Hope something in there is useful.
Regards, Dan.
Thanks Dan very helpful..
- In the typical application of the switcher the Cff is 0.022uF.. they did'nt provide calculation but this value in the schematic.
- I've changed the mounting holes to 3.2mm and put some silkscreen circle of 7mm .. thanks for the advice.
- thank you again, i will change the ULN part in the next design since im telling my company there will be second version of the PCB. i want to try different methods to teach my self as much as possible..
- about the leds, i saw arduino circuit which uses same uC using mosfet to turn led from GPIO, though the pin can drive to max 7mA if VDDIO supplied with 3v3. which should be enough to turn 2mA led with the ULN in parrallel.. thanks again..
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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!).
I didnt get this part, wouldnt the pMos design prevent reverse polarity + uni-directional TVS have smaller negative clamping voltage?
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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.
i will change but just to teach me something, if i assume the adc will be shorted to 24v:
this means the zener diode 3v3 rated power should be larger than 75mW right?:
current: 24/1000 = 24mA -> (split into 2k resistor 1.65mA and 22mA on the zener ?the 100pF is'nt calculated or somthing i just placed a filter cap, i will replace it to 100nF i dont know how fast is the change..
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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.
Sure, i will put isolated part on the next version. thanks. and the termination jumper.
- 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.
- 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.
can i know, why "reasonable in this thing, i really looked for chassis grounding but really i think its complicated i found different methods and opinions. when and how i should connect the chassis GND?
different people say different things..
i got this RC from Atmel Evaluation board for the same uC:
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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.
Noted, thanks..
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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.
this gives me great idea, although im using 5V LDO for RS485 part. i know it will have low drop so maybe 4.9V or something but i wanted to stabilize it but combining switching regulator and Linear regulator is best to elimenate the ripple voltages right? which is good for the adc of MCU right? i've read this somewhere from TI to use the LDO after switching regulator is best practice.
- thanks this give me idea of doing termination jumper of USB Vbus instead of mux so if i want to program it near the PC without 24V.
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?
since now i buy it allready, i will use it for this application and next design i'll use smaller power one.
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.
im using JST connector with crimping on the cable side, you mean to place some text with naming some connector like "Xn:Yn" female male connectors?