On Fri, 2012-01-27 at 12:26 -0500, gene heskett wrote: > The LM317T is a linear regulator device > and could be made adjustable so as to compensate for the wiring and > switching loss in your controller.
Judging from Viesturs' description in a later message: > Nope, I see 2 resistors in series for the middle leg. The LM317 is probably wired up as a current controller, not a voltage controller: it's providing a fixed *current* to the laser diode, not regulating the voltage across the wires. https://en.wikipedia.org/wiki/File:LM317_1A_ConstCurrent.svg In that mode, the voltage drop from controller to laser doesn't make much difference, at least within reasonable limits. What *does* matter is the voltage supplied to the controller (which sets the compliance it needs to regulate the laser current) and the current available from the raw +12 V supply (which must be greater than the laser current). Tweaking the resistors or substituting a voltage source for the laser controller will let the magic smoke out of the laser! The BD139 has a 1.5 A current rating, with a fairly low hFE = 40. That says it must have 1.0 / 40 = 25 mA of base current to saturate while carrying 1 A. More base current will be better. The 4N25 has a current transfer ratio of 20%, which means the LED current must be 25 / 0.20 = 125 mA. Anything less than that won't provide enough base drive, so the transistor won't saturate, so the laser controller won't get enough power, and the transistor will eventually overheat and die. However, you can't jam that much current through the 4N25's LED. At the risk of sounding like an Olde Farte, the easiest way to get this contraption working is a small mechanical relay: a few tens of mA in will switch an amp of DC on the output. No voltage drops, no muss, no fuss. The optoisolator won't have enough current capacity for the relay, so you will probably need the driver transistor to power the *relay* from the digital output. But there's no need for the optoisolator in that case. Or, of course, I could be completely wrong... -- Ed http://softsolder.com ------------------------------------------------------------------------------ Try before you buy = See our experts in action! 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-dev2 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
