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Change Pot in Mini Buck Converter

ThomsCircuit

Active Member
Got this BC and i want to change the pot with something more multi turn. Can you help me figure out what size pot to use?
My goal is to use this to make a simple power station to test circuits. It will have output pins for two adjustable voltages along with the full power (12v) from the input. So three outputs. 2 adjustable 1-12v and one 12v non adjustable. It will also have lcd voltage readouts and share a common ground.
  • Input Voltage: 4.75V-23V
  • Output Voltage: 1.0V-17V
  • Output Current: Peak 3A, long 1.8A
Alternatively if you think this is junk and want to make a better one then im prepared to do that as well

1646753068511.png
 

rjenkinsgb

Well-Known Member
Most Helpful Member
There are markings on the pot, but the photo is not quite detailed enough to see what they are?
Or just remove the pot and measure it.
 

ThomsCircuit

Active Member
Well whad-a-ya-know. A schematic. I did not think it would be that easy. So its a 220k pot.
Could you tell me what L1 (2uH) is?
1646755086728.png
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Could you tell me what L1 (2uH) is?
That's the inductor, the big ferrite-looking block on the module.

See this concept diagram:

BuckConv.png


The inductor (symbol L) is something like the equivalent of a flywheel in a mechanical system; it takes time for current to build up or die down, with energy being stored as a magnetic field.

When the switch is opened, the current through the inductor continues and has to be drawn through the diode, feeding the load. You could consider it as feeding the load using the back EMF from the inductor.
By adjusting the switching duty cycle and with a high enough switching frequency, the average current can be adjusted and in turn that controls the voltage across any given load (R in that).
 

ThomsCircuit

Active Member
When the switch is opened, the current through the inductor continues and has to be drawn through the diode, feeding the load. You could consider it as feeding the load using the back EMF from the inductor.
By adjusting the switching duty cycle and with a high enough switching frequency, the average current can be adjusted and in turn that controls the voltage across any given load (R in that).
Thank you. I may build one. If just for the experience.
 

schmitt trigger

Well-Known Member
Most Helpful Member
I once modified an identical unit, for the same reason that you mentioned: with the original p.o.s. pot, is impossible to adjust the voltage accurately.
A multiturn pot can be substituted. You will need a needle point iron because the solder pads are tiny and the main inductor is a mighty obstruction.
But with a little patience, you’ll be successful.

PD: you will appreciate the irony of the multiturn pot costing more than the complete unit.

Edit: plan on soldering additional input and output capacitance. I used 100u because it was available and reduced the terrible ripple of the original unit.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Why not just buy a different module?, one that already has a multi-turn pot - there are lot's of them available.
I've got to agree, if it's anything more than experimentation with the module itself.

This style with an LM2596 are very common and very cheap, rated for up to 3A max.
I've used quite a few in various projects and equipment.


 

ThomsCircuit

Active Member
Why not just buy a different module?, one that already has a multi-turn pot - there are lot's of them available.
I spent some time looking at different modules to purchase and in my interest to know what is a "better" module i ran into this article. He says his module is efficient and can handle about 3 amps. His design uses MP2315. While i dont know one from the other he built this following the datasheet specs then performed numerous tests.

I have learned what makes a good buck is its efficiency and low noise. This is 97% efficient and has low noise & ripple. He accomplishes the input noise issue with capacitors and ads an additional inductor and a pair of caps for the ripple.

I am able to acquire all the parts as through hole. The 35V caps were difficult but i did eventually locate them as Electrolitic SMD.

Ill post the link and schamatic here. Please provide your feedback.
DC DC Buck Converter
1132491215162t.jpg

1147192893878t.png
 

ThomsCircuit

Active Member
I've got to agree, if it's anything more than experimentation with the module itself.

This style with an LM2596 are very common and very cheap, rated for up to 3A max.
I've used quite a few in various projects and equipment.


LM2526 VS MP2315
Can you provide your opinion about these two modules? Are they interchangeable?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The LM2526 is a metal-tabbed package which appears far more robust & more likely to survive abuse.

None of the buck or boost modules I've tried that use tiny ICs have been able to stand even their supposed rated loads, without getting crazily hot or just failing.

That's may be down to the components used. However in my opinion, unless you must miniaturise something, it may as well be built to last with decently rated and capable parts.
 

eTech

Well-Known Member
I spent some time looking at different modules to purchase and in my interest to know what is a "better" module i ran into this article. He says his module is efficient and can handle about 3 amps. His design uses MP2315. While i dont know one from the other he built this following the datasheet specs then performed numerous tests.

I have learned what makes a good buck is its efficiency and low noise. This is 97% efficient and has low noise & ripple. He accomplishes the input noise issue with capacitors and ads an additional inductor and a pair of caps for the ripple.

I am able to acquire all the parts as through hole. The 35V caps were difficult but i did eventually locate them as Electrolitic SMD.

Ill post the link and schamatic here. Please provide your feedback.
DC DC Buck Converter
View attachment 136107
View attachment 136108

A few parts will have not so obvious critical values for the design to hold spec.
Some of these are C1,C2,C4,C5,C7, (esr) and L1,L2 (serial resistance).
 

ThomsCircuit

Active Member
The LM2526 is a metal-tabbed package which appears far more robust & more likely to survive abuse.
Thank you. Im glad the module is good. Im excited to build this. I spent a few hours and drew a schematic and PCB. While I do have 1/8 watt resistors (0805) and 100nf caps are also 0805 I am using Electrolytic Caps rated 50V for C1,2,4,5 & 7. L1 & L2 are through hole.
A question i have is would it be an improvement if i used 1/4 watt resistors?
 

ThomsCircuit

Active Member
A few parts will have not so obvious critical values for the design to hold spec.
Some of these are C1,C2,C4,C5,C7, (esr) and L1,L2 (serial resistance).
Meaning they can be rated less that 35 / 25 volts?
could you elaborate on the Inductors (serial resistance)
This is what im using. 1/4watt Through hole

1646865248947.png
 

ChrisP58

Well-Known Member
Most Helpful Member
Meaning they can be rated less that 35 / 25 volts?
could you elaborate on the Inductors (serial resistance)
This is what im using. 1/4watt Through hole

View attachment 136111
ESR is Equivalent Series Resistance. This is a real resistance that is inherent in all real world components. And, like a resistor, heat will be generated when current flows through this resistance.

For a capacitor, while no DC current flows through it, in a switching power supply like this, there are pulses of current that flow in and out of it. A capacitor with a higher ESR than another, will generate more internal heat.

For an inductor, the same is true. The current through the inductors will cause heat in the parts just like it would in a resistor.

Inductors also have two current ratings. One is the saturation current. That is the level of current where it no longer behaves like an inductor, and is just a piece of wire. The second rating is temperature rise. This is the point where the inductance falls off. The temperature of an inductor will be the combination of it's ambient plus the rise over ambient due to it's operation.

The operational current through the main inductor in a buck converter is not constant, but ramps up and down. It ramps up when the switch is on, then ramps down when the switch is off. This is known as the free wheel period. See the figure in post #4 above. The output current will be the average of these two periods.

The datasheet gives more details about choosing the actual components for a good design.

PCB layout is also very important is switch mode power supplies. It's not just about connecting everything up to match the schematic. You need to understand where all of the current loops are and make them as small as possible.
 

ThomsCircuit

Active Member
PCB layout is also very important is switch mode power supplies. It's not just about connecting everything up to match the schematic. You need to understand where all of the current loops are and make them as small as possible.
Thank you for the information. Ive made changes to the PCB layout. I do think this is better suited for the components functions. Please let me know if ive got this right.

BUCK PCB2.png
 
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