Just For LM2596 Users

[BJ43]

Here is the assignment for the day. On my Batt/PT I have different (in) voltages, 7.4v from two 14500, 12 to 18v from wall sources, 14.6v from cig lighter in the truck. So what new pot and resistors should I use on a 2596 that comes with a 10K for 3.5 to 6.5v out?

 

[TomCatt]

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[Java_Az]

So what new pot and resistors should I use on a 2596 that comes with a 10K for 3.5 to 6.5v out?

What is the number on the R1 resistor on the board. Need that resistor value to calculate the proper pot. You will also have to add a resistor with the new pot to get a 3.5 output.

after looking at the data sheet my guess is it is somewhere around 300ohm ,I don't think they used a 1K for r1 like the data sheet says.

 

[BJ43]

What is the number on the R1 resistor on the board. Need that resistor value to calculate the proper pot. You will also have to add a resistor with the new pot to get a 3.5 output.

after looking at the data sheet my guess is it is somewhere around 300ohm ,I don't think they used a 1K for r1 like the data sheet says.

Don't know the number or even where it is on the board, don't see anything that looks like a resistor with colored bands.

 

[Java_Az]

Don't know the number or even where it is on the board, don't see anything that looks like a resistor with colored bands.

It is a SMD resistor should have a 3 digit number on it. Tiny little thing next to the trimmer. At least thats where it is at on most of these lm2596 ebay boards. I dont have any here yet( slow boat from China is really slow) but that 3 digit code is what i need


Edit : here is a pic of the board i circled the two places the resistor usually is.

 

[WillyB]

Exactly, the OKR reg (assuming 7.4 V in) employ a 220 ohm resistor and a 200 ohm trimmer. From what I understand you could go with a single 500 ohm trimmer but you lose adjustment granularity...it makes sense that the smallest value trimmer that would give you your desired v out range is the best

There are lotsa choices for the Murata. But a limiting resistor in series should be used regardless, that will give you the correct, in spec, top voltage. CapeCad was the first to show it and he uses a 1K trimmer + the official 2.18Ω limit. But not using the limiting resistor with your trimmer to me is just hack modding. What do you save 5¢?

The datasheets on the Murata (and the TI) are pretty clear.

Programmable output voltage from 0.591-6.0 VDC

Do not exceed the specified limits of the output voltage or the converter’s maximum power
rating when applying these resistors.

TI's says it a bit differently.

The adjustment range is from 0.6 V to 5.5 V.

The minimum resistance between VOAdjust and GND is limited to 240 Ω. (note that is 5.5V)

I'll probably slap a Murata together using a ~245Ω resistor and a 100Ω trimmer. That will more than cover the range I use.

If you are playing around with the Murata you can use this as a quick, general guide to voltages.

 

[BJ43]

It is a SMD resistor should have a 3 digit number on it. Tiny little thing next to the trimmer. At least thats where it is at on most of these lm2596 ebay boards. I dont have any here yet( slow boat from China is really slow) but that 3 digit code is what i need


Edit : here is a pic of the board i circled the two places the resistor usually is.

I see the two spots on each side of the pot but no numbers.

 

[BJ43]

Tried to get a pic, I have to wait for some switches to finish my mod so if your ship gets in let me know what you find out.

 

[Java_Az]

I see the two spots on each side of the pot but no numbers.

Can you take some close ups of both sides of the board. There should a resistor somewhere on that joker

 

[Java_Az]

you beat me too it , still cant really tell but it looks like there might be on in the yellow circle

 

[BJ43]

you beat me too it , still cant really tell but it looks like there might be on in the yellow circle

View attachment 95738

It looks like one on each side but no numbers

 

[BJ43]

there is a large # JG-CZ005

 

[Java_Az]

It looks like one on each side but no numbers

That usually means it is a Capacitor. There should be a R1 resistor though. The voltage is set by using a voltage divider so there should be a R1 and R2 . In the case of variable voltage R1 would be a fixed value resistor and R2 would be a variable resistor ( AKA trimmer pot ). But thats going by the data sheet no telling how the circuit designer actually designed it. I suppose he could have left it off.

 

[BJ43]

There is something on each side.

 

[Java_Az]

should be something like this

I did cheat though and used my digital microscope

 

[BJ43]

Will put it under the microscope tomorrow.

 

[smp1990]

Will put it under the microscope tomorrow.

my lm2596s should be here any day, now i want to get a microscope to inspect them lol

 

[BJ43]

The R1 looks like IEE or 1EE

 

[Java_Az]

this might not be right still on the first cup of coffee. The max output of that reg is 25 volts ( Voltmeter LED display LM2596 DC-DC step-down adjustable voltage regulator module | eBay)

In order to get a 25 volt output with a 10k trimmer R1 would have to be 500ohms. So there should be a 500 ohm resistor somewhere on that board. Adding a 922 ohm resistor to a 1.2k pot would give you 3.5 volts to 6.5 volts. No such thing as a 1.2 k pot so i would go with a 1 k resistor and a 1k pot . That would give you 3.6-3.7 to 6.15 volts. You could more then likely get closer to your range if the R1 resistor was replaced to a different value. them things are tiny though not sure if you have soldered anything that small before.

 

[TomCatt]

What would be the range if the 10k pot was just replaced with a 1k or w/ a 5k?


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