I prefer to use a double-pole switch although it's not absolutely necessary. (My 25.2 volt/two-amp transformer measures 28.6 volts with no load.) This means that the actual no-load/peak output voltage for my 25.2 volt transformer is 40.4 volts! As you can see, it's always important to remember that the rated RMS voltages for AC power are substantially less than the actual peak voltages.įigure 2 provides a typical input power conversion and conditioning design. Often there is about a 10% increase in the rated output with no-load. Additionally, the voltage specified is the minimum voltage expected under full load. The output of the transformer is also specified as an RMS voltage. These are typically military or aeronautical devices and are generally not suitable for use on 50/60 Hz power (or vice versa). Occasionally, you may find 400 Hz transformers available. ![]() Additionally, the frequency stability of the power line is usually excellent and rarely a consideration. Transformers rated for 60 Hz will generally perform well on 50 Hz and vice versa. ![]() The input frequency is universally 60 Hz in the USA. Don’t forget that if you expect to see noise/spikes on the line, you need to add that noise/spike voltage to the peak voltage. The absolute minimum safe working voltage for your capacitors is 200 volts (250 volts is better). If you think the actual voltage is 120 volts, you may use 150 volt capacitors. This voltage becomes especially important when adding bypass capacitors to the main power lines to suppress noise from entering or leaving the power supply (a common situation). Therefore, the peak-to-peak voltage is actually 339.4 volts! The power goes from -169.7 volts to +169.7 volts each cycle. This means that the deviation around zero volts is actually 169.7 volts (for my 120 VAC power). He RMS conversion varies according to the wave shape for a sine wave, the value is 1.414. I just measured mine and found it to be precisely 120.0 VAC.) An RMS measurement of a sine wave is much lower than the actual peak voltage and represents the equivalent DC (Direct Current) voltage needed to provide the same power. (Note that I've seen ordinary household power specified anywhere from 110 VAC to 125 VAC. The first is that 117 VAC (Volts Alternating Current) is really an RMS (Root Mean Square) measurement. The first two are discussed in this article and the last in the next installment. A basic analog power supply consists of three parts. However, there are a couple of points that are important to mention.įIGURE 1. The input power conversion is typically a power transformer and is the only method considered here. It consists of three main components: input power conversion and conditioning rectification and filtering and regulation. Input Power Conversionįigure 1 shows the fundamental design for a typical analog power supply. A well-designed power supply is one that is never noticed. For that reason, it's critical to approach this design conservatively. It's always important to remember that the power supply - either for a particular product or as a general piece of test equipment - has the potential to electrocute the user, start a fire, or destroy the device it's powering. The design will focus on the ubiquitous three-terminal regulator and include a number of enhancements to the basic design. ![]() This two-part article will start from the beginning and explain every step necessary to build a basic analog power supply. However, they don’t teach the readers how to design a power supply by themselves. ![]() There's the old saying: “You can give a man a fish and he will eat for a day or you can teach a man to fish and he will eat forever.” There are many articles that give the reader a specific design for building a power supply, and there's nothing wrong with these cookbook designs.
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