Why are we using test loads on linear drivers?

[lasersbee]

Well... I'll tell you why I use a Muti-Turn Pot on all my Laser Drivers...

I don't adjust the driver's output directly...
I normally set the output of my driver by the LD's beam Power
as measured on an LPM... by adjusting the Driver's Pot...

I do use a Test Load to test that the Driver is functioning
within parameters first.

BTW... I have read too many posts of blown LDs because a
wrong fixed resistor was used or badly installed...

I always ramp up my drivers from minimum to where I need it
set to... And that requires a Variable Control device....
(ie. Potentiometer)


Jerry

 

[Hemlock_Mike]

I'm kinda like Laserbee. I like to exercise my driver to test its parameters before connecting to a $$$ LD. I look for min and max then turn it down to a safe level.
Having a bunch of dead LD's, I find that the LD which likely died of COD still functions as a LD load. It's good to check your driver in its expected opperating range of V and I.

HMike

 

[Deleted member 8382]

Well, the point is that although one can of course do it your way (and if one has an already build test load and a precision 1 ohm resistor I would also do it), once could bypass all this by just setting the pot with the MM and then checking the current on AMP mode. This would be way more precise and easy than using the classic method.

If we all agree on that, there's nothing else to discuss here xD

 

[netwave]

Wooah! Ive learnt allot today from reading this thread!
brains a bit baked now tho its all good.
:thanks:

 

[Deleted member 8382]

Wooah! Ive learnt allot today from reading this thread!
brains a bit baked now tho its all good.
:thanks:

Then the thread was worth it

 

[Eudaimonium]

Well... I'll tell you why I use a Muti-Turn Pot on all my Laser Drivers...

I don't adjust the driver's output directly...
I normally set the output of my driver by the LD's beam Power
as measured on an LPM... by adjusting the Driver's Pot...

I do use a Test Load to test that the Driver is functioning
within parameters first.

BTW... I have read too many posts of blown LDs because a
wrong fixed resistor was used or badly installed...

I always ramp up my drivers from minimum to where I need it
set to... And that requires a Variable Control device....
(ie. Potentiometer)


Jerry

You are right, but bare in mind that someone who follows the diagram with 2x 10 ohms and a pot (because he does not now how things work) probably does not have a laser power meter.
As I said, I developed an alergy to potentiometers since I've blown 20 good 16x DVD diodes because of trying to set them when power is applied.
"Turn until it starts glowing really bright" My a$$.

Wooah! Ive learnt allot today from reading this thread!
brains a bit baked now tho its all good.
:thanks:

LMAO. If you want to spare yourself of frustration I got through, just get the right resistor for your diode and it is all good. Although, just be sure (and not go against lasersbee's word) measure the resistor with a DMM , just in case the seller made a mistake. Happened on me once!
Also, make sure, if you haven't already, what is the resistance of your DMM's probes... just put your DMM to 200ohms measurement area and touch one probe with the other. Mine shows 0.5 ohms in short circuit of probes. Just a note not to go on your seller for getting the 1.5 ohm instead of 1 ohm resistor...

 

[Deleted member 8382]

The cheap Multimeters will show up to 1.5 ohms!

 

[Eudaimonium]

The cheap Multimeters will show up to 1.5 ohms!

My multimeter was the cheapest I could find, abour 12$ some supermarket, shows only 0.5 PRECISELY.
Well anyway, of there are ones that show 1.5 of test probes, only one more reason to measure them.
Double checking everthing FTWALLD (for the win and less LEDed diodes )

 

[HIMNL9]

@HIML: I suggested the 1ohm resistors for those who only have voltmeters, but discart this one. What if you use the amp mode only. I've done it and nothing happened! This also avoid the incorrections on the measurements due to the 1 ohm resistor tolerance!

Well, about this, it depend from what current you need from the driver ..... til is matter of 100 - 150mA, i think no problem ..... but over, is again matter of power dissipation and thermal protection circuitry.

Usual commercial DMM, have inside a low resistance circuit, for current measurement, and more low is the resistance, better is the instrument (and less influence the working of the circuti), so the manufacturers always try to keep it as low as possible.

Not speaking about lab instruments or old precision digital units (i have, as example, an old Beckmann pro one that have 0,1 ohm on 200mA scale, and a lab unit, the ones in wood boxes, that is 10 milliohm on 100mA scale, and 1 milliohm on 1A scale ), actually almost all the commercial DMMs uses 2 different systems, depending from the manufacturers ..... someone uses 2 diodes in antiparallel, usually BY255 or similars, and a correction circuit that derive the reading from the variation of junction voltages at different currents, and some others have usually an internal resistance of 1 ohm on 200mA scale, and 0,1 ohm on 2A scale, and read them as mV (like you usually do with the dummy load ) ..... this mean that, if you use the DMM directly, depending from the internal circuit, electrically is the same as if you connect a diode, or a low value resistor, at the end of the driver.

Now, always from the previous example, let me made some other hypothesis about regulating the current with 12V input, related to the better case, i mean the DMM with the resistance as reading probe, and keeping in mind that the maximum ideal dissipation of your LM317 in TO220 is 1W, or few more ..... if you're speaking about 100mA, connecting directly your DMM at the end of the driver (that have a dropout of 3V), is the same, electrically, as if you connect an 1 ohm load ..... the power dissipated from the LM317, in this case, is 9 (12-3) multiplied for 0.1 (100mA), so 0.9W, the driver work good, and you have no problems at all doing the regulation (LM317 is a robust chip, can hold also a 50% or few more power dissipation of the rated one).

But suppose now that you want to regulate it for 400mA for an 8X, and you don't use any dummy load ..... you connect the DMM directly to the driver, on 2A scale (cause you don't have other way for read 400mA), and the internal resistance at this scale is 0,1 ohm, practically a short circuit ..... the LM317, now, need to dissipate a power of 9*0.4, that means 3.6W, that is much more high of the power that a TO220 container can dissipate efficently without a heatsink ..... the junction temperature rise over 120 C in few seconds, and the regulator turn itself off for thermal protection intervent (in the worse cases, it burn itself, but usually the thermal protections of these chips are very efficent) ..... this mean that your driver is continuously turning itself off each few seconds, and on again in other some seconds (when the junction temp go under 90 C), and so on, again and again (until it give up and die ) ..... not so easy to regulate anything, in these conditions, right ? ..... especially cause also the current regulation change a little bit also, with high temperature variations .....

In this case, a dummy load that cause a considerable dropout and help dissipating some power, is not just better, but needed, otherwise you probably simply don't reach to regulate your driver ..... always the previous examples and values, if your dummy load have a dropout like a BR, say, 6V, the power dissipated from the LM317 is just 3*0.4 (12V, minus 3V dropout of the LM, minus 6V dropout of the dummy load), so 1,2W, and also if the LM317 become a bit more hot, it still work in good conditions, and you can do your regulation without problems, cause the rest of the power is dissipated from the diodes in the dummy load.

About the pot / trimmers, instead ..... this is not in the topic opened from the OP (sorry), but better punctualize also this, probably .....

I always use (and always suggest to use) two fixed resistors and a trimmer, and never just a trimmer (for variable drivers, ofcourse ..... no need of a trimmer if you set up a fixed current unit ), for safety reason and for not cook too much trimmers ..... cause, usually, the most part of the linear drivers users, don't realize, or don't care, about the fact that the current that is given from the regulator, pass also through the limiting resistor, with the obvious power dissipation, and possible cooking effects

Another pair of examples, about that (uhm, do i get paid for all the examples ? ..... LOL) ..... suppose you want to regulate 100mA, using just a trimmer ..... for 100mA, you need a resistance of 12.5 ohm (1.25/12.5) ..... with a 50 ohm trimmer, this is some millimeters of track, with a 100 ohm trimmer, maybe 1 or 2 mm ..... this current cause a power dissipation, inside the trimmer, of 1.25*0.1, then, 125mW, and the trimmer can also hold it (the most part of the trimmers are 250mW, the multiturn ones, usually, 100 or 150 mW) ..... now, regulate 400mA, with the same trimmer ..... you need 3.125 ohm ..... maybe 1 mm of track, or less ..... apart the fact that in this position, for pass from 400 to 800 mA, is enough probably just shift 0,1mm more, or also less, your end-of-the-track and cursor, now, are dissipating 0.5W, so, if you're not using an 1W wire trimmer, you probably are already seeing a nice little smoke and burning smell coming out from the trimmer, and at this point, you can also just throw it away

If instead you use the 2 resistors, with the trimmer, you have a good possibility to regulation, with less risks ..... and a more precise regulation, too ..... as example, you need 400mA, and have a trimmer of 50 ohm ? ..... use a main resistor of 4.7 ohm, and put in parallel of it your 50 ohm trimmer with another 4.7 ohm resistor in serie to the trimmer ..... this give you a regulation from, approximatively, 300mA to 530mA, with the trimmer that is dissipating only a fraction of the total power, having a more long course in the point where you are regulating the needed current, so more precision, and more stability (also if for vibration the cursor shift a little bit, the variation is less than the one that you have using only the trimmer)

 

[Eudaimonium]

I agree on you on reading the mA with dmm set to amps. Not good, cooks the chip. But I stand beside what I said. Why using thepot? Someone before said: To change the current if later needed, without soldering... In what possible case in this universe, whould you need to change current without soldering? If you have to change it, it means your diode is cooked, or needs to be replaced, or whatever different reason is: you still got to solder...
Well, pointed my point out here... Whatever somebody else says, I'M NOT USING ANOTHER POTENTIOMETER AGAIN!
Period. I'm through pullin' my scalp burning out a LDs beecause of stupid pot.

 

[HIMNL9]

Well, ofcourse if you only need a fixed current, no needs of trimmers at all ..... i use trimmers only on modules that i'm not sure what power i need for, as example, combine two colors when i want to obtain a different one, so i can test different regulations ..... and, ofcourse, when i need to change current on the fly, for experiments.

For fixed drivers in the modules, i also don't use them ..... also cause i usually fill the space through the driver and the module with thermal silicone, for maximize the dissipation of the driver and diode assembly, so, no reasons for put a trimmer on a module, and then dip it in glue

 

[jayrob]

Well, about this, it depend from what current you need from the driver ..... til is matter of 100 - 150mA, i think no problem ..... but over, is again matter of power dissipation and thermal protection circuitry.

... But suppose now that you want to regulate it for 400mA for an 8X, and you don't use any dummy load ..... you connect the DMM directly to the driver, on 2A scale (cause you don't have other way for read 400mA), and the internal resistance at this scale is 0,1 ohm, practically a short circuit ..... the LM317, now, need to dissipate a power of 9*0.4, that means 3.6W, that is much more high of the power that a TO220 container can dissipate efficently without a heatsink ..... the junction temperature rise over 120 C in few seconds, and the regulator turn itself off for thermal protection intervent (in the worse cases, it burn itself, but usually the thermal protections of these chips are very efficent) ..... this mean that your driver is continuously turning itself off each few seconds, and on again in other some seconds (when the junction temp go under 90 C), and so on, again and again (until it give up and die ) ..... not so easy to regulate anything, in these conditions, right ? ..... especially cause also the current regulation change a little bit also, with high temperature variations .....

If instead you use the 2 resistors, with the trimmer, you have a good possibility to regulation, with less risks ..... and a more precise regulation, too ..... as example, you need 400mA, and have a trimmer of 50 ohm ? ..... use a main resistor of 4.7 ohm, and put in parallel of it your 50 ohm trimmer with another 4.7 ohm resistor in serie to the trimmer ..... this give you a regulation from, approximatively, 300mA to 530mA, with the trimmer that is dissipating only a fraction of the total power, having a more long course in the point where you are regulating the needed current, so more precision, and more stability (also if for vibration the cursor shift a little bit, the variation is less than the one that you have using only the trimmer)

Also, what ever component you use to regulate a current such as 400mA's (your pot, or resistor), needs to be rated for at least .5 watts.

1.25 X .400(mA's) = .5 (watts)

Many pots are not 1/2 watt pots. That's why when I built Dave's 8X driver, I had to order a special pot for his 512mW blu-ray. (picture shown here)
http://laserpointerforums.com/f51/pioneer-8x-blu-ray-bdr-203-a-36790-5.html#post500740

 

[HIMNL9]

@ jayrob: exactly this ..... i mean, speaking about cooking trimmers .....

In that example, if you use the values that i gave you, you need half watt resistors ..... the fixed one dissipate 330mW, and the serie trimmer-resistor, in parallel with it, the other170 mW, at 400 mA (but, ofcourse, if you increase it, also the dissipation of the variable branch increase)

But, if you have only 150mW trimmers, and if you need only precision regulation in a short interval, you can calculate the resistors in different ways ..... with the same curent, as example, suppose you want to precisely regulate the current just around the 400mA ..... use a main 3,5 ohm resistor, with the 50 ohm trimmer in serie with a 10 ohm resistor, and let see the values ..... max trimmer value, 3,5 ohm in parallel with 60 (50 + 10), give you 3,3 ohm, for a minimum total current of 378 mA, and minimum trimmer value give you 3,5 ohm in parallel with 10 ohm, that is 2.6 ohm, for a maximum total current of 480mA (so you can regulate ONLY from 378 to 480 mA) ..... the fixed resistor is 3,5 ohm, and pass a current of 357 mA, dissipating 446mW, and the variable branch pass from 21 to 123 mA, dissipating from 26 to 153 mW ..... the regulation is more precise, and also a common 150mW trimmer can be used ..... and so on

 

[Eudaimonium]

Well, ofcourse if you only need a fixed current, no needs of trimmers at all ..... i use trimmers only on modules that i'm not sure what power i need for, as example, combine two colors when i want to obtain a different one, so i can test different regulations ..... and, ofcourse, when i need to change current on the fly, for experiments.

For fixed drivers in the modules, i also don't use them ..... also cause i usually fill the space through the driver and the module with thermal silicone, for maximize the dissipation of the driver and diode assembly, so, no reasons for put a trimmer on a module, and then dip it in glue

When doing multicolor experiments, I bet that you also have a fixed resistance in series with cap so you cannot overshoot, and destroy the diode. Right?

 

[Deleted member 8382]

okay, so this is at the end turning back xD

So at this point, you don't need to use diodes at all for a test load on a LM317 based driver, but you need a load. I'll ask you directly (HIML9), what resistor to place in series with the MM to disipate some heat? I mean a value that works for all the diodes we actually have

 

[lasersbee]

Hey Albert...

You don't even need to use an LM317 Driver Circuit....

You can take your PS/Batteries and put a "calculated" resistor in series
with your DMM on Amps and check that you have the Current you want
to run your Laser Diode... without a test load...

The choice is ultimately up to the builder...

I personally (through years of R&D experience) have learned that there
is always a better/safer/less expensive in the long run way of doing things...

I really don't care if someone wants to use a Test Load or not... It is
their choice...
Their Laser Diodes are not mine...


Jerry