Need help improving PCB layout design

[EPAIII]

Well, I admit I did not notice he was using the CMOS version. I have used that version less than the standard 555.

That's what's great about the BB; you get almost instant feedback when you post something. It is great and the OP probably winds up with a much better solution than any one of us can come up with.

[ElectricPower]

Thanks for all good help guys

I will check all your sugestions and make som changes. But my biggest question so far is:

Where can i find 2W 2512 resistors with value 20ohm and 30 ohm? I prefere to use AliExpress.

[Niklas]

One alternative to using a larger package is to split it up on several smaller packages. Instead of 50 ohms dissipating 500 mW, you can connect 2, 3 or 4 resistors packages in parallel and use standard rated 1206 parts. With short duration on the pulses you can use the full power rating of the resistor.

100R//100R
0.25W per resistor

150R//150R//150R
0.18W per resistor

180R//180R//180R//180R
0.14W per resistor

[Cerebus]

With short duration on the pulses you can use the full power rating of the resistor.

With short duration (and suitable duty cycle) you can use considerably more than the full power rating of the resistor. It's all down to quantifying what "short" means for the particular part in question. As long as you don't exceed the pulse power capability of the part, and the pulses are far enough apart for the heat from a pulse to flow to dissipation you're golden.

The trick is to look for parts that the manufacturer has characterised for pulse performance. For instance, this is from the data sheet for Vishay CRCW-37 e3 resistors, described on the data sheet as "Standard Thick Film Chip Resistors" i.e. nothing special. Sizes are 1206, 1210 and 2512.



The nominal power rating for the 1206 part is 0.25W, but you can see that for a short enough pulse you can put crazy amounts of power through the device (subject to breakdown voltage, ohm's law and pretty much nothing else). It will take ~100W (40,000% of the nominal rating) for 200us. All you have to do then is take the 0.25W rating into account in setting your maximum duty cycle based on average power through the device.

[fourfathom]

For instance, this is from the data sheet for Vishay CRCW-37 e3 resistors, described on the data sheet as "Standard Thick Film Chip Resistors" i.e. nothing special. Sizes are 1206, 1210 and 2512.
[...]

Nice chart!  So in this case a 0.5W nominal 1210 resistor can dissipate 1.5W during a 1 second pulse.  Given the OP's 1/25 duty-cycle 500mW pulses, there should be adequate margin with his single resistor.

So my caution was excessive.

[Cerebus]

But, my natural caution would be to derate things a bit. Also what's the betting that Vishay's figures are taken for resistors with good heat-sinking on the PCB - something like 80% of SMD resistor heat dissipation is conduction via the board [no pun intended] . There are callouts to various EN and IEC test methods that specify the conditions that the tests are performed under which may specify minimum pad areas for dissipation etc.

[james_s]

It's worth considering, but in this application I wouldn't be too concerned about it. If this were a design for some sort of life or safety critical device then sure, derating is a good idea, but to keep a power bank turned on? What's the worst that could happen? The resistor fails and the power bank turns off, nobody dies.

[Cerebus]

What, you think it's not worth making something reliable for its own sake, only if it's safety critical? Really?

[fourfathom]

What's the worst that could happen? The resistor fails and the power bank turns off, nobody dies.

Or the resistor and board catch fire, everybody dies?

[james_s]

What, you think it's not worth making something reliable for its own sake, only if it's safety critical? Really?

No, not really. You're not understanding me at all, I just meant I wouldn't obsess over it too much. Look at the data, add a reasonable margin, try it out and confirm it works as expected, it'll probably be fine. No need to analyze a simple hobby project to death. A tiny little chip resistor across 5V is not going to catch fire.

[ElectricPower]

I have tried the circuit on the breadboard with an 1/4watt resistor and the resistor was not very hot after about 30 minutes.

[Cerebus]

What, you think it's not worth making something reliable for its own sake, only if it's safety critical? Really?

No, not really. You're not understanding me at all, I just meant I wouldn't obsess over it too much. Look at the data, add a reasonable margin, try it out and confirm it works as expected, it'll probably be fine. No need to analyze a simple hobby project to death. A tiny little chip resistor across 5V is not going to catch fire.

I presume by "add a reasonable margin" you mean "derate it a bit" just like I said. So what was the fuss in the first place? You added all the guff about safety critical. If you didn't think it was really relevant and you didn't want us to understand that, then you'd be better served by not saying it. If you wanted to say that you thought it was overanalyzing then you should have said that, not something else. As someone else mentioned recently "we're not mind readers" we're only going to understand what you actually say. Personally I think giving a few seconds thought to derating a component that exists just to dump energy isn't overanalysing if you want a circuit that keeps working reliably.

And talking about what you actually said, I'd understand "little hobby project" as a bit unnecessarily dismissive of the OP. Sure, it's not the greatest project in the history of mankind. So what? If it's too insignificant for your consideration, why are you here? If it is worthy of consideration, why not help the OP to make the best job of it he can? I've noticed a tendency recently from some of the professionals on here to sneer at the hobbyists, or people they think are hobbyists, on here - it's not a very attractive attitude.

[ElectricPower]

First off all, thanks for all help!

I can say that i have changed the "design" and i have added an 2512 fottprint instead of 1206 for the load resistor. I have also ordered 2512 1W resistors.

[fourfathom]

I have tried the circuit on the breadboard with an 1/4watt resistor and the resistor was not very hot after about 30 minutes.

It sounds like you are OK, but do realize that a leaded 1/4W resistor may have thermal characteristics that are quite different from a small surface-mount resistor.  As Cerebus and I have mentioned, with the surface-mount part the circuit board layout (pad and adjacent copper area) will be a big factor.

[mc172]

Does it really need an entire second and that amount of current?

[ElectricPower]

Current, yes. It needs at least that current. One second? I really do not know. Maybe not. I trust on Mr Carlsons Lab here... i think...?

[james_s]

I presume by "add a reasonable margin" you mean "derate it a bit" just like I said. So what was the fuss in the first place? You added all the guff about safety critical. If you didn't think it was really relevant and you didn't want us to understand that, then you'd be better served by not saying it. If you wanted to say that you thought it was overanalyzing then you should have said that, not something else. As someone else mentioned recently "we're not mind readers" we're only going to understand what you actually say. Personally I think giving a few seconds thought to derating a component that exists just to dump energy isn't overanalysing if you want a circuit that keeps working reliably.

And talking about what you actually said, I'd understand "little hobby project" as a bit unnecessarily dismissive of the OP. Sure, it's not the greatest project in the history of mankind. So what? If it's too insignificant for your consideration, why are you here? If it is worthy of consideration, why not help the OP to make the best job of it he can? I've noticed a tendency recently from some of the professionals on here to sneer at the hobbyists, or people they think are hobbyists, on here - it's not a very attractive attitude.

The "fuss" it seems is entirely from you, ranting for multiple paragraphs over a simple comment from me of the nature of "don't over-think it". I'm sorry, I guess I should have included a trigger warning.

[Cerebus]

I presume by "add a reasonable margin" you mean "derate it a bit" just like I said. So what was the fuss in the first place? You added all the guff about safety critical. If you didn't think it was really relevant and you didn't want us to understand that, then you'd be better served by not saying it. If you wanted to say that you thought it was overanalyzing then you should have said that, not something else. As someone else mentioned recently "we're not mind readers" we're only going to understand what you actually say. Personally I think giving a few seconds thought to derating a component that exists just to dump energy isn't overanalysing if you want a circuit that keeps working reliably.

And talking about what you actually said, I'd understand "little hobby project" as a bit unnecessarily dismissive of the OP. Sure, it's not the greatest project in the history of mankind. So what? If it's too insignificant for your consideration, why are you here? If it is worthy of consideration, why not help the OP to make the best job of it he can? I've noticed a tendency recently from some of the professionals on here to sneer at the hobbyists, or people they think are hobbyists, on here - it's not a very attractive attitude.

The "fuss" it seems is entirely from you, ranting for multiple paragraphs over a simple comment from me of the nature of "don't over-think it". I'm sorry, I guess I should have included a trigger warning.

Still sneering I see. How do you think that helps?

[mc172]

Current, yes. It needs at least that current. One second? I really do not know. Maybe not. I trust on Mr Carlsons Lab here... i think...?

You mean this one? https://youtu.be/lOL3ic8msas?t=981

I scanned through the video to find the schematic and noticed that it says 2.5 seconds on-time. Seems excessive but without that exact power bank to try it on, who knows.

It's in your interest to make the pulse as short and as low current as possible, so you will need to experiment with on times, off times and load resistor values anyway. Also, each different model of power bank is going to have different sleep timers, I highly doubt that it's standardised and even if it were, that doesn't seem to stop manufacturers doing whatever they want anyway.
Just build one and find out, you can easily modify the on and off times by changing components and you can just put your finger on the resistor to see whether it gets unreasonably hot.

[fourfathom]

Just build one and find out, you can easily modify the on and off times by changing components and you can just put your finger on the resistor to see whether it gets unreasonably hot.

I know this is beating a dead horse, but at least *try* to calculate the power dissipation and temperature rise issue before you build the thing.  It's not that hard to get close enough.  Later on you can use your finger to verify.  I actually have an IR camera attachment for my cellphone that I use to catch any thermal surprises, but if it's going to be at all close, I first do the simple math.

[ElectricPower]

Yes, i have ordered the pcb now. 120 pcs for just 6 dollar, included chipping to Norway! Amazing

[mc172]

I know this is beating a dead horse, but at least *try* to calculate the power dissipation and temperature rise issue before you build the thing.  It's not that hard to get close enough.  Later on you can use your finger to verify.  I actually have an IR camera attachment for my cellphone that I use to catch any thermal surprises, but if it's going to be at all close, I first do the simple math.

When I said "just try it" I didn't mean to see whether the resistor can dissipate that amount of power, it has already been demonstrated that it is inadequate. I was merely offering up alternate paths to go down. If the duty cycle can be lowered, the power dissipation problem won't even exist. This information is not yet known and it seems like the OP doesn't have any equipment to try it. Just building one provides that equipment.

[Doctorandus_P]

I find the combination of wanting "the smallest possible" in combination with a DIP NE555 quite weird. Especially since all the other parts are already SMT.

[fourfathom]

I find the combination of wanting "the smallest possible" in combination with a DIP NE555 quite weird. Especially since all the other parts are already SMT.

True.  Even if he already has a bin fill of 555 DIPs, the surface-mount parts are about 10 cents each, and the hassle of mixed through-hole and SMD assembly would certainly drive me in the all-SMD direction.

[DBecker]

I find the combination of wanting "the smallest possible" in combination with a DIP NE555 quite weird. Especially since all the other parts are already SMT.

True.  Even if he already has a bin fill of 555 DIPs, the surface-mount parts are about 10 cents each, and the hassle of mixed through-hole and SMD assembly would certainly drive me in the all-SMD direction.

I expect that he is concerned about soldering the SO-8 package rather than the familiar DIP.

Once he tries it, he is likely to find that the SO-8 package is faster and easier to solder than through hole parts.  It's only when it gets to smaller pitch that the size becomes a problem.

I suggest reducing the number of unique parts.  There isn't a good reason to have both a 4.7uF and 10uF capacitors.  The power supply bypass is flexible, often optional, and can be whatever value is required elsewhere.  Similarly the gate resistor can be the same 470R part used elsewhere.  Or use  220R or 270R 0805 resistors everywhere.  Five 270R resistors in parallel will handle the current at 5V and nominally be in spec for power.  That also gives you an opportunity to experiment with smaller keep-alive current.  10-20mA should be sufficient, it shouldn't take 100mA.

I wouldn't flood pour 5V anywhere close to mounting holes or the edge.  At minimum there should be a ground ring around the edge.  Even if your design rule requires copper clearance to the router line, assume that your hobbyist board will be gripped by a grounded hacksaw blade and bolted down with serrated flange screws.

It doesn't apply here, but as a rule put the ground connection closest to the board edge and try to keep the power supply protected by outboard pins.

Hand soldering has more constraints than reflow.  SMD components should be lined up in even parallel lines.  They can be tightly adjacent, but leave a big gutter at the ends for a soldering iron.  Think of the angle the tip needs to make, and don't put a tall component at the end of the gutter.

On the plus side, hand soldering means that you don't need to be as careful about thermal relief gaps around pads and pins.  It only takes an extra second while soldering to add extra heat to a pad that would stubbornly refuse to reflow.  No thermal relief gap means that you can better use pin headers and connected wires to carry heat away.