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Topic: Why is there such a large difference in the current capacity of these vregs? (Read 6331 times)previous topic - next topic

oric_dan

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Well, I was calculating absolute maximums, not necessarily the average current the regs would be putting out.

In my particular setup for example, I would like to power a 3v vibration motor or a few lasers.  A massive 10G vibration motor or a green laser might need 200mA.  A smaller vibration motor or three red lasers might need 150mA.  And a small vibration motor or a single laser might need onl 60mA.  But in any case, they would only be on for brief periods of time.  A few seconds at most, usually.

One thing about those teensy-weensy v.regs is they might work ok in a dedicated app, like a cellphone/whatever where
you know what the load will always be, but when you have a more general-purpose board, you never know what might
be connected to it.

Just think, 150degC = ~300degF. That's almost the temperature most things are cooked at in the oven. If nothing else,
you run the possible risk of someone burning the heck out of their fingers. To me, even the 175degF I mentioned is
more than I want to reckon with.

The switch on/off bit is a problem, but I still might look for a more robust part with enable.

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Well, my plan as of this moment is to forget trying to run the 3.3v off the battery because even a NiMh doesn't leave me with much amperage to work with before the 3.3v reg would overheat.  So the plan now is to run both 3.3v regs on my board off the 5v regulator.  That will allow them to put put almost 350mA, which is 150mA more than what I ever expect the reg will need to put out, and even if does get to 200mA, it will only be for brief periods as I mentioned earlier.

Even running off 5V is a problem. (5V-3.3V) * 0.2A = 0.34W * 206degC/W = 75degC temp rise. That still gives 212degF
on the part. For me, that's too hot, boiling water hot, but it's your board.

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... but I don't really have the space for it, and it would make the way everything connects to the main board a lot more complicated for people.  Trying to keep things simple.  The board is already studded with pinouts:

http://shawnswift.com/arduino/layout2.png

Space is always a problem on pcbs. I can never get them as small as the original goal. Having burned up boards is
worse. In the end, you just have to make your own choice of tradeoffs.

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Fourthly, I don't even like the SOT223 parts [160degC/W], let alone the tiny ones you're using. I
go for the NCP1117 DPAKs [67 degC/W].

That looks like a nice part, but the problem is, that regulator doesn't have an enable pin, and the plan is to switch the motor and laser using the enable pin on the regulator.

Ok re on/off. I would still look around for another part, but that's just me/LOL. I'm sure they're there.

A lot of Arduino bds use the little SOT223 parts, and a few use the DPAK parts. Most don't even have a 3.3V reg. On
my own Arduino form factor pcb, I ended up with a TO220 5V reg and a DPAK 3.3V reg, both 1 Amp. That's my very
conservative idea of "general-purpose", where anyone using it will be less likely to burn the thing up.

At least you have some other considerations to think about. Good luck.

Last point, I might try buying 10 proto pcbs for \$100 first, and try out the circuit, before jumping in and

scswift

Dan:

I understand your concerns, and even though I still don't think heat will be an issue because the regulator will be off 95% of the time, (I can't control what people hook to the board and even a 5A reg will get hot if they short it, but it's not an Arduino so most shouldn't be connecting anything too crazy) I'll take another look and see what I can find as far as a 3v3 regulator goes.  I'm not against less heat and more power.

As far as size goes, the board's already at the absolute maximum.  It has to fit inside small props, and I've done a lot of research to determine exactly how wide and tall it can be and still fit inside most, and it really can't be any larger than it is now.  So at this point, my goal is simply to maximize the power output at that size, without driving the cost up too much.

And as far as prototypes go, while I don't plan to have 1000 of these made, (I wish! ) with the first batch likely being around 50-100 boards, I will of course have a prototype assembled first to make sure everything is in order.  Doing my best to ensure that the board works the first time though; that's why I'm asking questions to verify if my assumptions are correct or not. :-)

oric_dan

I always use the 10 protos for \$100 service first. My last 3 pcbs each had exactly "1" nontrivial
layout error, which makes me ever leary of getting more than 10 the first go. No matter how many
checks I do, some little error always creeps in. But then they're a little larger than yours, 3"x3"
to 3"x4" and 500-600 pads or so.

http://pcb4u.com/

takao21106

All the PCBs I have made so far had "errors", but all manageable, for instance no MCLR resistor.

I saw the photo from the board layout, having so many row headers is a lot of effort to solder, why don't you just only install a few row headers, and use flat cable connectors for the rest of the pins?

both the connectors and cable assemblies are available from a distributor I know, for good prices, I can add a link if you are interested.

Board size could become reduced considerably as well the efforts to solder all these pins one by one (flat cable connectors could be reflowed or row soldered).

scswift

I've had pretty good luck with my PCB's so far. :-)

I had one problem with the second circuit I designed where I miscalculated the current capability of the Arduino Pro Mini's onboard regulator (it was piggybacked on my board and driving an array of 64 leds) so I had to work up a solution by cutting a trace and wiring in an offboard regulator.

And with my third circuit, which was far more complex, (and similar to, but less complex than this design, and which used through hole parts) the only issue was with the pad sizes for a library part I downloaded for a 2mm connector.  The holes were too small.  Ended up having to wire that end directly into the board and put the connector on the board it attached to instead.

Not saying I won't have any issues with this board, but I'm cautiously optimistic.  Double checking all those pad hole sizes for sure.

Takao:

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I saw the photo from the board layout, having so many row headers is a lot of effort to solder, why don't you just only install a few row headers, and use flat cable connectors for the rest of the pins?

Several reasons:

1) I won't be soldering these.  They'll be assembled by the PCB manufacturer.  I don't have any experience soldering SMD components and my last board design which was half as complex as this took me 10 hours to solder each board together.

2) Flat cable connectors take up a lot of space.

Using them for the long LED pin headers would require me to leave .100" of room all the way around like the ISP header.  Except unlike the ISP header, which is just a pin strip, and thus leaves room for me to sneak parts under the socket, the shrouded headers I assume you're suggesting for the LEDs would require that space to be entirely bare.  I'd either have to make the board larger or put components on both sides to accommodate that.

3) Flat cable and IDC sockets are less flexible and harder to use.

When I say less flexible, I don't mean the cable can bend less easily than individual wires (though that is true, and a potential concern) but rather that you can't stick a six pin socket in one location, and an eight pin socket somewhere else.  It's all or nothing.  This board is designed to be modular, and I'd like someone to be able to attach a 10 segment bargraph to a 10 pin socket (plus 1 for a common anode), and another group of 3 leds to a 6 pin socket, and plug those in wherever they wish, separately.

It's also more difficult to separate and strip ribbon cable.  And with this setup you don't even need to strip anything.  You can just buy some sockets and wires from Pololu with crimped terminals on both ends, snip the leads on the LED down to 0.25", plug the LED into one socket, and plug the other end into the board.  I've used this setup myself on many occasions, and it works great.  Also, as an alternative, if you really want the LED to be permanently affixed, you can leave the terminals unsocketed, slip them onto the LED, and solder them in place.  Either of these is easier for a beginner than soldering bre wire leads on and crimping their own connectors.

It's more expensive, sure, but these boards are geared towards people who might not even own a soldering iron, so it's the perfect solution for them.  And for me, it's worth spending a few bucks on connectors and crimped wires if it saves me hours of stripping wires, soldering, crimping, and heat shrinking.

Also in regards to flexibility, the pins along the top need to allow for servo connectors.

scswift

Hm... I just found this regulator:

Can supply 1A, is \$1.30, and when mounted on 1 sq in of copper, the SOT-223 model has a thermal resistance of 70C/W, compared to 206C/W for the 150mA \$0.50 SOT-23 reg.

So for 80 cents more I get a much more capable part, which may survive if someone does something crazy like hook three green lasers to it.

But to be sure let's do the power calculations:

?j-a = 70°C/W
Tj(max) = 150°C

Pd(max) = (Tj(max) - Ta) / ?j-a
Pd(max) = (150°C - Ta) / 70°C/W
Pd(max) = 1.78W @ Ta = 25°C
Pd(max) = 1.07W @ Ta = 75°C

Then:

A = Pd(max) / (Vin - Vout)
A = 1.78 / (5 - 3.3) = 1.04 @  25°C (Piggybacking off my 5v regulator)
A = 1.07 / (5 - 3.3) = .629 @  75°C

So based on this, the part would likely survive if someone hooked three green lasers (~200mA each) up to it.  But I don't expect anyone will do that because green lasers are really expensive.

takao21106

It's also more difficult to separate and strip ribbon cable.

I assume, when I talk about flat cable, you think of these used for ATA hard drives, and legacy floppy drives?

I mean the type used for modern LC displays, and generally more and more found inside consumer electronics gadgets.

1.25mm pitch, through hole, not true SMD

0.5mm pitch, SMD

and the cable assemblies: http://radionics.rs-online.com/web/c/connectors/general-purpose/cable/

these assemblies are never cut, they are purchased ready-made.

The idea is to save time (the connectors can be soldered at once), space (they are much smaller than row headers), and in some cases, it's also cheaper to use a ready-made flat cable.

My idea was to have a number of conventional row headers, and if these run out, flat cables are used.

The technology is around since the 1980s but I have seen a lot of increase in recent years.
1.25mm is easy to use, still through hole.

oric_dan

From datasheet:
The SOT-223 package has a ?J-A of approximately 125°C/W when soldered down to a minimum sized pattern (less than 0.1 square inch) and approximately 70°C/W when soldered to a copper area of one square inch.

I think you're still being a little optimistic in choosing the 70 value versus the 125 degree value, since 1"
square of copper means just that, but all in all, probably a better thrmal choice than the others.

scswift

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I think you're still being a little optimistic in choosing the 70 value versus the 125 degree value, since 1" square of copper means just that

Well, it's true that the expansion board this will be placed on is a bit less than a full square inch, but I'll have thermal vias going to the ground plane like on my 5v regulator, so it should function as a reasonable heatsink.  Also, there's a mounting hole right next to it which is also a via connected to the ground plane and I have thermals disabled on vias, so the user could attach the board to their chassis to help with the heat dissipation if it does become an issue.

I've done the best I can with the limitations I've set for size and cost.  It's gonna cost me over twice as much for these regulators, but you're probably right that I needed the extra current capacity to be on the safe side.

scswift

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I assume, when I talk about flat cable, you think of these used for ATA hard drives, and legacy floppy drives?
I mean the type used for modern LC displays, and generally more and more found inside consumer electronics gadgets.

Ah, flat flexible cable.  Yes, I'm familiar with that.  I tried to use it on my last board but had to go with much more expensive 2mm headers because it couldn't bend the way I needed it to.

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The idea is to save time (the connectors can be soldered at once), space (they are much smaller than row headers), and in some cases, it's also cheaper to use a ready-made flat cable.
My idea was to have a number of conventional row headers, and if these run out, flat cables are used.

Yeah, I'd love to be able to use those, but they're just not suitable for this design.  Those don't work well when you don't know where someone will need to mount a part or how far from the board it will have to be.  This board could be mounted in the handle of a gun, and the leads to the leds at the tip might need to be 2' long.  Same thing goes for the expansion boards.  Main board might fit in the handle, but the expansion boards might need to go in the body, and FFC doesn't bend around corners sideways.

oric_dan

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Well, it's true that the expansion board this will be placed on is a bit less than a full square inch, but I'll have thermal vias going to the ground plane like on my 5v regulator, so it should function as a reasonable heatsink.

Full square inch of copper isn't the same as saying 1" square pcb. That's what the other number means:

"approximately 125°C/W when soldered down to a minimum sized pattern (less than 0.1 square inch)".

takao21106

@scswift

I understand your considerations. As well the end user may not always have access to a distributor carrying these cables.

Myself I try to move away from any kind of parallel bus or connector where possible.

I figured out, when I made PCBs for PICs, and included many headers/jumpers/extra parts, most of the time indeed just a blank PCB was needed.

However I do not know the depth of the design or the considerations for instance using it inside guns.

I even changed away from headers for the ICSP programming, just pushing a connector inside the holes, I also see this practice on many commercial PCBs.

What I do not fully understand is why many people actually are afraid of switching ICs, there are many different types, these big (I think DPAK) LM25xx running at low frequency, or also some smaller 500mA SOIC chips @ 2 MHz. I'd understand it for a DIL 2.54mm only design, but if you use TQFP/SMD anyway? Commercially, 7805 has been used in the early 1980s, there is also 3 Amp version, and somehow this regulator is prone to failure after a decade or so, due to heat stress on other components, even failing solder joints. There has been one in the SEGA Master System as well SEGA Megadrive.

I used these regulators too when I was younger, as a teenager.

Well somehow I don't get it the PCB design looks expensive I'd guess at least \$10 for each PCB, but you are concerned about this voltage regulator.

I'd never start calculations for a voltage regulator, anyway. I get some ideas from commercial applicances which I examine, and they do fail after some years, parts of the PCBs show heat stress, and all this. But, I think it is because they are cost optimized too much in some cases. One could think they do not always calculate sometimes just replace with cheaper parts and well, total lifetime becomes shortened.

Old capacitors from the 1980s still work fine but only if they did not experience heat stress...

scswift

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Full square inch of copper isn't the same as saying 1" square pcb. That's what the other number means:
"approximately 125°C/W when soldered down to a minimum sized pattern (less than 0.1 square inch)".

I don't follow.

The way I interpret that, it's 125°C/W if I just solder it to pads.

But if I have a ground plane, and attach it to that via thermals, that's going to be a big sheet of copper, and the thermal resistance should be much less.  (I guess the ground plane will be broken up a bit by traces on the main board, but that will be minimal on the expansion board we're talking about here.)

I guess if you feel thermals will be inadequate to transfer the heat the ground plane, I can place a polygon under the regulator to provide some additional copper as I have with my LED drivers per TI's specs.

jwatte

I believe products are deliberately designed to fail after a while. It drives more sales!
Commercial capacitors have a design life of 10,000 hours, which just happens to be about a month past a 1-year warranty period...

I find that switchers take more board space, because they generally need inductors and sometimes other parts.

+1 on avoiding parallel connections. Serial + Ground should be sufficient, if you put a bit of smarts on the other end :-)
Btw: You should be selling the connector cables, to make a profit. Don't send that business to another distributor!

scswift

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However I do not know the depth of the design or the considerations for instance using it inside guns.

I just want to clarify that I'm talking about toy sci-fi guns here, not real ones. :-)  This board is for people who want to add lights and sounds to their costume props.

And the reason I'm not terribly concerned about too much heat being generated is because these props are generally used in short bursts, and 95% of people will likely limit their use to less than 75 leds, and sound playback.  Which will draw about 1A on average, and 2A at peak.  If they add a vibration motor with this expansion module I'm designing now, that's another 200mA at most.  Three red lasers on a second module another 150mA.  And those would only be on for a few seconds at a time.

So while I am doing max calculations here to see what the board might survive, I don't actually expect people to drive it that hard.

But just for kicks, I did the calculations for a thermal resistance of 125C/W at 75C ambient, the worst possible case.  And for that I calculate the regulator can still take 380mA.  It would get really hot, but it would live.  And 380mA is almost twice as much as I expect anyone will ever try to drive that regulator at.

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