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What parts to select for simple circuits ?

Posted: 27 Nov 2012, 04:07
by Sid
Being new to electronics design, I've hit a roadblock.

I have done a lot of research to learn what to do and I have successfully designed and soldered a few microcontroller circuits on prototyping boards. My circuits include a power supply, some digital inputs, a few simple sensors read by ADC, PWM and digital output, as well as an ISP connnection to program the mcu.

Diptrace is the first program I have used to create a schematic and I find it easy to use. I am able to create good looking schematics that I refer to when I hook up my circuits. The circuits work right off the bat, much to the credit of the Diptrace schematic capture.

The problems I am facing seem to be related to my lack of knowledge of some sort of secret "code". Sure, I can find schematic symbols for most of the parts I use and something that looks almost sensible for other parts. But that's about it. There is no way I am going to be able to create a PCB. Some of the parts I have selected only have a schematic symbol, i.e. nothing that will show up on the PCB. Other parts probably don't have the right footprint.

I have done a lot of research to try to find out how to go about this, but to no avail. The diptrace tutorials don't help me either as there seems to be an assumption that you already know a lot from using other programs - and I haven't used any other programs for this.


So to start with what must be the most elementary question - which parts should I select for resistors, capacitors and LEDs ? For the schematics, I have used the RES, CAP and POLCAP symbols but these seem rather generic and if some of the footprints actually are the right ones it would be by accident.

Re: What parts to select for simple circuits ?

Posted: 28 Nov 2012, 10:16
by Alex
When you select a component from the list on left side of window, its pattern is shown below the list.

If you have already placed components to schematic, you can open component properties, click on "Attache pattern" button and check if component has pattern or not. If not, you can attach pattern on this dialog window.

Re: What parts to select for simple circuits ?

Posted: 28 Nov 2012, 14:54
by Sid
But how do I find out which component and/or pattern to select in the first place ? What do the numbers in the component names mean ? And what do the numbers in the pattern names mean ?

Do I need to measure all the components I use, or are there general rules that apply to selecting components ?

Re: What parts to select for simple circuits ?

Posted: 30 Nov 2012, 04:22
by Alex
Component names represent either manufacturer part number or sizes of generic components (resistors, capacitors, etc.). Pattern names represent pattern sizes.
Please let me know what components you can't find in standard libraries.

Re: What parts to select for simple circuits ?

Posted: 30 Nov 2012, 17:54
by Sid
There are several I can't find, but what I have asked about so far is much more basic. I don't know the "code" used to identify components, and I can't find any reference to it online.

For instance, what is the difference between RES, RES400 and RES500 ? I can't even figure out which one to choose for my 1/4 watt through-hole resistors.

The sizes in a pattern name - the size of what, exactly, is given ? What are the units ? There seems to be several numbers given, and I don't know how to interpret any of them.


I was hoping for answers that allows me to find not only 1/4 watt resistors, but any basic component. And what to do when there isn't one - the tutorial for creating parts is way too advanced for beginners like me. I understand what is said, but I don't need to design a component with 100 pins. I need to be able to select the appropriate component and footprint for for instance a standard LED - or the know-how to design one myself if it isn't there.

Re: What parts to select for simple circuits ?

Posted: 04 Dec 2012, 04:56
by Alex
RES500 is resistor with 500 mils distance between pads. I thought it is obvious.
Other components have different names because they have different body, number of pins, etc.

Re: What parts to select for simple circuits ?

Posted: 04 Dec 2012, 12:48
by Sid
I guess I must be real stupid to ask these questions, then. Even more so as I still do not have the knowledge needed to create even the simplest of PCBs.

What is a stupid beginner supposed to do to learn the basics ? Keep ordering PCBs until one comes out right ? Use Eagle or another package that has users willing to help beginners ?

I don't feel like I am entitled to help or anything else, I am willing to work hard to learn this, but as I stated in the first post I have hit a roadblock and I don't know where to look to find the information that is clearly needed in order to make a usable PCB.

Re: What parts to select for simple circuits ?

Posted: 04 Dec 2012, 18:25
by John77
First of all you need a design intent.
Plan it all out on paper.
Build it on a breadboard if possible, so your are confident it works before going to a board.
Decide if you are going to use through hole or smd parts.
Number of layers. ( I rarely use less than 2)
Resistor sizes depend on what you are using it for. If it is a microcontroller pullup, you can use a 1/8th watt or smaller. If it is a power resistor you will need much bigger. You will need to do your maths to calculate the power rating etc. If you need a 2 watt resistor for example, go to the RS website, search the part, open the datasheet and look for the recommended hole centres. If it is 800 mils then use a RES800 resistor. The same goes for capacitors, inductors etc. The datasheets are your friend.
If the part hasn't got a pattern assigned to it, then again, go to RS (or Farnell) find the part, open the datasheet, look for the footprint detail ie. SOT-223. Open the Pattern Editor, search the pattern, then with the datasheet to hand, right click on the component in the Schematic Editor, click on the 'Attach Pattern' option, goto to the previously searched pattern and assign the pins to the pattern. If you cant find the pattern, make one with the pattern editor.

When you are happy with your schematic and all your parts have patterns, then convert to pcb. Lay the pcb out manually. On a 2 layer board, I usually use the bottom layer as a ground plane. Apart from being good for EMC, the 0v tends to be the most common trace, so makes routing easier if it has its own layer.
I won't go into good board layout practice, thats a whole topic on its own.
When you think your board is fully routed, then use the Error checking features to compare to the schematic, check net connectivity, and general errors.
If you use SMD parts and have vias to the bottom layer, then move the vias off the pads, and place them close to the pad. If you don't then you can have problems with the solder paste being sucked away from the component, and end up in the via, leaving a weak joint.
Now, you can print your pcb on paper, and look over it. Check the traces are wired as you expected. If you have the components to hand, you can tape your printed copy of your pcb onto a piece of polystyrene, and poke all the parts through to make sure the pattern centres are correct. If not, go back, make adjustments, print again, repopulate on polystyrene, and keep repeating until your are happy.
A common problem is hole sizes. You dont want to drill out a plated hole because the component lead is too big. Any parts you cant find in Diptraces libraries are easily made using the Diptrace pattern editor.

Please dont take this the wrong way, but keep moving from one pcb cad program to another looking for a super simple pcb cad program is not the answer. Stick with Diptrace, practice with a simple board to start with, and keep tweaking it until you are happy with it. Patience is a virtue!

I hope this helps

Regards
John

Re: What parts to select for simple circuits ?

Posted: 05 Dec 2012, 13:14
by Sid
Thanks a lot for your very thorough response, John.

I particularly liked the suggestion of using polystyrene to "test" the layout - that would never have occured to me and I will certainly do that when I get that far.

I have done quite a bit of research on how to construct my circuits and I have been able to select suitable components to get them to work. I have successfully designed and soldered a few microcontroller circuits on prototyping boards. My circuits include a power supply, some digital inputs, a few simple sensors read by ADC, PWM and digital output, as well as an ISP connnection to program the mcu. I mounted the circuits on a solderless breadboard to verify that they worked before proceeding to solder them on prototyping boards (with a layout similar to the breadboard).

Diptrace is the only program I have used to create a schematic and I find it easy to use. I am able to create good looking schematics that I refer to when I hook up my circuits. The circuits work right off the bat, much to the credit of the Diptrace schematic capture. I don't want to use another package and I don't expect there is one that's easier to use - diptrace seems just fine for me. The only real roadblock has been understanding footprints and I am still struggling a bit in that department although your answer is very helpful. My fear was that I would have to use some harder to use program in order to find a user community willing to help a stupid beginner.

My short term plans are to keep making microcontroller circuits, with through hole components. I will eventually move on to surface mount, but I want to get more soldering experience first. My first PCBs will be double sided, with the bottom used for ground connections (and the occasional dropped trace segment when necessary).

My resistors are all 1/4W 1% Tolerance Metal Film Resistors. The kits didn't come with a datasheet, but I have looked up several datasheets for similar resistors at Mouser and others, and while all of them list dimensions there is no recommended hole distance. Let's say the coated part of a resistor is listed at max 6.8 mm. 400 mils is 10.16 mm, so that leaves less than 2 mm on each side - that is probably too small ? Do I go with 500 mils ? 600 ? Is there a "standard" for this ?

What about the hole diameter ? Do you use 0.9 mm for anything smaller than 0.9 mm ? Are there some guidelines somewhere for this sort of thing ? I'm thinking that a tight fit would be an advantage for many components to hold them in place while soldering, but too tight or too thin would obviously be bad.

Most of my caps came in a kit with no datasheet either. Those are a little bit easier to figure out, as the leads are already spaced for mounting. So I guess I can just use my slide caliper to measure the spacing and select a footprint based on that. But what do the additional numbers in the footprints mean ?

For instance, a CAP200RP has a footprint called CAPPR-5.08/7.62. Judging from the shape of the footprint, I'm guessing that this would be suitable for a cylindrical electrolyte cap mounted "standing up". From what Alex told me, the 200 means the lead spacing is 200 mil. The first number in the footprint must be the metric equivalent of 200 mil ? What is the second number - the diameter of the case ? Some CAPPR footprints have a third number preceeded by h - height ? Why would height be relevant to the footprint ?

One fool can obviously ask more questions than ten wise men can answer.

Re: What parts to select for simple circuits ?

Posted: 05 Dec 2012, 15:56
by John77
Hi Sid.
Generally hole sizes work better with some clearance. If you wedge the component down a tight hole, it can be a right pain to rework it without damaging the board. Also,
if you have to substitute a part, a slightly larger hole can help. An example. I have a regular sets of boards made and populated. A 3 way 3.81mm pitch pcb terminal block went on 16 week lead time, but because the holes were 0.3mm oversize, I was able to get a 3.5mm part to drop straight in.
It might be worth you making your own library of common used parts, then you know they fit. The component and pattern editors are pretty simple to use.
I'm not sure about the 3rd dimension, I rarely use through hole parts.
Generally, I tend to round up hole sizes to 0.6, 0.8 1.0 1.2 etc.