Why are we using test loads on linear drivers?

[Deleted member 8382]

I still don't get it. even assuming this is true (I don't understand why but anyway), aren't the imprecisions on a test load mount way bigger?

 

[jayrob]

@Jayrob: Mmmm... Interesting. Why is the other plug made for then?

Here's a quote from Igor explaining why the difference. But it probably doesn't matter which plug you use with a linear driver. Boost drivers are a different story. So just out of good practice, I always use the 10A plug...

DMMs measure current by measuring voltage across an internal resistor (called "shunt"). Just like we sometimes put a 1 ohm resistor and measure voltage across it.

On top of that, the long wires of the DMM itself introduce a resistane, which the current has to flow through. And all that requires some extra voltage, which is lost, by the time current gets from one probe, through the DMM, to the other probe. This is called a voltage drop, and depends on HOW MUCH current is flowing through a resistance (the higher the current, the higher the voltage drop across a resistance -> U = R x I). If you use a second DMM, you could measure this voltage drop across the first DMM.

So, the DMM introduces a resistance into the system, the resistance causes a voltage drop - the driver gets a bit less voltage than the battery actually has. If a switching driver gets less voltage it will compensate by drawing more current, to keep power (P = U x I) the same. It's the only way it can supply a constant output...


If you would measure the voltage drop across the first DMM with another DMM, you could then calculate how much less the actual current draw in the laser is. But since the 10A (or 20A - whatever the highest) range uses a thick metal rod as a curent measuring shunt, the resistance introduced is small, so the measurement is quite close to the actual current in the build.



The reason this effect is not noticable with linear drivers is, that they draw a constant current (not constant power), as long as they get enough voltage. Putting a DMM in between reduces the voltage the driver gets a bit. But a long as it is still enough, the driver will keep drawing a constant current.

If however, a linear sytem would be barelly above the minimum voltage required for regulation, adding a voltage drop could reduce the voltage to bellow the minimum, and cause the driver to drop out of regulation, while it could still regulate for a while, if there was no DMM in it's way.

At a certain point, a linear driver might still get just enough voltage to regulate when measured through the 20A range, but drop out when measured in 200mA range.


Due to the different behavior of linear drivers, the effects of the voltage drop are harder to notice there, and since this voltage is low, the above situations are unlikelly to happen when testing a linear driver setup.


A situation where the difference in DMM's resistance between the two ranges is very noticable is in the many cheap chinese unregulated green lasers. There, the drivers are out of regulation most of the time, and act as complicated resistors. Measuring the current with the high range on the DMM will show a much higher current than the low range on the DMM!

Another situation is, if you were to make a laser with nothing but a pot between the diode and the driver, set the current with the DMM in-line and then remove it. The actual current in the laser could be so much higher, that it woud kill the diode!

 

[Deleted member 8382]

Thanks. This was very explicit

 

[lasersbee]

AT THE END SOMEONE WHO UNDERSTANDS ME, THANKS GOD!

hey DrLava, you say in a ideal world only. why? I mean, I think it's even more precise to do it this way! 6 diodes are 4.9V drop, not 5. Just this little imprecision is higher than any other produced by my method right?

Thanks for explaining it the right way: "this is because the LM317 does not operate as a resistor, it has active feedback with high gain in order to maintain the 1.25V across its out and adj pins."

You all even made me go and test this lol

Hey Albert....

I just saw this Thread....

You could have saved the contributing members and yourself a lot
of communication problems if you had posted a schematic of the
EXACT circuit that you were talking about right from the beginning...

Even I was not sure if you were talking about the LM317 Voltage
Regulator App note circuit or the LM317 Current Regulator APP note circuit.

BTW... If you don't want to use a Test Load... that's your problem...
It is not a LAW....

I use test loads on all Laser Drivers I build... But that is my choice....
I have blown 1 LD so far (since joining this Forum) by my own errors...

Jerry

 

[Deleted member 8382]

I meant the DDL circuit or the simple current regulator using a lm317. I think I was explicit enough!

How would you use the lm317 as voltage regulator for laser diodes? I mean, the difference of voltage from 100 to 150mA on a laser diode is very small, isn't it? (aren't they working as a Z-diode?)

Yours,
Albert

 

[lasersbee]

I meant the DDL circuit or the simple current regulator using a lm317. I think I was explicit enough!

How would you use the lm317 as voltage regulator for laser diodes? I mean, the difference of voltage from 100 to 150mA on a laser diode is very small, isn't it? (aren't they working as a Z-diode?)

Yours,
Albert

If you can't draw a mental picture in another persons mind....
You were NOT explicit enough.... it is that simple...

I have more than 5 laser Diodes that use an LM317 Voltage regulated
circuit... so don't tell me that it can't be done...

I have already built a 5 Amp LM338 Variable Voltage Regulated Driver for
an 808nm IR Laser that works just fine... driving the Laser to almost 3 Watts..

You have sounded like a smarta$$ to some of the members on this Thread
and that wasn't called for...

IMO... Just because you had "some electronics classes" doesn't make you an
"expert" even if you think you are....


Jerry

 

[ElektroFreak]

You have sounded like a smarta$$ to some of the members on this Thread and that wasn't called for...

Just because you had "some electronics classes" doesn't make you an "expert" even if you think you are....


Jerry

You should see the PM the little shit sent me trying to make me out to be the bad guy.. all because I took the time to try to help him understand this stuff a little better. What a piece of work.. He's now the second person EVER to grace my ignore list.. right there with arenared.

 

[lasersbee]

I read the whole Thread.... Albert is not usually this way...
Maybe GF troubles or something...:thinking:

Jerry

 

[Deleted member 8382]

hey lasersbee. today everything is strange. It seems like if everyone was extremely sensitive, really. I've never tried to offend anyone and now suddenly everyone's offended with my posts. Just like you on this last one

so don't tell me that it can't be done...

Did I? I really don't think I ever pretended to be an expert on it lol.


@EF: If you are really going to ignore me just do it please. here's the PM in case anyone wanna see it. I've got nothing to hide. after having to hear an offensive statement about me being a foreigner:

they do have those things in Spain, don't they

This is my answer

What do have Spain to do with all this?

Quote:I freaking hate it when people ask a question, and then tell you you don't know what you're talking about

What I answered was not what I was responed about. And in fact the responses were wrong and so I tried to correct them. and you know I was right with the corrections I made.

From this point you just starting to flame me at no reason, telling me that you linked to images and not pdfs, when you just pointed me to the pdf page 19 at the botton, in which there was an image, just to make me look stupid.

Then you tell me that the DDL circuit is on the datasheet. I'll tell you something. This is the DDL circuit.

posted him an imgae

And this is what appears on the datasheets:

posted him an imgae

That was all I was trying to tell you. But you didn't listen. just atacking me. It's Ok. Bye.

I'll never understand people like you. You jumped on me wrongly and won't go back no matter what happens.

Ah, sorry for not being born in your country, I guess that was my biggest sin for you.

 

[ElektroFreak]

I don't care what the reason is, Jerry.. I have no use for anyone who would treat others (who were only trying to answer his vague questions) like this. He's burned this bridge... I have zero tolerance for that. There's always google if he's got a question.. he can go there from now on as far as I'm concerned.

It's funny to me that other people who have come along to this thread can see how he's been acting, but to him it's the people who tried to help him who are the problem.

 

[ElektroFreak]

EDIT: OK.. so I had a whole different post here, one which was pretty unfriendly about this whole situation and Albert here. When I got home from work today, I was playing with my daughter when I realized I had been in a terrible mood today due to a hard day at work and because I had to deal with some rough clients all day. Thinking about that made me realize how much the day's issues had clouded my head.

Now, let me be clear about this: I still don't agree with some of the ways Hallucinogenic handled things in this thread. I'm also not completely convinced that I was hearing the truth when I was told that there was no hostility present, and that the whole thing is a language-barrier misunderstanding. I am, however, man enough to admit when I think that I might be mistaken, so I'm going to assume that this has all been a misunderstanding, and I overreacted to it.

I've been here awhile and this isn't the first time I've thought Albert was taking a bit of a hostile tone, but he hasn't ever been a problematic member, so perhaps it is a language issue. I'm going to leave this thread on that note and hope that there are fewer misunderstandings in the future..

 

[lasersbee]

Yeah the Post #58 was a little "not you" ... but you are still correct and I stand by what I posted...

Jerry

 

[Hemlock_Mike]

I use a simple test load method. -- After burning out so many diodes, I find that when they become an LED, it's because of COD. The diode is still in tact and functional as a load -- no it won't lase any more but still a fully functional junction unless the diode portion was damaged.

HMike

 

[Mohrenberg]

I see what your saying, that since it's a constant current, it shouldn't matter what we use as a test load since it should always be the same current coming out.

but there is something your missing (i've no idea what it is...)
i'm assuming this because if you say set the current at 420mA with a test oad set on the "blu-ray" setting, and then look at then check the driver's current with the "red" settings, it's going to show a lot more than 420mA (i think it's 600 something, i did it once and killed a diode on accident)

...i don't know a whole lot, i just remember this instance of the reading on the DMM changing when going through 3, or 5 diodes on the test load.

Edit: Typed in a hurry so it kind of looks like a tyler rant, But I am too lazy to fix it..

 

[FireMyLaser]

Why you should use a test load:

Lets say you decide to use a wire as a load and check the current, you want 100mA, it reads 100mA. So far so good. Next you solder your diode in place and power up, you notice the it isn't as bright as it should be... hmm.
What went wrong was that you forgot to check the input voltage, it was too low. But why it did work with the wire is that is has no voltage drop, and that was enough to get 100mA. And even if you did check battery the voltage after that so you made sure that it would maintain regulation you'd still get a false reading because you're measuring the battery without any load on it. By now you will be scratching your head quite a bit asking yourself why it doesn't work... Btw the 1.25V under operation is your receipt that it regulates properly.

 

[drlava]

heheh that's a good reason, too.