PWMing verse DC output.

[random person]

While I like the fact that you are introducing new ideas (actually revisiting some old ideas, but they haven't been posted recently) laser diodes are a well-understood thing, and your tone of conversation isn't exactly friendly.. it's more like you're talking to a 9 year old.. I'm not 9 and neither is anyone else in this thread, so my suggestion to you is to rethink how you come at people.

Sorry, didn't mean to come off as a cocky ignorant idiot... Will make serious note of that in future posts. Thanks for the fair warning. Wasn't trying to stir up a fight or offend people. Like many fools I watched the kipkay video and wanted to build my first laser overnight and this was a question that was bugging me. I'm learning pretty quickly that there is alot more to it than that. Thanks all for being patient and answering newbie questions. Shows alot when respectable members are willing to spend their time posting in threads like these.

 

[Wolfman29]

Hey, we were all new once (hell, I still am! I just started this hobby in November). You seem like an experienced guy when it comes to electronics, but the fact of the matter is, unless you are making high-end boost DC-DC converters, then you really need to realize that the electronics in this hobby are actually fairly low level.

The most commonly used circuit around here is the DDL (or LM317) circuit as a current regulator. Essentially, it's a piece of cake and can be made small enough to fit into a hand-held device, even with a crappy soldering iron and PCB (like what I have!).

Hell, I suck at electronics, generally speaking (I still really don't understand transistors... damn things....) but, to me, the driver is simply a function box that I have gotten a peak into. I know how it works, I know why it works, but I don't look to find better ways to do it (besides just upgrading the chip to an LM1085 or whatever, but that's easy) because this way is easy and it just works.

The art of laser-building is making your heatsinks and drivers and diodes and hosts all work in unison. Sure, having a kickass driver is awesome, but it's only one part of the build!

 

[HIMNL9]

@random person: yes, it's supposed that the mosfet works in that way ..... it's the power element used for turn all the unused power in heat, and, except for boost switching-type drivers, there's no other way for do that ..... as any other constant-current linear driver do, anyway ..... regardless that you use a transistor, a mosfet, an LM317 or equivalents, you regulate the current through the load changing the output voltage, and all the "unused" part of it, must go somewhere, it can't just disappear ..... this is the reason for the power element, to turn in heat all the unused part of the power you feed the driver.

For this reason, too, all the linear drivers are better used keeping the input voltages near the one needed from the load ..... if your LD need 5V, keep the input voltage near it, plus ofcourse the dropout of the driver ..... say, 6V for a circuit like this, 9V for a LM317, and so on ..... and heatsink the power element.


@ EF: i have modulated them around 10KHz without problems (analog), but don't know about higher frequencies, cause til now i never needed them ..... must do some tests, but at the moment i don't have the time for that, sorry ..... anyway, according with the datasheets of the components, is probably possible to reach 100KHz and over ..... at least, eliminating the capacitor in parallel with the LD, or reducing it to 100pF .....

 

[ElektroFreak]

@random person: yes, it's supposed that the mosfet works in that way ..... it's the power element used for turn all the unused power in heat, and, except for boost switching-type drivers, there's no other way for do that ..... as any other constant-current linear driver do, anyway ..... regardless that you use a transistor, a mosfet, an LM317 or equivalents, you regulate the current through the load changing the output voltage, and all the "unused" part of it, must go somewhere, it can't just disappear ..... this is the reason for the power element, to turn in heat all the unused part of the power you feed the driver.

For this reason, too, all the linear drivers are better used keeping the input voltages near the one needed from the load ..... if your LD need 5V, keep the input voltage near it, plus ofcourse the dropout of the driver ..... say, 6V for a circuit like this, 9V for a LM317, and so on ..... and heatsink the power element.


@ EF: i have modulated them around 10KHz without problems (analog), but don't know about higher frequencies, cause til now i never needed them ..... must do some tests, but at the moment i don't have the time for that, sorry ..... anyway, according with the datasheets of the components, is probably possible to reach 100KHz and over ..... at least, eliminating the capacitor in parallel with the LD, or reducing it to 100pF .....

That's awesome HIMNL9!

I'll be building one to test very soon.. tnx for posting it!

 

[random person]

@random person: yes, it's supposed that the mosfet works in that way ..... it's the power element used for turn all the unused power in heat, and, except for boost switching-type drivers, there's no other way for do that ..... as any other constant-current linear driver do, anyway ..... regardless that you use a transistor, a mosfet, an LM317 or equivalents, you regulate the current through the load changing the output voltage, and all the "unused" part of it, must go somewhere, it can't just disappear ..... this is the reason for the power element, to turn in heat all the unused part of the power you feed the driver.

For this reason, too, all the linear drivers are better used keeping the input voltages near the one needed from the load ..... if your LD need 5V, keep the input voltage near it, plus ofcourse the dropout of the driver ..... say, 6V for a circuit like this, 9V for a LM317, and so on ..... and heatsink the power element.


@ EF: i have modulated them around 10KHz without problems (analog), but don't know about higher frequencies, cause til now i never needed them ..... must do some tests, but at the moment i don't have the time for that, sorry ..... anyway, according with the datasheets of the components, is probably possible to reach 100KHz and over ..... at least, eliminating the capacitor in parallel with the LD, or reducing it to 100pF .....

Ahh, my bad. That would explain the operational region you are using. My bad, it looked as if you were trying to use it as as a PWM switch rather than a regulator. Also explains the heat you were talking about. As you stated, in the linear/triode region the mosfet works almost like a resistor and thus your burning up power across it. Hence where the derivation of the term "Linear voltage regulator" came from since it is a device controlled by a mosfet operating in the linear region.

Wolfman29: Awww your really missing out. Electronics is quite fun and useful to know. Regarding transistors and how they "work" is still sometimes debated even to this day. The advance physics behind solid states can be fairly complicated. Knowing how they function is the key thing. Follow the basic equations and your good to go.

If you or anyone else is interested....
n-channel mosfets = 3 operational regions

cutoff region: (self explanatory) happens mainly when Vgate=~0 (technically when vgs<vtn)
application: Off switch

linear/triode region = Drain Voltage < Gate Voltage.
(Technically Vds<(Vgs -Vtn))
application: nicknamed the ohmic region, can work as a variable resistor for regulation...ect. Mosfets are not commonly used in this region. Will dissipate power over mosfet which can = meltdown.

saturation region = Drain Voltage > Gate Voltage
(Technically Vds>(Vgs -Vtn))
Works perfectly as a high voltage+high current switch. Better than a BJT switch because it requires no current in the gate/base. Also has a incredibly fast turn on time. Used as high power switches in power transfer and as low voltage switches in memory. All modern solid state memory primarily mosfets. (Flash, Ram, EEprom...ect). Also, very little if any power is dissipated over the device in this region and thus no heat.