jay123035
Posts: 32
Joined: Wed Nov 21, 2012 5:24 am

24v to 12v input 12v and 5v outputpower supply circuit

Wed Feb 12, 2014 9:12 pm

I am working on a project to add a raspberry pi console to some ATV s. a couple of them run 24v systems but most run 12v. my touch monitor (Lilliput FA1012-np/c/t) needs 12v and the Pi needs 5v.

in short I need a power circuit that can take 12v-24v input and output 12v and 5v.

my idea is to use a LM2376T-12 based circuit to power a LM2376T-05 circuit, as depicted in the schematic below.


I have these questions:
will this work?
is it a good idea?
is there a better way?
will i get 12v output if my input is 12v?
can i run this in an air tight enclose? (how hot will it get?)

i would like one configuration that supports both input voltages as i think it would be more versatile.

thanks
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12-24v in to 5v and 12v out.png
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joan
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Wed Feb 12, 2014 9:34 pm

Have you considered using a pair of BECs, one to convert to 12V and one to 5V? They only cost a few UK£ and may be more efficient.

eBay search for 5V and 12V BEC.

jay123035
Posts: 32
Joined: Wed Nov 21, 2012 5:24 am

Re: 24v to 12v input 12v and 5v outputpower supply circuit

Wed Feb 12, 2014 11:14 pm

joan wrote:Have you considered using a pair of BECs, one to convert to 12V and one to 5V? They only cost a few UK£ and may be more efficient.

eBay search for 5V and 12V BEC.

No because i do not know what a BEC is or how i would use it. goolgle makes it seam like something RC hobbyists use a lot.

Tarcas
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 6:34 am

If you have 12v on the ATV, you can get a very inexpensive cigarette lighter adapter to output the 5v that the Pi needs. Even if there's no lighter jack on the ATV, you can still extract the circuit from the housing and wire it into the electrical system to get your 5v USB out. All it is is a 12v to 5v voltage regulator with a USB output and possibly an indicator LED. It's exactly the circuit you're looking for. Look at your local pound shop. My local dollar stores have a decent selection of them.
It won't work on the 24-volt models though, unless they have cigarette lighters, or... If the 24-volt models are supplied by two 12v batteries, you could connect the voltage regulator across the contacts of one battery only to get 12v for your equipment. I'm no electrical engineer, but if I understand that portion of a circuit correctly, if you "tap into" the circuit like that, with one lead between the two batteries and the other across one battery from the first lead, you should be able to use the 12v from one battery while it still adds its extra 12v to the rest of the ATV circuit. If someone who knows electronics better than I can confirm or reject this, I'm all ears.

hampi
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 8:52 am

Once you get it running you can contribute to the wiki

http://www.elinux.org/RPi_5V_PSU_construction

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Burngate
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 11:45 am

Why buy off-the-shelf when you can build your own, which does what you want rather than what someone else thinks you should want?
When you've done your research, you'll see that a BEC is merely the second half of what you've designed, as is the cigarette-lighter adapter - they take 12v in and give out 5v.

The major problem with your design - and with the off-the-shelf BEC or cigarette-lighter adapter - is the 12v output, when the input is only 12v.
From what I can see on the datasheet (from which you grabbed the diagram) the minimum Vi-Vo is about 1v so the input will have to be above about 13v. If the ATV is using a lead-acid battery, it may turn out not to be a problem, but if is, you may have to start with a circuit capable of boost as well as buck.

achrn
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 1:52 pm

Burngate wrote:Why buy off-the-shelf when you can build your own, which does what you want rather than what someone else thinks you should want?
Personally, because off-the shelf is cheaper quicker and easier. I can buy a BEC from ebay that will be in my hands as fast as if I buy the parts from Farnell, be significantly cheaper than the parts, and the BEC will be working five minutes later. If I'm just blindly plugging parts together following a specimen design on a datasheet (as I would be in the case of something like a DC-DC buck switcher) there's basically no interest in it, and I'd rather have it quicker easier and cheaper by buying it assembled. There's little satisfaction in assembling such a thing - I'd rather be assembling something that I've actually worked out.

But I agree that to meet the application described a buck-boost stage is needed, not just a buck. Personally, I'd have one power supply module for the 24V cases and a different one for the 12V ones.


jay123035
Posts: 32
Joined: Wed Nov 21, 2012 5:24 am

Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 5:28 pm

joan wrote:You can get buck/boost BECs.

e.g. http://www.helipal.com/bec-for-fpv-syst ... v-out.html

i would like to build this myself not just strap on parts. my project has many components, GPS, analog Logic, I2C sensors, and WiFi. i am trying to build/fit this all onto one board that i can mount in a NEMA box.

that said i like the idea of this BEC. except that it looks to take AC input (11.1~30V / 3S-8S???) and my input would be DC

if i could find a diagram for this type of BEC i could use it as for the power circuit . (if it will take DC input)

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rpdom
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 6:24 pm

It does take DC input. The "11.1~30v" is a clumsy way of saying the input should be between 11.1V and 30V DC, typically supplied by a battery pack containing 3 to 8 LiPo cells connected Serially (3S-8S).

achrn
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Re: 24v to 12v input 12v and 5v outputpower supply circuit

Thu Feb 13, 2014 6:27 pm

jay123035 wrote: i would like to build this myself not just strap on parts. my project has many components, GPS, analog Logic, I2C sensors, and WiFi. i am trying to build/fit this all onto one board that i can mount in a NEMA box.
It's all a matter of degree. You're proposing to just strap on a LM2576T, when you could build a switch mode power supply from parts...

Anyway, that unit will take DC. It probably won't accept AC. They used a tilde to mean 'about', not AC. It's input is for between 3 and 8 lipo cells, and lipo cells produce DC.

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