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
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
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
On Fri, 26 Dec 2025 18:36:57 -0500, ehsjr <ehsjr@verizon.net> 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.
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
Gentlemen (IOW not you, Bill),
On 12/26/2025 7:35 PM, Cursitor Doom wrote:
On Fri, 26 Dec 2025 18:36:57 -0500, ehsjr <ehsjr@verizon.net> 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
On Fri, 26 Dec 2025 21:01:34 -0500, ehsjr <ehsjr@verizon.net> wrote:
On 12/26/2025 7:35 PM, Cursitor Doom wrote:
On Fri, 26 Dec 2025 18:36:57 -0500, ehsjr <ehsjr@verizon.net> 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-adc0-0098b2af7c4e.sf/en_GB/?ObjectPath=/Shops/05c54fb6-7778-4d36-adc0-0098b2af7c4e/Categories/Background_and_Projects/Setting_the_Laser_Driver
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