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

Tue Jan 31, 2012 10:02 am

Although the move to use a 5 V phone charger was beneficial for 90+ % of everyone, some got disappointed as they had other plans.

Unfortunately I don't know the first thing about making electronics, but I do know that making a voltage regulator is next to trivial by itself.

So, why not make a community project around a tiny PCB (as small as possible) with a microUSB connector in one end to plug directly into the Rasberry-Pi and a simple screw terminal for two wires in the other end and the electronics in the middle? Then one can apply any DC voltage (within the regulator's range) to power the R-Pi in an elegant way.

I imagine something of the size of a small USB memory stick. Anybody think it would be fun to design?

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

Tue Jan 31, 2012 10:22 am

Something along the line of this?
http://search.digikey.com/us/e.....ND/2259781
It's a 3-pin device with a voltage input of 7 to 36 volt, and a voltage output of 5volt at up-to 1.5 Ampere.
Ground is shared on the middle pin.
Would probably be a good idea to add a small electrolytic capacitor on the output line to filter the voltage.
Otherwise you just need a small PCB to mount in on and the screw terminal + a ordinary USB A female plug.

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

Tue Jan 31, 2012 10:34 am

Baconator said:


It's a 3-pin device with a voltage input of 7 to 36 volt


Not quite "DC-anything to 5V".

I suspect the OP is thinking of something which can work with Li-Ion batteries (below 5V), 6V SLA's, packs of AA batteries, vehicle supplies; almost anything put into the input gives 5V @ 1A out.

I also suspect that would be non-trivial but I think we need an expert in PSU design to give a definitive assessment.

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

Tue Jan 31, 2012 10:36 am

Yeah, if it is something anybody can buy one unit off it looks fine.

Then only the PCB, capacitor and the two connectors are missing

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

Tue Jan 31, 2012 11:07 am

kme, it's not really worth the trouble, as you can buy $4 USB phone chargers that can deliver the required 1A.

If you want to power the RPI in your car or something like that, consider a car-charger, for a similar amount. If you want to power it off a battery, consider the same car charger thingy, and buy "about 12V" of batteries. i.e. about 10 NiMh, 3S LIPO or 6S lead-acid.
Check out our raspberry pi addons: https://www.bitwizard.nl/shop/

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

Tue Jan 31, 2012 11:14 am

rew said:


kme, it's not really worth the trouble, as you can buy $4 USB phone chargers that can deliver the required 1A.


Correct, but that isn't the point. A lot of people want to run their R-Pi from a solar panel, a car/boat battery, wind turbine and what not and they can't just plug into a 230 V wall socket. The original alpha board could accept a wide voltage range, but for cost reasons this feature was slashed (and correctly so, IMHO).

So the idea was to design a hobbyist project where people could find a PCB layout and component list and go on solder their own solution.

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

Tue Jan 31, 2012 11:17 am

This could be a great project...one tricky thing is to keep the cost low enough to do everything and keep it efficient.

Would be interesting to see what considerations are required for a power-supply.

Something like the MintyBoost is probably suitable though.

Is a single circuit able to do step down and step up voltage conversion at the same time, or would they need to be switched.

There may be advantage in creating two separate boards if not.

I guess there could be some advantage in by-passing the linear supply on the RPi for extra efficiency, if at all possible.

Consideration needs to be given for when running off rechargeable batteries to cut-off supply when it drops too low (or it'll ruin them).  Perhaps circuit for monitoring the input voltage and cut-off relay, the RPi can control it.

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

Tue Jan 31, 2012 11:28 am

The thing is the simplest switched mode converter topologies are "buck" and "boost". Buck converters only convert down, boost converters only convert up.

There are other topologies that can convert both ways but they have their own issues, for example "buck-boost" is inverting, "flyback" requires a transformer (which isn't bad per-se, in fact it can be very useful sometimes but probably will increase the cost) and "sepic" has quite a few extra components over other topologies.

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

Tue Jan 31, 2012 11:38 am

meltwater said:


This could be a great project...one tricky thing is to keep the cost low enough to do everything and keep it efficient.


Maybe this in reallity is two projects:

1) A very simple one taking 7+ V from a robust source.

2) A somewhat more complicated (and expensiver) one taking undervoltage and nursing delicate rechargeable batteries.

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

Tue Jan 31, 2012 12:01 pm

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?

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

Tue Jan 31, 2012 12:13 pm

As the solution should be able to match the 1000 mA of R-Pi's polyfuse, I'm a bit skeptical about the Zener diode solution. The alpha R-Pi did use a active regulator and the R-Pi team probably considered this short cut too.

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

Tue Jan 31, 2012 12:24 pm

Fair enough, I was only made aware of the method, the other day when trying to identify a blown component on a HDD (yeah come to think of it, reverse-bias didn't get handled very well).  A typical 3W diode doesn't support much current (600mA) even at 5V (is that input or output voltage though?).

A dumb question here...what happens if you use several zener diodes in parallel?
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Re: Suggestion: Tiny PCB for DC-anything to 5 V DC

Tue Jan 31, 2012 12:32 pm

meltwater said:


A dumb question here...what happens if you use several zener diodes in parallel?


This is where my knowledge of electronics ends.

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

Tue Jan 31, 2012 12:32 pm

kme said:


Maybe this in reallity is two projects:

1) A very simple one taking 7+ V from a robust source.

2) A somewhat more complicated (and expensiver) one taking undervoltage and nursing delicate rechargeable batteries.


I think it's actually three projects

1) a buck converter for the high voltages. Say starting at 7-8V and going up however high the components you chose can tolerate. The LM2576-5 suggested by meltwater seems good here at least for a hobbyist soloution.

2) a low dropout regulator to cover the 5.5 to 7V or so (more with a heatsink) range cheaply and simply. Something like the L4940V5

3) a boost converter for running from voltages below 5V.

I don't think protecting the batteries is too big a deal, afaict most lithium ion packs have overcurrent and overdischarge protection built in anyway.

You COULD build a sepic or flyback converter that could do it all but i'm not convinced the complexity is worth it.

meltwater said:


I guess something using LM2576-5 would work then for the 7+V range.  http://www.ti.com/lit/ds/symli.....lm2576.pdf


Looks a pretty good choice for the hobbyist. The low switching frequency should make it pretty tolerant of poor board layout. Downside is that low switching frequency will mean a relatively large inductor.


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





It'll probablly work if you can find a beefy enough zener and you can tolerate the high input current but it's a pretty horrible soloution.

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

Tue Jan 31, 2012 1:03 pm

kme said:


meltwater said:


A dumb question here...what happens if you use several zener diodes in parallel?


This is where my knowledge of electronics ends.



This is not 100% accurate but probably the best way to understand:

Pretend each zener diode is a 'resistor' with certain 'curve'. The system will settle where all the curves have the same voltage. Because each zener diode will have a slightly different resistor value at that point, they will not all have the same current. The closer the zeners are (e.g. all of the same production batch or all of the same corner of the wafer) the more the currents will be the same.

Last remark: Don't! Not because it will not work but because just like with linear regulators you are burning power. You need very big zeners to get rid of the power (heat)

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

Tue Jan 31, 2012 1:11 pm

plugwash said:

....
2) a low dropout regulator to cover the 5.5 to 7V or so (more with a heatsink) range cheaply and simply. Something like the L4940V5

....


As I posted elsewhere I did that on my Gertboard. Take 7.5V input into a 5V LDO (Low Drop Out) regulator. The regulator got piping hot. I added a heat sink which hardly made a difference. I never knew how much heat 2.5 Watts was until then. (Oh and the Gertboard used very little power 90% was going into the Pi).

hzrnbgy had this link which I liked:


http://uk.mouser.com/ProductDe.....49qqIt4%3d


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

Tue Jan 31, 2012 1:19 pm

plugwash said:

I think it's actually three projects.
I would agree. It's much simpler to design for a particular solution than a universal one.

As a related aside the R-Pi is being promoted in the FAQ and elsewhere as 'being well suited to running off 4 x AA batteries'. I think that should be better clarified before people take it at face value, wire up their 4 x AA and connect to the R-Pi. I may be wrong but I'm not sure the R-Pi will work supplied with 6V input. If it does that's great, but if not ...

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

Tue Jan 31, 2012 1:33 pm

Just for a laugh lets run through some numbers.  Assume input range of 7V to 36V, output of +5V, fixed current into the RasPi of 200mA (1W at 5V).  Also assume a minimum Zener current of 5mA to keep it in the active region.

Minimum voltage @ 7V


Volts across dropper resistor = 7 - 5 = 2V
Current through dropper resistor = 200mA + 5mA = 205mA.
Dropper resistor value: V/I = 2 / 0.205 = 9.76 Ohms.
Power dissipated in dropper resistor = V x I = 2 x 0.205 = 0.41W.
Power dissipated in zener diode = 5 x 0.005 = 25mW.
Total power input = V x I = 7 x 0.205 = 1.44W

Not too bad.

Maximum voltage @ 36V


Volts across dropper resistor = 36 - 5 = 31V
Current through dropper resistor = V / R = 31 / 9.76 = 3.18A
Power dissipated by dropper resistor = V x I =  31 * 3.18 = 98.6W
Current through zener diode = 3.18 - 0.2 = 2.98A
Power dissipated by zener diode = V x I = 5 x 2.98 = 14.9W.
Total power input = V x I = 36 x 3.18 = 114.5W

As they say on "Big Cook Little Cook": Hot hot hot!

Note for the pedants: yes I know there's no such thing as a 5V0 zener diode.  The closest value is 5V1, but I'm keeping things simple, m'kay?

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

Tue Jan 31, 2012 1:36 pm

Gert said:


As I posted elsewhere I did that on my Gertboard. Take 7.5V input into a 5V LDO (Low Drop Out) regulator. The regulator got piping hot.


They are meant to. The regulator I linked has a maximum junction temp of 150C and without a heatsink case temp will be pretty close to junction temp (since thermal resistance case to ambient is much higher than thermal resistance junction to case) . So running at maximum non-heatsunk load the case temp will be more than hot enough to boil water.


hzrnbgy had this link which I liked:


http://uk.mouser.com/ProductDe.....49qqIt4%3d



Yeah those modules are a nice easy solution if you don't want to deal with laying out a switcher yourself.

hippy said:

As a related aside the R-Pi is being promoted in the FAQ and elsewhere as 'being well suited to running off 4 x AA batteries'. I think that should be better clarified before people take it at face value, wire up their 4 x AA and connect to the R-Pi. I may be wrong but I'm not sure the R-Pi will work supplied with 6V input. If it does that's great, but if not …
mmm, afaict on the pi it is likely to trip out when fed from four good alkaline AAs. For NIMH AAs would probablly work but any devices running directly off the 5V line may get flaky as the batteries run down.

Running stuff reliablly off batteries is actually a PITA because you really want to tolerate anywhere from 1.7V or so per cell (nife fully charged) down to 1V or so per cell (practically dead).

Personally if I had to run off AA batteries i'd probablly use 6 of them feeding a buck converter.

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

Tue Jan 31, 2012 5:27 pm

I"m going through all this myself. At the heart of most of my projects is now a RasPi. Trouble is most of my projects are green energy powered, that is to say unregulated. I"ve just started looking into the Buck/Boost transformers and favored them over energy wasting regulators.
However my electronics experience is limited so I"ll be keeping a close eye on this thread.
ric

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

Tue Jan 31, 2012 6:05 pm

I tried lots of ways to power a Beagleboard-Xm from 12-18v for a project, and the only cold running solution I found was http://www.ebay.co.uk/itm/2707.....K:MEWNX:IT (Hope its not against forum rules to post an Ebay link)

This powers the Beagle and a GPIO interface board that tests PCB's (It also powers the PCB's)

By cold I mean if I touch it, its room temperature...

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

Tue Jan 31, 2012 6:43 pm

ricbob said:


I"m going through all this myself. At the heart of most of my projects is now a RasPi. Trouble is most of my projects are green energy powered, that is to say unregulated. I"ve just started looking into the Buck/Boost transformers and favored them over energy wasting regulators.
However my electronics experience is limited so I"ll be keeping a close eye on this thread.
ric



Sorry if I repeat myself so often but there are sooo many forums. If you really want to be green you should first replace the 3V3 regulator (RG2) with a switch mode one. Ultra low power is then also to replace the 1V8 regulator (RG1) with a switch mode.

The following is speculative and I do not know if it works: There are situation where you could run the whole thing of 3V3. (That also requires a board mod). If I have enough time plus spare Raspis I will give it a try.

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

Wed Feb 01, 2012 10:46 pm

mmm given how little runs off 5V I wonder if the sensible soloution for running off 4 AA batteries operation would be a buck to 3.3V followed by a boost to 5V for the network chips etc.

Given that you have access to both the 5V and 3V3 lines on the expansion connector I presume you could just pop the regulator off with hot air and supply the two lines seperately from the expansion connector.

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

Wed Feb 01, 2012 10:48 pm

I've been looking at power supplies for my project which will be a Pi daughterboard as well. Let me back up what Gert is saying: you're not going to want to use a linear regulator or linear LDO for much more than say 7V input. You're just going to make a tremendous amount of heat.

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. It comes in a TO-220-5 package and goes for $2.50/q1. I think I may add a polyfuse but here's what I'm looking at:


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

Wed Feb 01, 2012 11:24 pm

hippy said:


plugwash said:


I think it's actually three projects.


I would agree. It's much simpler to design for a particular solution than a universal one.

As a related aside the R-Pi is being promoted in the FAQ and elsewhere as 'being well suited to running off 4 x AA batteries'. I think that should be better clarified before people take it at face value, wire up their 4 x AA and connect to the R-Pi. I may be wrong but I'm not sure the R-Pi will work supplied with 6V input. If it does that's great, but if not ...


It will do just fine.  6V is not that much over the max USB (and still within specifacation, 5 to 5.5V +/- 5%) voltage of 5.775V.  Also batteries are not a regulated power source so the voltage will lower under load, and as they are drained.  I would be supprised if you read over 5.6V with the device turned on.  Chances are likely that with an SD card, Wireless, KB, Mouse you would read 4.8V from alkaline batteries.

A better fit though would be to use 3 Nickel-Zinc AA rechargables.  They produce an average (nominal) of 1.6V per cell.  So 4.8V.  The only dangerous characteristic of Nickel-Zincs are that they have low internal resistance so under low resistance loads they can put out as much as 2.1V per cell, but the Pi is well above that threshold so I would not worry about it.  Or just use a V reg and run 4 of them.

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