3.7 Power Supplies
To build anything electronic, you are going to need a power supply. We have already seen how we can rectify AC current, smooth it, and even regulate it slightly; however the devil is in the details, so the question is what exactly to use for a power supply? You choices are simple:
- Battery power.
- Buy a power supply.
- Use a wall transformer and a voltage regulator circuit to derive the voltage you need.
All choices are appropriate in the right circumstances. For example, once your design is complete and you want to make it mobile, you might want to make your project battery powered. This design is nothing more than solution 3 with DC battery power rather than AC converted to DC with a wall transformer or bridge rectifier: You will still have to regulate the battery power since the voltage on the batteries will change as you load them. Solution 2 is what you will need for most of your designs; that is, you may need many different voltages in your system designs and you may need to adjust voltages and experiment. The only way to do this is to build an adjustable voltage supply (not a good idea yet) or to just buy one for $50–200. The last choice if you know what voltage you need is to simply build a power supply into your system or on another board that you can use; let’s focus on that one (which is also applicable to 1).
3.7.1 Voltage Regulators
All right, step one is to obtain a good starting point, and that’s the AC outlet. We need to get power from somewhere, so the first thing you need to do is buy or obtain a 9–12V DC wall transformer; if you can’t find a 9–12V DC wall transformer then a 12V AC wall transformer will do, but we are going to have more work. We need to first get the power into our system: To do this there is a connector on the end of most wall transformer cables; you need to buy a mate to this connector. Figure 3.48 shows a typical 2.1mm wall transformer male end. You can also just cut the wires coming from the wall transformer if you really want to.
Figure 3.48 Wall transformer cable connector for 2.1mm.
22.214.171.124 Stage 1: Rectification and Filtering
In either case, let’s first deal with the AC/DC part. If you have a AC transformer then the first step is to rectify and filter it; if you have a DC supply then rectification and filtering won’t hurt it, so we can use the same input stage for both AC and DC power feeds. Figure 3.49 shows the first stage to our power supply: AC or DC current is brought in, full-wave rectified, and then filtered through a capacitor and feed to the output stage. Of course, I highly suggest you use a DC wall transformer since the bridge rectifier and 20,000μF capacitor are pretty large and a waste of space if you have a DC Vin.
Figure 3.49 Stage 1 of the power supply design.
Also, notice the SPST switch S1. This is to turn the power on and off; I can’t tell you how many designs and consumer products I have in my room that have no power on/off switch. The famous Iomega "ZIP Drive" is a perfect example; to turn it off you rip the power cable out of it!
126.96.36.199 Stage 2: Regulation
Now, we are ready to regulate the power to our desired voltage. This is accomplished with a "voltage regulator" device. We have actually dabbled with designing regulators in the previous sections, but let’s leave the analog design to the professionals at Texas Instruments, National Semiconductor, Fairchild, etc. and use one of theirs. Before getting specific, let’s take a look at how a regulator fits into the system. Most regulators are 3 terminal devices (however some have many inputs): one input for Vin, one for ground, and one for Vout. Figure 3.50a shows the common block diagram schematic symbol for a 3-terminal voltage regulator. Additionally, most regulators require filtering capacitors on the input and output at very least, so you might see a regulator with the extra components Cin and Cout as shown in Figure 3.50b.
Now, before discussing details, let’s cover the general types of voltage regulators, so you know they exist and don’t try to re-invent the wheel.
Figure 3.50 Block diagram of 3-terminal voltage regulator.
- Linear Regulators—These are the standard 3-terminal regulators that are used most commonly; they regulate a higher voltage down to a smaller voltage. Typically, they come in voltages 5–24V: Some are fixed, some are variable. The input voltage usually has to be 2–3V greater than the target voltage to regulate to. These regulators come in a number of packages including the TO220/202, TO92, and TO3, as shown in Figure 3.51. Linear regulators are usually DC-DC, meaning DC in DC out.
- Switching Regulators—Switching regulators are more exotic and can either step down a voltage or step it up (and even invert it). These regulators usually come in surface mount packages and are highly efficient. Additionally, they may have many more pins than linear regulators and may need external capacitors and inductors. Internally, they operate by generating an AC signal from the input DC signal and then using it to generate any voltage needed via capacitive and inductive networks. Again, switching regulators are DC-DC.
- Charge Pumps—Charge pumps are usually used to generate high voltages from small voltages and are usually DC-DC converters. However, they have very little current drive and thus are used in applications where very large voltages are needed, but very little current, such as LCD applications. In general, we will use linear and switching regulators in all our work, with the emphasis on linear regulators since they are so easy to hook up.
Figure 3.51 Regulator packages.