of the beam reflected and got into the corner of my eye.

I heard a "click" inside my head and saw a shimmering spot of silvery-white blindness out of the "corner of my eye".

Later that day (at night) I started to get "floaters" (the little shadow particles that drift around the eye).

After a week or so the floaters faded away, and I stopped noticing the "bad spot" (I don't think it healed - I just stopped noticing it).

Thankfully, there was very little damage and now I don't notice it at all and it doesn't hinder me in the least.

But I'll tell you I was scared the beans out of me when it happened! A CLICK... right inside my head! VERY weird.

Eww... sounds scary indeed. 25W is a pretty powerful laser, lucky that you didn't have any more damage.

This may be useful but it does say the limit for your method of modulation is around 30k. But does talk about other methods.

https://www.laserglow.com/page/modulation

Mark