From: ehsjr@verizon.net   
      
   On 12/28/2025 9:26 AM, Cursitor Doom wrote:   
   > On Fri, 26 Dec 2025 21:01:34 -0500, ehsjr  wrote:   
   >   
   >> On 12/26/2025 7:35 PM, Cursitor Doom wrote:   
   >>> On Fri, 26 Dec 2025 18:36:57 -0500, ehsjr  wrote:   
   >>>   
   >>>> On 12/26/2025 12:56 PM, Cursitor Doom wrote:   
   >>>>> Gentlemen (IOW not you, Bill),   
   >>>>>   
   >>>>> I've got a bunch of green laser diodes which are specified for 370mA   
   >>>>> current draw. I've been using a straight 5 ohm WW resistor rated at   
   >>>>> 10W as a dummy load, but it's crude and inaccurate. Is there something   
   >>>>> better I should be using? I've got some 2W blue ones to do later on as   
   >>>>> well so something which could be adapted for those would be a plus.   
   >>>>> Ideally something which mimics the knee you get as it starts to   
   >>>>> conduct.   
   >>>>>   
   >>>>> Cheers,   
   >>>>>   
   >>>>> CD   
   >>>>   
   >>>> If you want something better than a 5 ohm resistor dummy   
   >>>> load, see the LM317 datasheet figure 8.8   
   >>>>   
   >>>> As you did not specify your supply voltage, I don't know   
   >>>> how close your 5 ohm resistor dummy load is to drawing   
   >>>> 370 mA when connected directly across the supply.   
   >>>>   
   >>>> If you put the LM317 circuit in series with your 5 ohm resistor,   
   >>>> you compute the current drawn by by  1.2/R1.  So, for example,   
   >>>> if R1 is 3.25 ohms the current drawn will be ~369 mA.  This of   
   >>>> course assumes a supply of enough "grunt" and within Vmax   
   >>>> for the 317.   
   >>>>   
   >>>> Some more detail:   
   >>>> R1, at 3.25 ohms, will dissipate around half a watt. Use   
   >>>> at least 1 watt. I don't know what you have on hand - I'd   
   >>>> use power resistors - a 3 ohm in series with a .25 ohm.   
   >>>>   
   >>>> Ed   
   >>>   
   >>> Okay, many thanks. Yes, I know there were scant details provided but I   
   >>> only wanted vague suggestions I could maybe develop myself. The other   
   >>> idea I had was four diodes in series with a one ohm resistor so as to   
   >>> mimic the Vf of the laser diode. Fortunately I have a good selection   
   >>> of WW power resistors in my stash here.   
   >>   
   >> There's a problem with that as I understand what you said. I   
   >> suspect I'm not understanding what you have in mind. Here's   
   >> what I see as the circuit from what you said:   
   >>   
   >> Supply +---[D1]---[D2]---[D3]---[D4]---[R]---Gnd   
   >>   
   >> The 4 diodes in series provide a 2.4 volt voltage drop, assuming   
   >> .6 volts per diode. Call that Dd (Diode drop). The total voltage   
   >> drop is the Diode drop (Dd) plus the drop across your 1 ohm R.   
   >> So you need to know the current through R to compute its voltage   
   >> drop.   
   >>   
   >> Your R is 1 ohm.   
   >> Your circuit looks like this:  Vs---Dd---R---gnd.  The voltage   
   >> across R is Vs - Dd.  Current through R (1 ohm) is found by   
   >> I = (Vs-2.4)/1 = Vs-2.4 . That means I varies as Vs varies - I   
   >> is not fixed.  Thus we cannot say what the voltage drop is   
   >> across R. That means the total drop cannot be established as   
   >> equal your laser diode Vf using that circuit.   
   >>   
   >> So in general, you need an active current limiting circuit   
   >> for what you want to do.  If we can get more specific - say   
   >> a regulated supply of some specific or settable regulated   
   >> output voltage, then we can use your circuit with a computed   
   >> load resistance   
   >>   
   >> Maybe you could post a schematic if I've misunderstood?   
   >>   
   >> Ed   
   >>   
   >   
   > The power supply is sorted out, but before I apply it to an expensive   
   > laser diode, I wanted to check I had the current in the right   
   > ballpark. These fuckers are real current hogs!   
   > I found this on the Odic Lasers website. They're suggesting something   
   > curiously similar to my earlier idea: four diodes plus a 1 ohm   
   > resistor:   
   >   
   > https://odicforce.com/epages/05c54fb6-7778-4d36-adc0-0098b2af7   
   4e.sf/en_GB/?ObjectPath=/Shops/05c54fb6-7778-4d36-adc0-0098b2af7   
   4e/Categories/Background_and_Projects/Setting_the_Laser_Driver   
      
      
   Thanks, now I get it! Your supply, in the circuit below, already   
   has adjustable current limiting:   
   Supply +---[D1]---[D2]---[D3]---[D4]---[R]---Gnd   
      
   So what you want to do is connect a volt meter across   
   the 1 ohm resistor and adjust the supply to provide a   
   constant current through the resistor so the meter   
   reads 370 mV. (Or whatever current you are after.)   
   The current through the 1 ohm resistor will be the   
   same number as the voltage reading.   
      
   Ed   
      
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