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[on:]

I'm a beginner at electronics, and I'm building Arduino based clock. I had some issue with voltage regulators. Linear are very inefficient and generate lots heat (which interferes with thermal sensor that clock have). Switching regulators are expensive and add complexity to PCB. Is it so bad to just provide 5 Volt switching power adapter that will power everything? Quality wall warts cost $5-6.  What do you think?  Am I doing a foolish thing here?  Maybe there's some kind of simple over-voltage/current protection I can add just as extra insurance?

P.S. not sure if it's important, whole clock thing takes about 200-300mA of current.

[Reply #1 on:]

One option: go with 1A wallwart, cut the end off a USB cablle and connect to the board that way.
http://www.dipmicro.com/store/DCA-0510

2nd option: use a 7.5V adapter into the barrel jack, the onboard regulator doesn't generate lots of heat creating just 300mA of current from a lower voltage source.
http://www.dipmicro.com/store/DCA-07510

[Reply #2 on:]

One option: go with 1A wallwart, cut the end off a USB cablle and connect to the board that way.
http://www.dipmicro.com/store/DCA-0510

2nd option: use a 7.5V adapter into the barrel jack, the onboard regulator doesn't generate lots of heat creating just 300mA of current from a lower voltage source.
http://www.dipmicro.com/store/DCA-07510

Thanks! For the 1st option I can just use barrel DC jack right?  What would be the reason to use USB jack?

[Reply #3 on:]

No, for the Barrel Jack you  need 7.5V to overcome the reverse polarity protection diode voltage drop and to give the regulator enough voltage to regulate down to 5V.

With 5V into the USB connector, the voltage only goes thru the autopower select FET, same as if you'd been powering the board from a  PC.

Or, you can connect 5V  to the 5V header; be sure to put a 1N4001 diode from 5V (anode) to Vin (cathode) to protect the regulator.
See Figure 25, page 10 of attached.

[Reply #4 on:]

Well you didn't state what 'raw' DC voltage your starting with, but I would have issue that modern switching regulators are too expensive to use in small projects: I bought five of these recently and they should work well for small projects using just about any DC raw voltage I can dig up.

http://www.ebay.com/itm/251066005460?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

 When I need to power up my arduino boards with an external +5vdc I like to use an old USB cable, lop off the PC end and fish out the + and - power wires and wire them to my external +5vdc power source and then just plug into the arduino using it's USB connector.

Lefty

[Reply #5 on:]

Well you didn't state what 'raw' DC voltage your starting with, but I would have issue that modern switching regulators are too expensive to use in small projects: I bought five of these recently and they should work well for small projects using just about any DC raw voltage I can dig up.
http://www.ebay.com/itm/251066005460?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Wow! That's pretty neat, a lot cheaper than DC wall wart and can use various voltages. I like that idea, now I just have to figure out how to incorporate it into my enclosure somehow...

Crossroads I think I mislead you stating that it's Arduino board. It's not. I just build custom PCB for ATMega644P so it has no USB port, just DC barrel jack, so I assume if I just plug in 5VDC adapter it will work (DC Jack wired directly into ATMega's VCC, and other components.  Maybe I'll add PTC fuse as well, and I heard zener diode might be a good idea too, but otherwise I just wanted to doublecheck what are the dangers. I'm attaching my schematic.

[Reply #6 on:]

How's the output look on a scope Lefty?  These Basic Attributes are a little confusing. Are they read across the chart? As individual groups? Does it do step-up & step-down conversion?

Module property    Non-isolation buck     Rectification mode    Non-synchronous rectification
 Input voltage           4V-35V                   Output voltage          1.23V-30V
 Input current          3A(maximum)          Conversion efficiency  92%(highest)
 Switching frequency 150KHz                  Output ripple            30mA9maxmum)         << what does this mean?
 Load regulation        ±0.5%                    Voltage regulation      ±2.5%
 Work temperature   -40℃- +85℃            Overall dimension      48*23*14(length*width*height)(mm)

And just gotta love that 1 month delivery time:
Delivery: Estimated between Tue. Jan. 15 and Fri. Feb. 1  Please note the delivery estimate is greater than 11 business days
Guess that's one "benefit" of Free Shipping.

bratan,
If you want to use barrel jack as the connector for 5V on custom board, that's fine then.  Low ohm PTCs only seem to be available in surface mount, 1206 size package.
Zener is overkill if you use external wallwart, they have built in short circuit protection as well.

[Reply #7 on:]

bratan,
If you want to use barrel jack as the connector for 5V on custom board, that's fine then.  Low ohm PTCs only seem to be available in surface mount, 1206 size package.
Zener is overkill if you use external wallwart, they have built in short circuit protection as well.

Awesome! Thanks!
I found some 250mA PTCs on Digikey, added them to the cart but didn't "pull the trigger" yet, still thinking if I really need it   From description resistance is 1.250 ~ 1.950 Ohm, is this acceptable?

[Reply #8 on:]

Well mine arrived in 10 days via Air as does most all the small stuff I buy on e-bay from Asian sellers, but I am on the west coast. I did check the output on my scope and could see about 20mv pp switching ripple which is about standard for switchers. I haven't load tested one to it's max current rating yet but they do adjust smoothly through the voltage range and I was able to get down to that 1.23vdc output they stated. These are very useful and the cost is outstanding.

It is strictly a step down regulator so the raw input voltage has to be higher then desired output value by a couple of 2 or 3 volts or so. I ordered 5 and got one single unit and four were on a single PCB with a score mark to just snap off each one. I snapped one off, it was easy.

Lefty

[Reply #9 on:]

"From description resistance is 1.250 ~ 1.950 Ohm, is this acceptable? "

With 250mA, that will be 0.5V of drop across the fuse.

I'd go for something more like this
http://www.digikey.com/product-detail/en/0ZCA0050FF2G/507-1480-6-ND/2165355

or this
http://www.digikey.com/product-detail/en/RXEF050/RXEF050-ND/1045827
if you needed thru hole.

You want this value nice & low:

R Min/Max 0.500 ~ 0.770 Ohm

[Reply #10 on:]

Nope to OP. I've read your schematic, but i didn't find any decoupling on the chips... Place 100nF cap across supply rails on each chip. And make sure to place the Xtal close to the Atmega.

// Per.

[Reply #11 on:]

I assume if I just plug in 5VDC adapter it will work

Your assumption is correct but risky: your wallwart may output higher-than-specified voltage, with lots of ripples, or you may reverse the polarity of it, etc.

You need some protection / filtering. I would put a forward connected diode + a tiny (1/8w) 4.7 - 22ohm resistor there, followed by a small electrolytic / ceramic capacitor (4.7u/0.1u). The resistor is there really to burn up in case of over current (kind of like a fuse).

[Reply #12 on:]

Nope to OP. I've read your schematic, but i didn't find any decoupling on the chips... Place 100nF cap across supply rails on each chip. And make sure to place the Xtal close to the Atmega.

// Per.

Thank you ! Does it matter where in the circuit I add decoupling caps?  I'll add one (C4) near power jack and second one near output to LED Matrix (C5). Is this ok?
Dhenry, that's an interesting idea about resistor!  I'm a little worry that diode will drop voltage (even .4 V drop might be an issue)...

[Reply #13 on:]

You want the decoupling caps right next to the power pin being decoupled.
For the uC, that is right next to pins 10 & 30.
What is HT1632? Is there a reason not to connect all Vcc & Gnd pins?

For your buttons, it preferable to use the internal pullup resistors and use the buttons to connect the pins to Gnd when pressed.
If you put a 1N4158 across R4, with cathode to Vcc, it will help prevent any odd latchup conditions when Reset is pressed.
What happpens is Reset may cause a high voltage spike that the chip interprets as going into high voltage programming mode, and then appears to lock up.
The diode dissipates that spike, and is found on current Arduinos.

[Reply #14 on:]

You want the decoupling caps right next to the power pin being decoupled.
For the uC, that is right next to pins 10 & 30.
What is HT1632? Is there a reason not to connect all Vcc & Gnd pins?

For your buttons, it preferable to use the internal pullup resistors and use the buttons to connect the pins to Gnd when pressed.
If you put a 1N4158 across R4, with cathode to Vcc, it will help prevent any odd latchup conditions when Reset is pressed.
What happpens is Reset may cause a high voltage spike that the chip interprets as going into high voltage programming mode, and then appears to lock up.
The diode dissipates that spike, and is found on current Arduinos.

HT1632 is external 32x16 bicolor LED Matrix (based on HT1632 chips).
As for buttons, it's a little bizarre but I could not get internal pullup resistors working on ATMega644P chip. It worked fine on Uno and Mega boards, but not on my breadboard 644P, so I gave up and just used external resistors.
Will 1N4148 work? I have few on hand  
I'm attaching 3D model of my PCB...