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Fixed voltage on cheap buck converter (MP1584) conversion with single 0805 resistor

Everywhere I look on the Ali and Ebays I see these step down converters based on MP1584.
I bought a couple and actually they are not bad at all.

The output voltage is set by the trimming-resistor in the left top corner of the picture.
This works OK, but.. it is dangerous because it is rather sensitive to the touch.

I decided that I wanted fixed output, so I had to figure out how this thing worked.
The datasheet looks like this :

R1 in the datasheet is what I call "R feedback" in my image.
The value of R2 is mostly 8.2K Ohm in the boards I have seen. 

So to set some common values for output voltage:
change R1 to 27K for 3V3 output (actually 3.4 volt, but 27K is a standard value)
change R1 to 43K0 for 5.0 Volt output. (43 K is a standard value)
A standard 0805 size resistor fits precisely, how convenient ;-).

Make sure the other resistor is really 8.2K because that determines the feedback ratio.

This way you can not burn up your circuits by accident (or some funny clown turning the potentiometer)

There is an online tool to help you figure all the values out :


JHM said…

I've been looking at running one or two of these off a PS from a Sun array that has a very solid 12V/5V output at ~23A and 30A respectively to get a couple of additional variable outputs for a bench, with a heatsink on the other side of the the PCB, of course.

I know it is an old post but ...
Do you recall the value of the variable resistor?

I was wondering if it could be substituted with a multi-turn wire pot.


OETEL-X said… will help you.

But as a rough ballpark value... I would use a 50k pot, that should get output values from about 1 volt upto just above 5 volts, which is what I need most of the time.

Good luck with experimenting. If you have any succes please post a reply.
JHM said…

Thanks for taking the time to write back. =-)
And for the link.

But it takes me to a place where I am rather over my head: I'm just a hardware tinkerer who manages to solder some, albeit not too well.

My idea is to keep this thingy well within it's specs, replacing the original SMD pot with a multi-turn one of the same value so as to get roughly the output the manufacturer of the board *says* you can get but with the possibility of a finer and more reliable adjustment and a higher thermal stability.

I guess a 12V input can get me a decent range from 5V to 11.5V and if I find I need a range below 5V, I could maybe add a second board for that range.

Page 14 of the spec sheet indicates that the chip cools through thermal conduction from a pad on it's underside exposed to the PCB and the ground plane through some tiny holes on it, thermally connected with some solder.

But I doubt the copper is the proper gauge for that, at least not at this price point.

I've seen some tests on-line that show it is not good over 1.0A as the built-in thermal protection will kick in, but maybe a decent sized heatsink taped to the underside of the board could help.

Nevertheless, I forsee that mounting these things properly will be a challenge.


OETEL-X said…
Ok, I will take a look at this.
So we assume input is >= 12 Volt and below 15 V.
Output current is up to 1.0 Amp.
Output voltage is between 5.0 and maximum, about 11.5.
We need to adjust the UVLO (under voltage lockout, so if voltage drops below required output voltage the thing will shut down instead of hicking and behaving badly)
What is the exact load/application you want to use this thing for?
JHM said…

> Ok, I will take a look at this.
Thanks a lot ... =-)

> ... assume input is >= 12 Volt and below 15 V
This is a FDK PEX737-40 PS made in Japan for Sun Microsystems.
Has no SMD components on the board so it is rather large by today's standards but it is impressively well built. Like the HP/IBM iron from long ago.

Label states 'total max. continuous 360W' +5V 30A (L3) and +12V 23A (L5).
Given the OEM and quality of Sun hardware, I expect the output tolerances and regulation to be very tight, so maybe 15V is too high.

> ... between 5.0 and maximum, about 11.5.
Yes, I have 5V from one rail and 12V from the other.
Don't think the converter will go over 11.5V.

WRT the current, if using a heatsink will get a stable 1.5/2.0 that would be ideal. I will probably be adding a polyswitch to the variable output and 20mm fuses to the 2x12V and 2x5V outputs.

> What is the exact load/application ...
Just for the usual tinkering stuff, nothing specific.

My RPi3B+ complained at boot because the crap PS I was using dropped a bit for just a under 1.5 seconds as noted in dmesg but then recovered.

The cause was the old 40Gb portable IDE HDD drawing power from the same PS and I did not have a bench PS to test that, which led me to get this out of the closet to see if I could repurpose it. I try to recycle any hardware that comes my way.
OETEL-X said…
Ok, I think you want to give every critical device its own POL (point of load) regulator. So the Raspberry-3B its own 5.25 Volt (yes a bit higher) POL power supply, do not use a polyfuse in series, it will drop too much voltage. The little MP1584 board will survive a short circuit. Also use separate POL regulator for other parts. Connect ground wires in a star topology. Keep wires short. Use decoupling capacitors on the devices, eg. small capacitor added between GND and 5 Volt rail of the Raspberry. Do not feed the raspberry using the USB (bad cables), use the pin headers. Keep voltages stable. Use wires of good thinkness, like AWG 22, real tinned hookup wire. (no thin recycled flat cables)
OETEL-X said…
I think this has been done before... wait... it was me : Here on this blog
JHM said…

I'm sorry, my text was rather confusing: the exSun PS is not for the RPi, it is to have a general purpose PS to be able to check problems like the one I had with the RPi and just test hardware/stuff to see if it works.

ie: 2x+5V, 2x+12V and the +5V -> +11.5V variable output, if possible up to 2.0A.

The RPi PS problem got solved with a small 5V/9.5A 12V/1.2A 50W PS from a router to which I had to add a 10W 10Ω load so that it would put out a stable 5.25V. I connected the outputs to the remains of an old laptop plug-in type USB1.1 hub, shorting the data lines and adding 30V/1.5A polyfuses to each of the four ports. The power cable to the RPi is a short (20cm) charging cable from one of my Blackberrys. It's AWG28, works a charm.

Only thing I don't have is the decoupling capacitors.


JHM said…

I'll have a look ...

And yes, I was hoping for something more from my RPi3B+
But besides the fact that it is 64bit, my Asus 1000HE does more or less the same and has 2.0Gb instead of 1.0.

BTW: bright white lettering on a bright yellow background is very hard to read.


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