Buck converters

Hi experts.

Being new to electronics, I got this unanswered question about adjustable buck converters (dc step down converter modules) - see Buck Step Down LM2596 Power Converter Module DC 4 0 40 to 1 3 37V LED Voltmeter | eBay.

They are usually built around a LM2596 IC, which uses 2 resistors to adjust the output voltage.

On my module, the first resistor is fixed and the second one is a multi-turn potentiometer.

The module should be able to handle 37V / 2A, but the potentiometer is only rated 1/2 watt. So why is it that 1/2 watt is enough for this application?

I want to replace the multi turn potentiometer with a rotary potentiometer. Does anyone have a clue how much resistance the potentiometer should provide? I guess the higher the better?

Kind regards,

Klaus

Parents

jw0752 over 9 years ago

Hi Klaus,

The small blue board mounted potentiometer will have a number on it. It is coded just like the small components. For example it will be a 3 digit number. The first 2 numbers are significant and the third digit is a times ten multiplier. In this way 1 0 2 becomes 1000 ohms, 1 0 3 becomes 10,000 ohms, 5 0 2 becomes 5000 ohms, and so on.

John
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Former Member over 9 years ago in reply to jw0752

It has 2 numbers printed on it.

One is 123 T and the other is W 503.

So I guess either the max resistance would be 12000 ohms or 50000 ohms. Guessing its 50000.

R1 on my module is a 1800 ohm resistor.

Using the vout formula from the LM2596S datasheet:

vOut = vRef * (1 + R2/R1) where vRef is 1.2V, I get that the max output voltage would be 34.5V.

That sounds reasonable.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
jw0752 over 9 years ago in reply to Former Member

Hi Klaus,
The 50,000 ohms is the correct guess. The ones I have used have also all been 50,000 ohms. The LM 2596 chips does the heavy work of supplying the voltage and current to the output. All the resistor / potentiometer divider does is provide a reference voltage that tells the LM 2596 what to do. The LM2596 itself uses switching technology to send the proper duration of pulses to the output to create the output voltage. Feedback of the output voltage is compared to the reference voltage you have selected with the potentiomenter and adjustments are then made by the LM 2596 to keep things stable. If you want to learn more about switching converters check out this nice book by Recom:

http://www.recom-power.com/de/americas/downloads/book-of-knowledge.html

John
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Former Member over 9 years ago in reply to jw0752

Thank you for your fine explanation
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel

Reply

Former Member over 9 years ago in reply to jw0752

Thank you for your fine explanation
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel

Children

No Data