kme
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 02, 2012 12:15 am

CapnBry said:

I designed around the Pi pulling 500-700mA and 2x peripherals pulling 500mA each, which means you need at least 1.7A of output. I settled on the LM2576 5V 3A switching buck converter, which gives 77% at 1A given a 12V supply.
Looks interesting. As I already stated I know very little about electronics, but isn't 77% efficiency a bit disappointing? It's going to need a largish heat sink, especially when using more that 12V input. Also shouldn't C4 be rated more than 25V when the LM2576 is rated for 40V input? Many solar panels are 36V and I think trucks (real trucks, not pick-ups) also comes with 36V systems.

But such a system apparently isn't going to be as tiny as I hoped for.

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Chromatix
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 02, 2012 12:38 am

One switching topology of interest would be the Cuk converter - which is a little weird since it inverts the voltage given it, but otherwise fulfills everything you could want out of a power converter.  It draws a steady current from the source and presents a steady voltage at the output, and the input and output voltages are essentially independent.

The main trick is figuring out how to implement the feedback across a voltage inversion.  The easiest solution might be an optocoupler driving an integrator, which in turn drives a comparator attached to a sawtooth oscillator (all easy to implement provided you have a known minimum input voltage).  The comparator's output is then the PWM input to the switching transistor.

It is entirely possible of course that someone has already put all that on a chip.
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error404
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 02, 2012 2:40 am

kme said:


Looks interesting. As I already stated I know very little about electronics, but isn't 77% efficiency a bit disappointing?


Pretty typical for a cost-optimized DC-DC converter I would say. It should be possible to get to 85+% for a reasonable cost and with a fairly simple circuit.


It's going to need a largish heat sink, especially when using more that 12V input.


As a power converter, the input voltage doesn't have a huge effect on dissipation. There is some change due to switching losses and whatnot, but mostly the efficiency will stay within say 10% vs. input voltage. Might get up to a couple watts under load though, so it's probably wise to run the thermal data…or just use a more efficient converter.


But such a system apparently isn't going to be as tiny as I hoped for.


I'm not sure how tiny you were hoping it'd be, but it can be pretty small. LM2576 modules are readily available on eBay for a couple bucks that are like 1.5" square. Smaller is probably possible if you're willing to spend a bit more on nicer components and can handle the SMD packaging.

A quick run on National's WeBench tells me the LM3150 plus a decent coil, some MOSFETs and other passives will get a supply that is > 90% efficient for 7-24V input at loads > ~300mA. I'll look further at a design for this in a bit...

hzrnbgy
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 02, 2012 2:58 am

save yourself the trouble of designing your our switching regulator

Mouser has one with max 1.5A output with minimum efficiency of 89% at 12V input. Includes the necessary protection circuitry and has the same footprint as a T0-220 package.And it only cost $4 (shipping not included).

http://www.mouser.com/ProductD.....845nWHA%3d

Here is a link to the datasheet

http://www.murata-ps.com/data/.....i-78sr.pdf

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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 02, 2012 4:29 am

kme said:


Looks interesting. As I already stated I know very little about electronics, but isn't 77% efficiency a bit disappointing? It's going to need a largish heat sink, especially when using more that 12V input. Also shouldn't C4 be rated more than 25V when the LM2576 is rated for 40V input?


77% is actually about average. 77%@12V means that given a 12V input you're going to get 12/5 * 0.77 output so at 1A you'll get 1.848A out. Unlike the linear regulator wattage waste posted, you're not going to burn anywhere near that sort of power. As a generalized formula a linear regulator is 100% inefficient over its voltage drop. So at 12V -> 5V you're looking at 42% max theoretical efficiency.

I used a 25V cap because my source in my project is a 12V regulated supply. That murata looks pretty sweet though. There just hasn't been enough data posted about the Pi for me though, to know exactly what's needed for it so I can fully spec out my board. It would be awesome to just need a TO-220 footprint.

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meltwater
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Sat Feb 04, 2012 8:34 am

Ok. I think for the low voltage side (below 5v), there may be some value designing for 2.0v to 3.7v range.  This would allow the use of li-ion cells as well as 2xAA.

To keep it simple, no charging circuit, but a circuit can be built in for indicating low voltage (manually calibrated by the user for the battery type) or disabled if non-rechargable ones.. The RPi can monitor the indicator (via digital input) and perform shutdown and power-cut off.. Could even have two levels a low power warning and a shutdown level if required.

Also, if fitted 4xAA in 2 parallel sets in theory you could swap out a new set live.

Could even build a sensor circuit with a ADC for more advanced applications.

Possible?
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slacker
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Sat Feb 04, 2012 2:36 pm

Yeah that should be possible, using something similar to the Miny Boost, like you posted about earlier.

You could do a simple battery level switch using a comparator running off the 5 volt supply. One input would take the voltage from the battery and the other would be a reference voltage, this could be generated using a couple of resistors or a trim pot wired as a voltage divider. When the battery voltage dropped below the reference voltage the output of the comparator would switch and this could be detected by the Raspi using one of the GPIO pins.

You'd need to do some sort of software debouncing to stop it triggering due to noise or brief dropouts on the supply, but the basic idea is there.

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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Tue Feb 07, 2012 5:19 pm

meltwater said:


Possible?


Sure, pretty standard boost converter.

Be aware that at low voltage input the current will get very high. Especially if you are going to boost to 5V only to let the inefficient linear regulator on the pi drop it down to 3.3V.

Popping the 3.3V regulator off the pi and putting a switching one on the add-on board would be an option.

Question for the PI guys, is the large three terminal device next to the GPIO header the 3.3V regulator?

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Gert van Loo
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Tue Feb 07, 2012 7:21 pm

I don't have the full data here but I suggest you find the schematic Liz put in a forum and check the RGx numbers. I think RG2 is the 3V3

Caliber Mengsk
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Tue Feb 07, 2012 10:31 pm

I didn't really read everything, but why not use a simple LM7805. It's got a large range (5v-35v {higher voltages need heatsinks}) and outputs up to 2.2amp. They are fairly cheap at $.50 (usd) from mouser. I made my own version of arduino and used this without any issues. It even ran the arduino fine for hours off of two button cell batteries. (the standard watch battery size)

Unfortunately, PCB's aren't cheap in general. Cheapest small batch I've found was iTead studios at $16 for ten 5cmx5cm boards (just over 2 inches square) which is actually really cheap (that was $12 for units, and $4 for shipping to the US). Depending on the size of the board, if you bought bulk (in the thousands with vscoring) you can get the individual PCB cost down to less then a buck.

http://iteadstudio.com/store/i.....cts_id=173

They dropped the price on iTead since I bought it also, so it's only $9.90 + shipping for 10 pcs.

Either way though, you don't really "need" the pcb, and the LM7805's are automatically regulated without capacitors. This means you only need the 7805 and maybe a diode for extra protection. The chip has built in voltage and current regulation also.

Just a suggestion is all. If needed, I could whip up an eagle file for a basic PCB that could be used. I'm not a professional by any means. I just got it to work with arduino and was happy, but I don't see why something like this wouldn't work.

(On a side note, I've compressed my arduino's size down further. I have a full 20 I/O pin, 5 ground, and FTDI connection arduino into a 25mmx25mm size. Roughly an inch square. That's with a voltage regulator and everything. Don't know why, but I felt like saying it XD)

plugwash
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Wed Feb 08, 2012 12:29 am

Caliber Mengsk said:


I didn't really read everything,


Why not?


but why not use a simple LM7805. It's got a large range (5v-35v {higher voltages need heatsinks}) and outputs up to 2.2amp. They are fairly cheap at $.50 (usd) from mouser. I made my own version of arduino and used this without any issues.


How much current did this "own version of arduino" actually draw? I'm guessing less than 100ma.

Have you ever actually tried pulling significant current through a 7805?


It even ran the arduino fine for hours off of two button cell batteries. (the standard watch battery size)


What was the nominal voltage of these batteries?

Did you monitor the voltage on your 5V line during the process?

The 7805 is an old linear regulator.

At the low end of the votlage range you get the problem that it's typical dropout voltage is high. On a fairchild or national LM7805 it's specced at 2V and they don't even tell you a maximum. Dunno if other brands are better but I doubt it.

That means you really don't want to be running it with less than 7V input if you want to reliablly deliver a solid 5V. At low currents you may get away with less but according to the graphs on the national datasheet you are still talking a dropout voltage of 1V even at no load at all.

At the high end of the voltage range you run into the problem of power dissipation. If you are running a 1A load from a 15V source through a 7805 then you have to get rid of 10W from the 7805. That means a fairly large chunk of metal is required.

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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Wed Feb 08, 2012 1:06 am

Gert said:


I don't have the full data here but I suggest you find the schematic Liz put in a forum and check the RGx numbers. I think RG2 is the 3V3



OK done that, it seems the really big (well relative to the size of the other stuff on the board) reg down by the power connector is the 3.3V reg. The medium sized reg by the GPIO header is the 1.8V reg and the small reg by the GPIO header is the 2.5V reg.

Question for those who know the hardware: what is the relativel load on different rails? If trying to improve power efficienty would it more sense to remove the 3.3V regulator or the 1.8V one?

charithjperera
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Wed Feb 08, 2012 2:56 am

I dont think there is much of a project to be made here with so many cheap regulator boards out there.

The simplest and cheapest option would be to head to ebay and buy something pre made and mass manufactured already, any DIY solution is almost definitely going to cost more.

Examples:

Step Up / Boost (~3-5V in) - Solar Boost DC-DC 3V-5V Adjustable Power Supply Module
http://www.ebay.com.au/itm/250.....92664171 


Step Down / Buck (~6V to 40V) – LM2596 Step Down Adjustable DC-DC Power Supply Module
http://www.ebay.com.au/itm/170734419936

And a step up and step down together one one board (even though I wouldn't recommend it) - http://www.ebay.com.au/itm/250990148807

The latter look like they require some soldering to get the power leads connected to the R-Pi and a multimeter to set it to 5V but no major electronics knowledge needed

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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Wed Feb 08, 2012 9:34 am

I think there is a project here for a few applications, in particular using sense circuits to protect over-discharge of rechargeable power sources.  I'll put it on my list of things to do if I can, when I get to it (not at the top though).

@plugwash Good point about the current.  And it'll be interesting to see how feasible it is to change the supplies to switchmode too.

@charithjperera The suggested modules may well be ideal to form the basis of what is needed in one neat package (the first being well suited to what I have in mind).
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 09, 2012 12:37 am

meltwater said:


And it'll be interesting to see how feasible it is to change the supplies to switchmode too.


All should be feasible to change

The 3.3V reg is the easiest. Just desolder it from the pi (should take a few seconds with hot air, a little longer with more primitive tools but still perfectly doable) and make your own arrangements on an expansion board connected to the expansion header

The 1.8V is a little trickier because the 1.8V line isn't brought out to the expansion header but the pads for the regulator should be more than big enough to solder a wire to.

The 2.5V reg would be a PITA due to it's small size but from posts i've seen here in the past and the fact the regultor is so small I don't think any significant current is drawn from that rail in the first place.

The question is which would save more power to replace? the 3.3 or the 1.8…….

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jojopi
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 09, 2012 3:35 am

plugwash said:


The 3.3V reg is the easiest. Just desolder it from the pi (should take a few seconds with hot air, a little longer with more primitive tools but still perfectly doable) and make your own arrangements on an expansion board connected to the expansion header


I think it would impossible to remove with a low-powered iron due to the big copper plane it is on. Just cut its legs and leave it in place.

The question is which would save more power to replace? the 3.3 or the 1.8…….
Definitely the 3V3. It supplies at least the LAN9512 -- up to 231mA with Ethernet in use. According to Eben's blog the 1V8 is low current and was an LDO even on the alpha boards.

Now, I wonder whether it might be feasible to power the board from 3V3 only -- assuming you did not mind the 5V lines on the USB and HDMI being missing or out of spec.  Does the support circuitry for the SoC's internal switcher need to be varied for different input voltages, or would it be able to cope?  Is there anything else on the board that uses 5V?

wallabybob
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 09, 2012 3:42 am

The dropout voltage for a regulator is the minimum difference between input and output voltage at which the regulator is still in control of the output voltage.

The National Semiconductor datasheet for the LM2956 discussed in some replies suggests it has a dropout voltage of around 0.9 at 1A load and about 1.3V at 3A load.

There are switchmode voltage regulators with considerably lower dropout voltage than the LM2956, for example the MP2307DN used in some assembled regulator boards such as the KiS-3r33 available on eBay (example http://www.ebay.com/itm/ws/eBa.....OC:US:1123 - the title says LM2956 but the photo shows a MP2307DN) and the AP5002SG used in a switchmode regulator circuit published in the Feb 2012 issue of Silicon Chip magazine (http://www.siliconchip.com.au). PCBs for the Silicon Chip design are available from Silicon Chip and the components appear readily available from Element14. I don't know of anyone producing kits for the Silicon Chip design. Silicon Chip claim a typical dropout voltage of 0.1V at 1A so somewhat longer battery life should be possible using their switcher rather than one based on the LM2956.

The LM2956, MP2307 and AP5002 all have enable terminals which could be used to disable the regulator if input voltage drops too low. This could be used to  help prevent damage to a battery from over discharge. The Silicon Chip design makes the enable terminal readily accessible but none of the (small sample) of LM2956 or MP2307 boards I looked at did.

error404
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Feb 09, 2012 6:48 am

Switchmode regulators are also available that will go into a passthrough mode when the input voltage falls below their dropout voltage.

onzoom
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Fri Mar 16, 2012 5:27 pm

Well I think you've all missed the most elegant solution.

The micro usb connector on the pi board will accept 5v from a standard USB port. It just so happens there is a standard USB port on the opposite side of the board about 85 mm away.

Luckily dabs.com do a USB standard to MICRO  part number UUSBHAUB6IN  this is an A to micro B lead 6 inchs long  - just the right length to connect the ports on the pi board.

Now of course this doesn't work just like a switching regulator thrown together without experience or test equipment wont work  - but unlike the switching regulator this solution wont blow anything up.

Now you can plug the standard USB plug into that laptop with the broken screen and coffee stained keyboard but working battery and charger. With the lid closed you even get a plinth to display your pi project. When the laptop finally dies you can use a mains plug to USB adaptor I have even bought one from a pound shop (yes it cost £1) that claims to have a 1000 mA output.

curiousengr
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Thu Mar 22, 2012 5:09 pm

Sorry to butt in guys, just liking this techy stuff- finally seems like someones getting real on this forum.

I will be using a usb power adapter for RasPi when I'm at home but am looking for something for when it is possible to go portable - (display's the problem).

i have used LM3805 type in the past and Zeners but only on mains powered units as they get a bit warm and need heat sinks- not very good for battery power.

If you are wanting to use a zener the high power rated ones get a bit big and pricey, better to use a regulator/power transistor with the base tied to a low rated zener then there is less power lost to heat as the transistor only passes the current that the Pi needs to maintain 5v.

Why are you interested in the 3.3v and 1.8v supplies? surely the load on them won't change as they are only needed for the components on the RasPi board.

The USB 5v is supposed to be 5v or pretty darn close otherwise any other USB devices you plug into the ports (Memory sticks/WiFi dongles) are likely to get fried.

The modern regulator chips LM2596 look like a good bet for my purposes i just need to work out how to link a mobile phone style battery.I am looking at using a LED/Zener combination as an indicator to light as a warning when battery power is getting low.

PaulTech
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Fri Mar 23, 2012 7:33 am

I have built switchmode supplies for project for over twenty years, but now thanks to ebay I can buy a LM2956 ready built module, with adj voltage down to 1.6V @ 1A for about $1.50 delivered. I can't buy the IC for that, forget about the rest of the components or board. They're very small modules, all SM, so they can fit anywhere. I used to replace 7805s with a LM2975 shoved into the 3 holes and the other few components just hanging off, hey they have lasted 20 years. Took power consumption in one device from 600mA to 200mA.

vikram1
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Sun Aug 05, 2012 5:25 am

I wanted to power my Pi from a car battery.

1. First tried with 7805, and as expected it was heating up badly.
2. Tried with 7808 -> 7805. Still the heat was too much on both.
3. LDO is also not a good idea from 12V to 5V.
There's >50% loss, which is a big deal for batteries.

Using a LM2576 here is the better option. Sometimes I've lost my microcontrollers because 7805 fails and destroys my circuit.!!. So use a 12V to 7V switching regulator and 7V -5V linear regulator in series. (This is what I do to be safe).

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joan
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Sun Aug 05, 2012 7:08 am

I'm not interested in boosting so I use a UBEC.

http://www.ebay.co.uk/itm/RC-Helicopter ... 0801032200

£1.60 including postage.

arbuz
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Sun Aug 05, 2012 8:51 pm

I guess that anyone can build something like that: http://sudoxanthippe.wordpress.com/2012 ... -module-7/
There is datasheet of regulator I used (I replaced 7085 to 78s05 today):
http://www.st.com/internet/com/TECHNICA ... 000449.pdf
You'll find schematics on my blog.

Greetings!
arbuz

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r4049zt
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Mon Aug 27, 2012 4:06 pm

meltwater wrote:I guess something using LM2576-5 would work then for the 7+V range (it is what is in my car adaptor).

http://www.ti.com/lit/ds/symli.....lm2576.pdf

However, would a Zener diode regulator work, or would the current draw be way too much?

The circuit which you sketched could just about get it to boot, but try working out the right sort of value for the resister and you'll see where it goes wrong ..
Given a typical draw of 0.3 to 0.4 Amps at 5V, plus up to 0.4Amps for optional other usb devices, to work off a flattish car battery at 10V and provide 0.8A at 5V requires that 5V be dropped in your resister, so six Ohms. Now go to a very full car battery at 14V and consider what that will do. Through six Ohms we'd get 1.5 Amps down to five volts, and the rPi might only use 0.3A of that. More than 1A overflows down the Zener, so it would have to be a big one and it would get hot. This makes your circuit Not Recommended because it wastes more power than it provides.

If you have time, try reading up on buck pwm controllers. You'd still want your zener in place, but the resister in your cicuit is replaced by an inductor and fast switch.

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