Adjusting current in a module contains LM2596 and 3 pot

Hi.
I was trying to adjust the output of this LM2596 3 POT to 1.8VDC and 20mA. The voltage potentiometer(left pot)was not a problem to be adjusted to 1.8VDC,but i couldn't adjust the current potentiometer(right pot)to 20mA. The instructions says to connect the output to ammeter(dmm)at 10A scale and adjust the current(without aload),i did that,but even if i am turning counterclockwise the screw all the way left,the current is not get lower than 20mA(while the voltage is setted on 1.8v).The only way to get the 20mA was to raise the voltage above 1.8v and then the current potentiometer let me to decrease the current to 20mA.And if i want to decrease the current more,i have to increase the voltage more.Since that,i can't adjust the 20mA current to the voltage that i would like to adjust(1.8v).
Why does it behave like that?Is it normal?How can i achieve the 20mA at 1.8V?
Thanks.

Hi,

it is not the fault of the module.

It´s the problem of your setup: wires, meter, connectors...

Show us the whole story.

Klaus

Your considering the conditions stipulated in the Iq specs ?



Note 5.

Current limit, what is the feedback network R value ?

Schematic of your connections to pots, etc.... ?

How you measuring current , AC or DC ?

You trying to make a bench supply that goes to 0V, 0 Amps ? This would not be
the approach recommended. A more linear solution would be usually applied, or a compound
approach.

Regards, Dana.

I reviewed LM2956 module description at AliExpress. Current range 3 A, using a PCB trace as measurement shunt. Simply assume that small currents can't be regulated due to OP offset voltage.

Thank you very much for all the help.I guess that FvM answer solved my question.
And yes,it is rated 0-3A and it has a pcb trace that apparently is used,as FvM
said,as measurement shunt.
Klausst,i don't have something to add,but i can say that,if i connect the output
directly to a 5mm led(instead to the ammeter),then i also can reduce the current
to 20mA and even less(though the instruction say to make the adjustment only
while the output are connected to the ammeter),but when the output is connected to the ammeter(10A scale,as the instructions say)i can't reduce the current to 20mA or less,unless i increase the voltage.
Thank again.

xchcui said:

,if i connect the output
directly to a 5mm led(instead to the ammeter),then i also can reduce the current
to 20mA and even less(

Click to expand...

How do you know it's less?
Are you going by the brightness of the LED?

crutschow said:

How do you know it's less?
Are you going by the brightness of the LED?

Click to expand...

Hi,crutschow,i think i know you from somewhere
I connect the led in series with the ammeter and i see the
current decrease less than 20mA,(without the need to increase the voltage),while i turn counterclockwise the current trimpot screw.
I can't achieve that if i connect the output directly to the ammeter(making short,as the
instruction says).So,i can get low current at 1.8V,but this is not the right way to do that,
according to the instruction.

How many LEDs? what color?

D.A.(Tony)Stewart said:

How many LEDs? what color?

Click to expand...

Tony,i was using the led only to check the module.
It was 1 clear red 5mm led.
I just saying that follow the instruction you can't have lower current,
but when connecting the output to a led,the current automatic decrease
and you can lower the current even more,but againthis is not the right
way to do that,according to the instruction and there might be a reason to make
the adjustment without a load,but is connected to the ammeter(making short).

If a Red LED is 20 mA at 2~2.1V then at 1.8V it will certain be much less than 20mA as a 5mm type ESR is about 10 ohms above 1.85V and depending on quality ESR might vary 50% and determine the value if rated Vf @ 20 mA.
Verify your Vf and If with an accurate method.


I know that besides thermal effects Vf of any LED depends on the threshold voltage at 10% of rated current and the quadratic current characteristics are negl’ble compare to the linear bulk Rs or ESR. The bulk Rs or ESR is always related to max power rating such that Rs=k/Pmax at 85’C=Tj where k=~1.0 for tiny LED or cheap diodes and can be as low as k=0.25 for high quality power transistors, Diodes & LEDs.

Thus low power or cheap LEDs and all diodes will have a tolerance for Vf of ~50% tolerance is only due to Rs and not the Vt threshold at 5~10% Idc max. Yet modern ultra bright processes have much lower tolerance and k factor. You won’t find this anywhere in journals. It came as a result of inspecting the best and worst LEDs 16~32 kcd white 5 mm 30 deg. from my experience sourcing over 1 million LEDs for one client only to custom specs. for ofver 10 yrs.

Hi,

I´m sure there is a reason ... but one needs to have the complete schematic to investigate in detail.

Klaus

Thanks again for your help.The most important thing is that i know now,that the behavior of the module is normal for that type of module and this is not a fault(as FvM suggested).If i could find a schematic,then i would certainly attached one,in order to get more information,but i couldn't find one(excluding the module with just one trimpot-voltage).
BTW,Tony,my question was more related to the module itself than
the led information,but your explanation about the led helped me to understand some facts that i wasn't aware of before,about the led.
Thanks.

That was my reason for details. Either your voltage or current was wrong
It cannot be 1.85V and more than 20 mA

D.A.(Tony)Stewart said:

That was my reason for details. Either your voltage or current was wrong
It cannot be 1.85V and more than 20 mA

Click to expand...

Yes,it gave some clue,why the module can be adjusted to lower current with the led,while in short with the ametter it couldn't.Thanks.

xchcui said:

.If i could find a schematic,then i would certainly attached one,in order to get more information,but i couldn't find one(excluding the module with just one trimpot-voltage).

Click to expand...

Is it really that hard?
I mean we know it is an "LM2596 module".
And since a search gives mayn unsuitable results .. wen need to refine the search.

So what is special to your module? What makes it different to the ones you found?
It is that you can adjust voltage AND ADDITIONALLY CURRENT.

In short: CV stands for constant voltage and CC stands for constant current (the operating modes of your modules)

So a search could be "LM2596 CC CV module schematic".
--> try it.

Klaus

KlausST said:

Is it really that hard?
I mean we know it is an "LM2596 module".
And since a search gives mayn unsuitable results .. wen need to refine the search.

So what is special to your module? What makes it different to the ones you found?
It is that you can adjust voltage AND ADDITIONALLY CURRENT.

In short: CV stands for constant voltage and CC stands for constant current (the operating modes of your modules)

So a search could be "LM2596 CC CV module schematic".
--> try it.

Klaus

Click to expand...

It is really hard when you don't type a specific sentence in google engine.
You have much better knowledge in electronic and circuit from me,you probably know how this circuit work,so it easy to you to choose the right sentence(based on this module)in order to find the schematic in google.
And yes,you are right,when i typed"LM2596 CC CV module schematic",i get a schematic that i think,fits to this module:

btw,the component in the black circle is the third trimpot(charge control,right?
There is the CV trimpot,the CC trimpot and the middle one:charge-control.

xchcui said:

the component in the black circle is the third trimpot(charge control,right?

Click to expand...

How do you come to "charge control"? ... it says "LED".

I don´t know the module ... but if "LED" is correct, then I guess it´s LED brightness.

*******

xchcui said:

You have much better knowledge in electronic

Click to expand...

I don´t think general internet search has something to do with electronics.
For example:
If I want to find something about a bottle (just randomly).
You do a search for "bottle" .. then a lot of unsuitable results pop up.
So you need to refine.
Your bottle is 500ml --> so do a search for "Bottle 500ml"
your bottle is blue --> "bottle 500ml blue"
stilll too much unsuitable results? --> manufacturer, cap form, bottle form, what´s the liquid inside ...
and so on.

Was there anything electronic related? NO.
Am I into bottles? NO

Klaus

KlausST said:

How do you come to "charge control"? ... it says "LED".

I don´t know the module ... but if "LED" is correct, then I guess it´s LED brightness.

*******


I don´t think general internet search has something to do with electronics.
For example:
If I want to find something about a bottle (just randomly).
You do a search for "bottle" .. then a lot of unsuitable results pop up.
So you need to refine.
Your bottle is 500ml --> so do a search for "Bottle 500ml"
your bottle is blue --> "bottle 500ml blue"
stilll too much unsuitable results? --> manufacturer, cap form, bottle form, what´s the liquid inside ...
and so on.

Was there anything electronic related? NO.
Am I into bottles? NO

Klaus

Click to expand...

I came to charge control(see my photos in my first post,middle pot),
since the module has 3 pots and even that it its says led at the scheme,i pretty sure that it is related to the middle trimpot at the photo(charge control)as it is 10K pot like the 2 others.
In regard to the internet search:yes,the procedure that you described
is the standard way to find something in the web,but if you are specialized in the specific field,you have more vocabulary to use in order to increase your chance to find an hard to find information.But we are deviating from the topic.Thanks for the schematic.