On Friday, January 20, 2012 12:07:16 PM Viesturs Lācis did opine: > 2012/1/20 Kirk Wallace <kwall...@wallacecompany.com>: > > On Thu, 2012-01-19 at 17:32 -0500, Przemek Klosowski wrote: > >> On Thu, Jan 19, 2012 at 5:08 PM, Viesturs Lؤپcis <viesturs.la...@gmail.com>wrote: > >> > Hello, folks! > >> > > >> > I would appreciate some advice or insights, what am I doing wrong. > >> > > >> > What I want to do is to switch on and off a power to laser engraver > >> > diode. It needs 12 VDC supply and it consumes ~1 A currrent. > >> > > >> > Here is schematics of what I currently have: > >> > http://picpaste.com/DSCF6164-vX7heUEq.jpg > >> > > >> > The problem is that it is not working: > >> > Voltmeter was showing +12 VDC on both leads, going to laser module > >> > all the time, regardless if I had turned the output pin "true" or > >> > not. > >> > >> آ What is the voltage of the 'low' state? could it light up the LED? > >> > >> As Steve said, your resistor is in the wrong place. I'd put it on the > >> collector of the phototransistor.; then, the base current of your > >> power transistor would be (12V-0.7V-0.7V)/R; assuming the power > >> transistor beta is 20, you'd need 50mA, i.e. the resistor must be > >> 200 ohm. > > > > Just to add my two cent's worth, see attached (if it makes it): > Thank You, guys! > > Kirk, I repasted Your scheme here: > http://picpaste.com/scheme-3V0w43QM.png > > Do I understand that includes also Steve's and Przemek's sugestions, > if R2 is 200 ohm? > > The optoisolator is 4N25, the transistor is BD139. > It says 4N29 in that link. I am not familiar enough with either, so when I say r1=1k is too high, its a swag only, check the docs on the device for sure. When calculating that current, you don't have 5 volts though, subtract first the logic zero voltage the controlling card can supply, often in the 0.15 volt range, and then subtract the on drop of the led in the 4nxx from that. IOW at 1k, you are likely getting only perhaps 3.5 milliamps to light the led. That doesn't seem sufficient in my mind at least, to get the job done.
The rest if this modified drawing looks as if it should fly. Today however, I don't think I would use a 2n3055 due to its high on state forward drop , costing the laser a bit of its supply voltage. Rather, I think I would feed the photo transistor to a cmos schmidt trigger to get a good, fast rail to rail switch, and drive the gate of a hexfet with its output. The hexfet in my case would be whatever is in the first 'dead' computer power supply I have a grocery sack full of. ;-) Any of those will likely have a similar or higher current rating than the 2n3055, hundreds of volts greater off state withstand, and 10% of the 2n3055's on state voltage drop, a genuinely better way to get that job done. Depending on the hexfet some sort of a negative bias of -2 or -3 volts might be required to achieve a total turnoff, but with only a 5 volt supply, the off state leakage really should not be a factor. > Gene, I think that I have spare 330 or 470 ohm resistors to replace > the 1K ohm R1, I will try to test it. These slot interrupters I am using need 10 to 20 milliamps to light them adequately. There is no doubt lower current stuff being used in old ball mice but the logic in those can be a bit of a mystery. Its a valid complaint that my junk box never has what I need. OTOH, it seems to be in better shape than most. ;-) > Viesturs > > ------------------------------------------------------------------------ > ------ Keep Your Developer Skills Current with LearnDevNow! > The most comprehensive online learning library for Microsoft developers > is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, > Metro Style Apps, more. Free future releases when you subscribe now! > http://p.sf.net/sfu/learndevnow-d2d > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> Chinese saying: "He who speak with forked tongue, not need chopsticks." ------------------------------------------------------------------------------ Keep Your Developer Skills Current with LearnDevNow! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-d2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
