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dentex
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24V to 5V DC to DC conversion

Mon Dec 15, 2014 5:04 pm

Hello.
I'm almost ready to use a LED fixture I've built with my RPi (for PWM). I'll be using a 24V, 5A (60W) power supply and three driver (700mA each) to drive some LEDs. So I'll have spare 2,9 amps left. I was thinking about using a step down DC-DC converter to power the RPi with the 5V needed (using the micro-USB port anyway) in order to remove its basic power supply and simplify a bit the whole project.
I saw something similar to this: http://www.aliexpress.com/item/DZ199-DC ... 87146.html ... available through various resellers around the world at similar prices.
bc.jpg
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What do you think about? Is it feasible? Does it have any drawbacks?
To recap some of its features:
Module property: Non-isolation buck
Rectification mode: Synchronous rectifier
Input voltage: 4V-35V
Output voltage: 1.23V-30V
Input current: 3A(maximum)
Conversion efficiency: 92%( highest )
Switching frequency: 150KHz
Output ripple: 30mV( maximum )
Load regulation: ±0.5%
Voltage regulation: ± 2.5%
Thanks in advance for any input.
Last edited by dentex on Wed Dec 17, 2014 8:48 am, edited 1 time in total.
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

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joan
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 5:11 pm

It should work fine.

However I'd prefer a UBEC as it does the same job but always outputs 5V.

The unit you link to will have to be calibrated to generate 5V from your input voltage by turning the screw. If the input voltage changes (I think) it will need to be recalibrated.

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dentex
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 5:19 pm

Thanks joan,
The regulation screw is something I was thinking about too... is it stable? I also don't know how could the input voltage change.

In the mean time I'll also check what is an UBEC. ;)

EDIT: found it; so you meant this: http://www.aliexpress.com/item/Universa ... 31355.html
or this: http://www.adafruit.com/product/1385
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

Stupotmcdoodlepip
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 6:02 pm

These types of switching regulators typically have a pot-down from the output into a feedback pin, which is normally set at around 1.23-1.25V, so a change in the input voltage should not effect the output voltage. However, those potentiometers aren't brilliantly stable over large temperature ranges. Also if it accidentally gets knocked it could inadvertently increase the output voltage.
It would probably be fine depending on the application, but it would be pretty easy to replace the pot with a couple of 1% 0805 SMT resistors. You'd just need to calculate the correct values.
For example, assuming the feedback voltage is 1.25V and you want 5V output, you're feeding a quarter of the voltage back in, so you'd need a 4:1 pot-down. E.g., 15k from output pin to feedback pin and 4k99 from feedback pin to ground (those are both available values in the 1% (E96) range).
Check the feedback voltage for the device first though before using those values :P

Cheers,
Stu

*Edit:
Based on the specs you provided, I'd guess that's an LM2596 regulator which has a typical feedback voltage of 1.23V under "normal" operating conditions. A 4k87 and a 15k resistor would give you 1.23 / (4.87 / (4.87 + 15)) = 5.02V output.

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ilguargua
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 6:16 pm

I've used one of this, very small and easy : just 3 terminal, in same way and order of the 78XX linear regulator IN-GND-OUT, nothing to adjust. It's rated for 1 amp, and tahat was ok for my raspy B. RS has it in stock (811-1423), maybe can be found even from cheaper supplier.


Cheers, Ale.

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joan
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 6:24 pm

Stupotmcdoodlepip wrote:These types of switching regulators typically have a pot-down from the output into a feedback pin, which is normally set at around 1.23-1.25V, so a change in the input voltage should not effect the output voltage.
...
Thanks. I have several similar myself, just have never got around to trying different input voltages after calibration.

As the OP says there is no real reason for the input voltage to change in any case so it would be suitable. It does have the advantage over a UBEC of offering a different output voltage if needs change.

Stupotmcdoodlepip
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 6:41 pm

ilguargua wrote:I've used one of this, very small and easy : just 3 terminal, in same way and order of the 78XX linear regulator IN-GND-OUT, nothing to adjust. It's rated for 1 amp, and tahat was ok for my raspy B. RS has it in stock (811-1423), maybe can be found even from cheaper supplier.


Cheers, Ale.
78XX linear regulators are fine as long as you don't have an input voltage that is much larger than the output voltage. Otherwise they dissipate a lot of power inside the regulator itself and you need a heatsink. Say for example, you had 12V input and 5V output, the power dissipated by the regulator itself would be (12V-5V)*1A = 7 Watts!
The beauty of switched mode regulators is that they dissipate much less power in the regulator itself and thus can have large differences between the input and output voltages.

Cheers,
Stu

Edit:
Apologies, I missed that you were linking to a DC/DC converter that can be used to replace a 78XX type regulator.

ame
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Re: 24V to 5V DC to DC conversion

Mon Dec 15, 2014 8:08 pm

Many of the "cigarette lighter plug" USB power supplies are 24V capable, and they seem to be easily available.

I see nothing wrong with the module you have identified however.

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dentex
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Re: 24V to 5V DC to DC conversion

Wed Dec 17, 2014 9:13 am

Thanks everyone for all the inputs! Really useful.

Looking at the whole picture, I think I'll use an UBEC. It's cheap, really small (fits easily in my project's enclosure) and can go up to 3A.
I found this one (the same item it's on the adafruit store) or this one.

While the linked Recom converter it's a bit undersized (1A) for my RPi (USB rootfs, webcam and possibly wi-fi dongle, later on), also the "cigarette plug" idea seems good. I have one of them to tear apart so I'm going to happily disjoint it and make some measures, as it doesn't have any specs. :D
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

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dentex
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Re: 24V to 5V DC to DC conversion

Wed Dec 17, 2014 9:38 am

Oh, another thing.
Would be bad to connect in series: the 24V main power supply -> the adjustable regulator (from the 1st post) to 12V -> and then the UBEC to have 5V (for the RPi)?
The reason is that I need anyway the 12V and also the UBEC accept 4.75V to 23V input voltage (containing an MP2307).
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

Stupotmcdoodlepip
Posts: 21
Joined: Sun Jan 26, 2014 6:25 pm

Re: 24V to 5V DC to DC conversion

Wed Dec 17, 2014 11:30 am

That should be fine. Just bear in mind that whatever the Pi and anything else draws from the 5V will also be drawn through the 12V DC/DC converter. However, that converter is quite beefy.
With your USB devices attached, you're likely to be looking at 1.5-2A. At 5V that's 7.5-10W. Lets say 10W. At 12V, that's roughly 10/12 = 0.83A (ignoring losses / efficiency, etc). So your 12V 5A converter should have loads of grunt left.
Just remember to keep all your grounds connected and make sure your ground connections are substantial (not skinny little wires or tracks).

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dentex
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Re: 24V to 5V DC to DC conversion

Wed Dec 17, 2014 1:10 pm

thank you very much Stupotmcdoodlepip,
this kind of calculations about Watts, Volts and Ampere is exactly what I needed, due to me substantially being a newbie in DIY-electronics and RPi-hardware.
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

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dentex
Posts: 180
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Re: 24V to 5V DC to DC conversion

Fri Dec 19, 2014 8:41 am

Bought the converter from the 1st post + the same UBEC sold by adafruit (they say has been tested and it's fine).
Yeah. :mrgreen:
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

Stupotmcdoodlepip
Posts: 21
Joined: Sun Jan 26, 2014 6:25 pm

Re: 24V to 5V DC to DC conversion

Fri Dec 19, 2014 9:18 am

Something to play with over the Christmas break :)

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dentex
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Re: 24V to 5V DC to DC conversion

Fri Dec 19, 2014 9:24 am

Although I'm sure I'll receive them next year. (China is far away :) )
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

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Offcenter
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Re: 24V to 5V DC to DC conversion

Sat Dec 20, 2014 1:38 am

dentex wrote:Thanks everyone for all the inputs! Really useful.

Looking at the whole picture, I think I'll use an UBEC. It's cheap, really small (fits easily in my project's enclosure) and can go up to 3A.
I found this one (the same item it's on the adafruit store) or this one.

While the linked Recom converter it's a bit undersized (1A) for my RPi (USB rootfs, webcam and possibly wi-fi dongle, later on), also the "cigarette plug" idea seems good. I have one of them to tear apart so I'm going to happily disjoint it and make some measures, as it doesn't have any specs. :D
I have my Pi running off of the second UBEC that you linked to up above.
My Pi has been running my webserver 24 hours a day off that UBEC for over a year now.
No problems.
George in New Jersey.
(learning a little bit every day.)
(and darned confused most of the time!)

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dentex
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Re: 24V to 5V DC to DC conversion

Sat Dec 20, 2014 8:05 am

Thanks for the feedback!

Actually I took already the 1st one and the the other converter from 1st post.
;)
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

User avatar
dentex
Posts: 180
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Re: 24V to 5V DC to DC conversion

Sun Aug 02, 2015 4:24 pm

Hello.
Just as a follow up, to anyone may be interested: my aquarium controller project's Pi has been running for months now, out of this sequence made by the 24V/5A power supply -> the 12V step down converter -> down to the 5V output UBEC.
All is good and healthy, as the time being.

Cheers.
One of my projects with the Raspberry Pi:
--------------------------------------------------------
(Raspberry) Pi Aquarium Controller: https://github.com/dentex/piac
...and its dashboard: https://freeboard.io/board/O17ACD

Samspencer
Posts: 1
Joined: Tue Dec 19, 2017 7:45 pm

Re: 24V to 5V DC to DC conversion

Tue Dec 19, 2017 7:53 pm

Is there a "Pre-made" power adapter that will start with Bare Wire 24v and end with the Raspberry Pi 3 input Mico USB?

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OutoftheBOTS
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Re: 24V to 5V DC to DC conversion

Tue Dec 19, 2017 11:34 pm

I have used the exact same DC to DC buck converter posted in the original post many times fro powering my RPi.

They have an adjustable pot to change the output and you just need to screw the pot till it outputs 5v.

The RPi further regulates the 5v input down to a 3.3v so the 5v input isn't that critical as long as the it is within the 3.3v regulator specs.

AJB2K3
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Joined: Sun Aug 11, 2013 3:25 pm

Re: 24V to 5V DC to DC conversion

Tue Dec 26, 2017 4:48 pm

I Have used those modules with no problems to drop 12v to 3v6, the pot screw tends to be iffy on them but they are far safer and run at a much lower temp then their linear counterpart. (burnt my hand on a linear module!)
Back in the old days, we were smarter then the technology.
Nowadays technology makes us look like villages fools in front of the children!

davef
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Re: 24V to 5V DC to DC conversion

Wed Dec 27, 2017 5:36 am

The RPi further regulates the 5v input down to a 3.3v so the 5v input isn't that critical as long as the it is within the 3.3v regulator specs.
I believe that you want to be within the recommended voltages for USB devices ... as well.

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