stress relief DO NOT ROUTE TRACKS BETWEEN THE PINS OF IC’S
13.4.8 Make the Negative Printout
(Remember the text on the PCB appears reversed)
* Open TDA2822verA.brd in Eagle
* From within the Eagle Board Editor start the CAM Processor
* select device as PS_INVERTED * Scale = 1
* file = .ps
* make sure fill pads is NOT selected this
13.5 Extra PCB making information
Grids
An important point to note is that the rulers and grids in Eagle are generally in inches, this is because IC’s (such as the TDA2822) and other components have legs that are 0.1 inch between centres.
The current grid spacing is shown in the layout window most likely as 0,05 inch, if you want to see the actual grid, type grid on. For all layouts we will use inches because that is the spacing of component legs. Although when we specify a drill size wew ill use mm. Also never change the grid size, we will use 0.05 inch (50 thou). If you want ot start squeezing things together – well don’t especially in your first few boards. it just makes the boards hard to etch and to solder.
Track width, copper thickness and current ratings
The board we buy is 2oz (ounces), that means the amount of copper in one square foot of pcb is 2oz, That equals 0.0028 inches thick (2.8 thou – or just to confuse you PCB people often say 2.8mils). We generally use 0.032 or 0.04 inch tracks on our boards in the classroom as they print and etch easily.
Even though tracks are made of copper and are a conductor, they are not perfect conductors and have some resistance. This means that as charges move through the circuit the tracks get warm! The thinner they are the higher the resistance and the warmer they get. If they get too hot they will burn up (and smoke and possibly flames will appear).
A track of 0.04 inches width on the boards we use is about 0.006 ohms per inch will when carrying a current of 4 amps will rise in temperature by around 10 degress which is ok. Our circuits don’t in general need to carry 4 amps but its good to know this sort of thing. If you want to carry 10amps then go to about 0.15 inch to be on the safe side!
Grounding
The ground connection is a circuit is the path for current back to the power supply, and the bigger and the more of it we can make the better. We almost always make single sided pcbs so its a good idea to put a ground right around the whole circuit board. There is an example of using polygon fill later on.
Forwards and Backwards
You must always have your schematic and layout open at the same time, if you have only one open then any changes you make to one will not appear on the other. Then when you open them both Eagle will complain and say that no forward-backward annotation will happen, now you are stuffed, it can actually take longer to fix annotation problems tha starting all over again!
13.6 Component Forming Codes of Practice
Component leads are bent at least 2 mm away from the component body, not bent close to the body as this would stress the component and reduce its life expectancy.
The component is placed firmly against the PCB.
This helps mechanical rigidity. (Components would only be put up off the board if during normal operation they would become warm enough to damage the PCB itself)
If there is not enough room to lay the component flat on the PCB then one leg may be bent over.
Under the pcb the component leads are bent over slightly to hold the component in place during soldering, they are not bent flat as then it would be difficult to remove the component later on.
Component leads are cut off after soldering; during soldering they act as a heat sink and keep excess heat away from the component.
13.7 TDA2811 wiring diagram
13.8 SKETCHUP Quick Start Tutorial
1. From the menu select Window then Model Info and then select units, set up units as shown in this picture.
2. Close this dialog box
3. Select the Rectangle tool in the toolbox (the set of tools on the left hand side of the SketchUp window).
4. Click the nouse mouse pointer once on the origin and move it right and upwards to start drawing a rectangle (do not click again to stop drawing).
5. In the bottom right hand corner the dimensions of the rectangle are shown; without
clicking there just type on the keyboard 200,100 and press Enter. The rectangle will take on the dimensions you have typed in.
6. Your rectangle may well have disappeared because you are zoomed out too much. From the tool box, identify the zoom extents tool by hovering the mouse pointer over the buttons. Get use to the other zoom controls now and zoom out a little.
7. From the menu select Window then Display Settings and change the Edge Color to By Axis (now you can see whether what you are drawing
lines up with the axis you want it in).
8. Under the menu is the tool bar identify the Iso view button (isometric) and click it.
9. In the toolbox identify the Push/Pull tool and then move the mouse pointer over the
rectangle, the rectangles surface will change in appearance. Click once on the surface and drag the rectangle upwards along the blue axis into a 3D box; type 75 as a dimension and press enter. Your box should be aligned to the three axes and the edge colours should match the axes colours.
10. Select the Tape Measure from the toolbox and
click on the upper front right corner and then move along the green axis, type 30 and press enter, a grey construction point will appear.
From the same corner place another construction point 50mm down the blue axis.
11. From the toolbox choose the line tool and draw a line between the two construction points, notice how the cursor snaps to the construction points as it nears them (it also snaps to edges, ends and centre points and each has a different colour).
13.9 Creating reusable components in SketchUp
Creating a component that you can reuse in other SketchUp drawings is simple if you follow a few simple steps 1. You need a large surface on which to create the component. For
example, if we are to make a breadboard, create a flat horizontal surface larger than the breadboard to start with
(e.g. 300 x 300mm).
2. Create the base for the breadboard component (e.g. a rectangle 165 x 55 mm).
3. Extrude the breadboard 10mm.
4. Use the TapeMeasure button to mark out the two points for the groove in the centre of the breadboard 5. Then draw two parallel lines.
6. Extrude downwards 3 mm to make the slot
7. Select all of the entities you want to include in the component.
Then right click and in the drop down menu select Make Component.
8. The Create Component dialog box opens:
o Name. Type a name for the component.
o Description. Optionally enter a description of the component.
o Glue to. Select a glue-to
alignment. The most flexible choice for components you want to glue is "Any."
o Cut opening. Select this if you
want the component to cut an opening in the face to which it is being glued. For example, you would
typically use this option for a window.
9. You need to view the components in your model. From the menu select Window then click Components. In the Components window click the “In Model” button (little house),
10. In the components window right click the component and save it somewhere you can find it again.
Adding a component to another drawing:
1. In the new SketchUp drawing
2. From the menu choose File then Import 3. Select the component you want to import 4. It should ‘glue’ onto faces of your model.