Electronics > Beginners
i need a design Review
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hsn93:
Hello, i was working on my first PCB for past few weeks .. (i've read and watched tutorials, asked questions to achieve this) ..


I did ask here in a previous post but got only 1 reply which was helpful... but i think i had post it in early design stage ..
Previous Post: https://www.eevblog.com/forum/beginners/pcb-design-help-110518/

now i think i've completed the design i want review from you guys as im new to the field and this is for industrial product ..
mostly its pneumatic control system

the plan is to send this to PCB manufacturer next week.. ( thinking of JLCPCB )

DONT GET ME WRONG:
im not posting the whole design for someone to check and review the whole design but if someone can spot any big mistake in it...
and would be helpful to get comments on it..

so I'll start with the schematic:
Top Level:



Power:
Sw regulator:

mux and LDO:



Microcontroller
USB:

MCU SAMD21G18 (i've choose this bcz its arduino compatible if i couldnt program it through ATMEL ASF) i will use ARDUINO to generate HEX:


Solenoid valve driver

i ve got this darlington chip advice from previous post thanks to @Hero999

sensors:


Communication: RS485


now to the PCB:
Ground planes : top + bottom layers.. (didnt put them bcz its easier to see without them)

planes on layer3:
mcu plane : 3v3
left side plane: 24v <-- should i put it on the top plane?
planes on layer2:
5V output from mux



all


rs485


regulator (should i use electrolytic cap or ceramic is enough these days?)
small piece is TVS .. Vbr (28 - 30 - 32) [min typ max]
Pmos for polarity has MAX VDSS -30V


sensors and usb


3d view
i changed to USB type B i think its better and more industerial .. + its for programming only ..

------------------------------------------------------------

is there any test pad tips i should i do somthing ? because im going to order pcb, solder and test my self .. should i put the vias because i hide them all (better looking)


should i put some capacitors here .. bulk? right ?? what type and what range of capacity would be enough?
i've read and watched davids videos that electrolytic type get damaged and leak .. so ceramic is good ?
capt bullshot:
I did just a quick scan over your design:
- You have ground planes, that's fine, ensure they don't have too long and too many cutouts and slots.
- Your RS485 doesn't have a ground. RS485 without ground reference (third wire or shield connected to ground at both sides) doesn't work reliably.
hsn93:
Hello, yeah thank you very much good note ..

the plan was to use a cable with 4 wires


2 pair 22AWG wires for 24V and 2 for RS485 ..

which both come from different controller that i need to design later .. or is it bad idea? i dont really know, i've done some research on this topic but really didnt find any clear answer .. its not long distance, around 10 meters ..


i have read that RS485 should have special impedance controlled signal of 120ohms .. but i dont know should i really care for such a distance specially with cable prices that i've found, and one cable would be easier to install than 2 different cables (power + communication) ... im not sure really what i should do.. i just went for this option is it critical ?


------------------------------------------



actually your comment made me post this two ground planes.. i've forgot to post them ..





ok about cut outs.. i dont know how long is long .. but i will start placing vias between grounds every where ..
dmills:
Cff1 in the switcher is suspicious, I would have expected 22pF here not 22nF, but I have not checked the datasheet.

Have a look at the head diameter of your mounting screws, I usually use an M3 screw in a 3.2mm hole and assume a 6.5mm head diameter, I think your mount near the USB connector is probably way too close. Also since you are not grounding thru the mounting screws (reasonable in this sort of thing) you might want to either make the annular rings much bigger or place a keepout to chase the ground pours away from the screw heads.

Any reason not to just use a second ULN for the valves? It saves you 11 parts which is probably more valuable then the cost of the chip?

A resistor and LED across each solenoid connection will be something you (during software development) and your users (during commissioning) will worship. On that subject a power indicator is well worth the cost in reduced frustration. If you have a spare pin or two on that micro then LEDS and test pads are worth the time, just having an led that you can make flash to indicate the processor is still up is a good and worthwhile thing.

Since the 24V line connects to the sensor wiring and also the solenoid wiring, a polyswitch or fuse or something might be a good idea, maybe a 1A or so polyswitch on the sensor supply (With a red led and resistor across it to indicate a fault), and whatever makes sense for the solenoids. This is as much to protect the cables as anything, you probably don't want a few amps or tens of amps accidentally ending up on a damaged sensor cable, for the same reason a fuse or polyswitch at the input is a good plan, as it avoids destroying the input transorb when bubba hooks the supply up backwards (Or make the input transient protection a bidirectional part!). 

The 1k resistors on the sensor inputs should be sized to tolerate a short to 24V, so need to be rated for 0.5W or so, also 100pF filter cap? The pressure change is THAT FAST? I would have thought 100nF more like it. 

The RS485 needs to be a three terminal connection with a ground (Or better in an industrial controller, an isolated RS485 part, with the cable ground connected to the isolated ground on the chip). R5 needs to have an option jumper so it can be taken out of circuit except for the boards at the ends of the lines, and please label this jumper 'termination' or something similar.

What is that RC network doing on the USB shield connection? There is all sorts of tosh done with these, but really IMHO it should just be tied directly to chassis.

If you make the three wire sensor connectors have +24 on the middle pin, then it is likely that a sensor will survive being plugged in backwards, which is nice.

Given the USB is just for configuration, why bother to do anything much with VBUS? Instead power the board from the 24V rail all the time.
It is just extra parts that buy you nearly nothing, and you could then run the switcher at 3.3V output directly, eliminating the 3.3V regs on both the mux page and the one on the RS485 sheet.

The switcher is overkill, I figure maybe 100mA @ 3.3V on a bad day, there has to be something smaller and cheaper that can do that?

Finally, look to your silk screen, label connectors (Right down to what pin does what) and LEDs, and if you are fitting connectors rather then soldering wires (please tell me you are fitting connectors!) then some text saying what the mating half needs to be is always a good thing, "Mating Plug Molex Mate-N-Lock XXX-YYYY" can save a lot of cursing in 5 years time.  Also, PCB Name and revision number are good as is some text saying what it does.

Hope something in there is useful.

Regards, Dan.
tron9000:
The 24V plane, it necks down too much, I see what you're trying to do by keeping it away from other circuitry by going round the outside, and as a result your necking the plane down. This should really be avoided. let the plane flow round the pins of the solenoid connectors to allow the pins 24V/Vsupply pins to connect. It would be a good idea to move this plane to an outside layer. Some bulk capacitors in the areas you've highlighted won't hurt and would take the sting out of any transient from the solenoids turning on.
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