Kits: Plug-in Bread-Board Power Supply
--- v1 building instructions ---
- Soldering Iron and Solder: A 15 Watt iron will get the job done, but it'll be easier with a 25 or 35 Watt
iron. Don't lose sleep purchasing your first iron; any cheap RadioShack one will do. We like small chisel or screw
driver shaped tips (say 1/16th of an inch wide), but personal preference plays a big role. The goal is to use a tip
that's about the same size as the pad you're soldering. This way you get you get as much surface area contact as
possible (better for heat transfer) but are still small enough to avoid touching neighboring pads.
For solder, we recommend .02 or .031" diameter, rosin cored flux, tin-lead solder, either 63/37 or 60/40, whatever's cheaper. See our soldering guide for explanations. Lead-free solder is a little bit harder to use since it "wets" metal somewhat slower and has a slighty higher melting point. If your kit has sat on the shelf for a while (a year), some steel wool or a pink eraser is useful to remove oxidation from the components and board before starting.
- Needle Nose Pliers: Used to clinch or bend component leads, and also during component removal.
- Flush Cutters: Used to trim leads close ("flush") to the board.
- Solder Sucker or Solder Wick: Used to remove solder.
- Clamp: A table top will be fine for most of the job, but a clamp can be handy, especially when removing components.
- Multimeter: To check your work.
David (the kit designer [uCHobby.com]) recommends starting with the shortest components first, and then progressing by height. This way, the component you're currently on will be the tallest on the board, so when you turn the board over to solder, that component will be held in place by the table. Other approaches are to start from the center, or to do the harder components first so that easier components don't get in the way later.
We'll start with the switch:
To keep the switch in position while soldering, you can either rest it against the table or clinch (bend) the leads with pliers to hold it in place. In mass production, machines automatically clinch leads to hold components in place while they're passed over fountains of molten solder (wave soldering).
First tin the tip with a small amount of solder, then hold the tip so that it can heat both the component and pad at the same time. Apply solder to the opposite side--the component lead and pad should do the melting. The golden rule of soldering is to apply solder to the components, and not the iron, but there is a catch to this: a "dry" iron tip does not conduct heat very well at all. So, some molten solder is needed in between the tip and component in order to transfer heat quickly. When I solder, I first try to melt a little solder in between the tip and component to establish a "heat bridge" and only then apply solder to the opposite side.
After soldering the first pin, double check the component's alignment. If it moved a little, it's easy to reheat the single pin and adjust the position, but once multiple pins are soldered you'll probably have to remove the entire component to make adjustments.
Now trim the leads with the flush cutters. For extremely critical applications, leads are not permitted to be trimmed after soldering since there is some risk the joint will develop small cracks that eventually lead to failure, but this is unlikely; trimming afterwards is OK for just about everything except NASA circuits.
Note the notch on the rectifier and make sure it aligns with the notch on the board when you install it. In general, pin 1 on an IC (integrated circuit) is the first pin counter-clockwise from the notch. It's also common for board designers to use a square pad instead of a round one to designate pin one on the board.
Clinch leads on opposite corners of the IC with your finger nail or pliers to hold the chip in place while you solder.
The side pin on the barrel jack may need to be bent in a little before installing. Once again, either clinch the leads or use the table to hold it in place while you solder.
Ideally, the solder will form a smooth ramp that connects the board to component lead. If it looks like the solder is just sitting on top, and not adhering to the joint, it's probably a bad joint. Officially, the angle between the solder and board should be less than 90 degrees.
Many how-to guides state that the joints should look shiny, but this isn't always the case. Lead-free joints typically look dull, but are actually fine. Even some lead-based solder may not come out shiny depending on the type of flux used. The main determining factor is whether the solder looks like it's clinging to the surfaces, and not beading up like water on a waxed car.
It doesn't matter which direction the smaller ceramic capacitor is installed, but the electrolytic caps will fail and possibly explode if installed backwards. The longer lead is usually positive, and the negative lead is indicated by a minus sign on the body.
In general, components should be inserted as far as possible. It's OK if some solder touches the meniscus (yellow plastic covering) of the ceramic cap, just as long as the meniscus doesn't interfere with the joint on the other side of the board. The only exceptions to pushing components all the way in are ones that have metal casings that might short circuit traces (like crystals). Also, some components might require clearance for heat dissipation. Clearance should also be given if the component might seal off the hole and prevent solder from filling the barrel.
Clinch the leads to hold the caps in place, then solder and clip the leads.
| Vout | R1 (Sel A) Ohms | R2 (Sel B) Ohms |
|---|---|---|
| 9.1V | 120 | 750 |
| 5.98V | 200 | 750 |
| 5.08V | 270 | 820 |
| 3.3V | 240 | 390 |
| 2.99V | 240 | 330 |
Use the above table to choose an output voltage and its corresponding resistors. The formula is:
Vout = 1.25*(1 + R2/R1) + .00005*R2
(see the voltage regulator spec sheet under the applications section)
If one of those voltages doesn't suit you, you can choose a different pair of resistors. Or, if you'd like the circuit to remain flexible, leave R2 off and simply place a new resistor between J5 and ground on your bread board. The above picture shows a potentiometer (variable resistor) in this position.
Pre-form the resistor leads to fit into the hole so that both sides can be inserted at once without having to forcefully drag one side through a hole. The by-the-books method for doing this is to hold the resistor lead with pliers so that the connection to the resistor body isn't stressed while the lead is being bent (In critical applications, serrated pliers aren't permitted because the serations could scar the lead and leave it susceptible to vibration damage). Damage is somewhat unlikely, though, so fingers work fine in most applications.
If the hole spacing is somewhat larger than the resistor, hold it against the board to determine the bend location with pliers. Or, if you have a lot of resistors to bend, a lead-forming tool sometimes called a Christmas tree can be used.
Resistors should be inserted flush to the board, and for ease of reading later on, they should all be oriented in the same direction (ie, the gold tolerance band is always on the right) so you can read off values without having to rotate the board.
Ideally, solder will be drawn through to the other side of the board and form a ramp on the top side as well as the bottom (see top resistor). This, while stronger, isn't necessary. Even in critical applications, IPC / military standards don't require solder to fill more than 75% of the barrel height, or even wrap completely around the circumference.
Install the headers on the opposite side by resting the board against the table to hold them in place. After soldering one pin, re-check the alignment. If you can't use a table (because other components are in the way), you could also pre-apply solder to a hole, and then re-heat it while inserting a row of headers to get started.
The LED should go in with the positive lead towards the middle of the board as shown. Another way to tell is to look at the back of the board and see that the outer pin is connected to the ground plane.
When we soldered the first pin of the screw terminal, it was a little crooked. The picture shows reheating of the first pin to re-align it.
Be sure to install the voltage regulator with the heat sink facing towards the outside.
