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test load

Xplorer877

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This is what I use.



Also make sure that your 1 ohm resistor is indeed 1 ohm! I like these sandstone resistors because they have higher tolerances. But those peanut shaped resistors often can be off by 20%. In short, that means your reading will be off by 20% if you assume it's exactly 1 ohm. That can prove lethal for some diodes.

Always measure and use the exact value! Don't assume!

-Tony
 



HIMNL9

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nice test load ;), you can use it til 1,5A without problems, just the diodes becomes a bit hot (can go til 2A, no problems, but over this they become too hot :p)

About the resistor value, it's right, but there's a little problem ..... normal DMM, and also lab standard ohmeters, don't have enough precision for measure resistances less than 40 or 50 ohm in a decent way ..... the tolerances of the instrument, the parasitic resistance of the wires and plugs, the maybe-not-perfect contact from leads and resistor terminal, may give you very different results (i have tried it ..... taking a sample 1 ohm resistor and trying all my multimeters, i found that the better one was measuring 1,46 ohm, the "decent" ones 1,74, 1,99, 2,15 and 2,2 ohm, and the cheapy "battle" ones 2,9 and 3,2 ohm :p)

You need a milliohmeter, for do similar measures, but not all the hobbysts have some spare 500 $ for buy a cheapy one, not speaking about "labby" 4400$ ones ..... i was lucky, i'm working in electronic from a life and half, and i'm also a DIY maniac :)p), so i built a decent desk one for myself, and also a small "add-on" box for use it "on the battlefield" with a common DMM, for a relatively low price (they can't measure microohm range, but are decently precises at milliohm and ohm range) ..... but not all the hobbysts want to do, or have the possibility to do, the same, so is difficult to "measure" precises 1 ohm resistances .....

A solution, for those that don't want to buy or build a milliohmeter, can be using a stable and known current source (and, after all, we're full of current sources, as laser diodes drivers :p ..... also my "add-on" one uses the same principle) ..... take a stable and known 100mA current source, that can be built easily using an LM317, any 2 to 5 ohm 1W resistance (not need to be precise, this one, it's just used as "buffer", or whatever you call it), and your 1 ohm resistance to measure, and connect in serie the "buffer" resistor and the 1 ohm one that you want to measure, as load for the LM317 ..... with a milliamperometer, check that the current in the circuit is exactly 100mA (or regulate ir until it's 100mA, precise), then measure the voltage at the leads of the 1 ohm resistor ..... if it's exactly 1 ohm, it must be 100mV ..... any difference causes a different voltage, and you can easily calculate the real value of the resistor using ohm law, where R=V/I ..... with as example 115mV, and knowing that the current is 100mA, it's easy to see that 0.115/0,1=1,15 ohm, instead 1, and so on ;)
 

Xplorer877

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^Good point! But IMO your average DDM is more than capable of measuring the resistance to within a few percent which is much better than not measuring at all. Like you said if you're calculating resistance with miliohm precision then other factors like parasitic resistance come into play. Hell even temperature can affect resistance on a precise enough scale. But at that point the precision is negligible. The average person at best will only be able to set the diode current to within 3-4% of what they want with a dummy load calibration. Which IMO is good enough for all intents and purposes. I'm just saying it's better to get the error down to a few percent than to forget that most resistors have pretty large tolerances.

-Tony
 

qumefox

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Actually most DMM's do fairly well at low resistances. But you have to remember to compensate for the resistance of the test leads your using. Short the leads together, see what resistance they are, then subtract that from what your measuring and the result will be more than accurate enough for the results required for this particular purpose.
 

izzzzzz6

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It seems that this new member opens a thread to ask a question..
and when he gets an answer... opens another thread to ask a
question about the answer....

@dtucker21... we can see all the Threads that you open...:thinking:

You should really do some research on the Forum for yourself...
We've been there and done that...........
Trying the [Search] button only once will not help...


Jerry
I have been searching for the same answers but i just can't find the explanations. I am about to set a :

2A Laser Diode Driver Analog Modulation Bias Standby SBS 445nm 638nm | eBay

for a:

Nichia NDB7875 445nm 9mm Laser Diode in A Copper Module w Leads 405 G 2 Lens | eBay

I know what a dummy load is, i just don't understand weather i can use any diodes for example i have 1N4001 diodes and i have 1ohm 5W resistors.

Also i don't see or understand how many diodes i should wire in series.

This is the same info i have been searching for. Please help if you can explain how many 1N4001 diodes i need to connect to make a dummy load for these LED projector diodes and why i need to connect a certain amount of diodes for different LD's. I understand that the number diodes are relevant to the LD but i don't understand how to calculate this.

I am planning to hit a mountain with my laser tomorrow for New Year. I will try to setup the driver myself if i don't get help but it would be a shame to blow the diode if i get something wrong as it cost me quite a bit of money :)
 

izzzzzz6

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I just want to point out that i have found it very easy finding info on dummy loads and that i understand fully true concept and how to use one, however i just can't seem to find the posts which refer to the amount of diodes used for different LD's. For example, which LD's require 9 diodes in series? I am hoping that by using 6 1N4001's with a 5watt 1 ohm resistor will be a good test load for my Nichia NDB7875 LD. I'll let everyone know after New Year how it went.
 

crazyspaz

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Please don't necrotic post...the last response to this thread was over 2 years ago.
 

Isos

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I appreciate all the good information here. Thanks!

I've done a bunch of searching around, and I still have a couple of little questions if you'll indulge me. Most sources seem to say 6 1N4001's for 405nm, 4 diodes for 650nm, etc.

1. I gather that the diodes simulate the voltage drop across the laser diode. Do all laser diodes of a given wavelength exhibit the same voltage drop characteristics (i.e. same dummy load to test 100mW or 1W 405nm diodes)? If so, why?
2. Why is a 1ohm resistor suitable for all dummy loads?
3. Could you look at a laser diode spec sheet and immediately calculate the required dummy load?

Thanks for the answers! I'm trying to learn my way into the electrical world from a mechanical background :)
 

Jmillerdoc

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Last I checked you could order extremely accurate, or close tolerance, resistors directly from Mouser or Digikey. They are a bit more costly than your run of the mill resistors found at RatShack or similar, but you will get what you pay for. Also, I am fairly certain Percy Audio still sell Boutique resistors with ultra tight tolerances. For my test load resistor I used one with a 0.01% tolerance, a "naked" foil type. I don't have a multimeter that can measure anything that accurately but it does read 1 ohm and a am confident it is as close to 1ohm as I will ever get. Source your resistor like this, measure it with what you've got on hand, and you will be in the clear and more accurate than the average person out there. You certainly shouldn't have an issue with being so far off you kill a diode.
 

Jmillerdoc

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I appreciate all the good information here. Thanks!

I've done a bunch of searching around, and I still have a couple of little questions if you'll indulge me. Most sources seem to say 6 1N4001's for 405nm, 4 diodes for 650nm, etc.

1. I gather that the diodes simulate the voltage drop across the laser diode. Do all laser diodes of a given wavelength exhibit the same voltage drop characteristics (i.e. same dummy load to test 100mW or 1W 405nm diodes)? If so, why?
2. Why is a 1ohm resistor suitable for all dummy loads?
3. Could you look at a laser diode spec sheet and immediately calculate the required dummy load?

Thanks for the answers! I'm trying to learn my way into the electrical world from a mechanical background :)
I can't answer all your questions but #2 is fairly easy. This is to measure your voltage drop across a known resistance to calculate the current. Using a 1R just simplifies the equation. You can use any relatively low valued resistor and simply adjust the calculation based on the resistor value. In the end it's not that 1R is ok for any diode, 1R is ok for testing any current in this range of testing. Try to remove the diode from your thought process and just think "how do I measure a current with an Volt-meter?". Because volt-meters are cheap, generally accurate, and just about everywhere many of us use them. Using 1R when measuring amps makes the calculation easy from I=V/R. Hope that helps.
 




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