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制造商零件编号 0191640013
CONN SPLICE 18-22 AWG CRIMP
Molex
Rectifier Component Selection
2023-03-24 | By Jack Hannum
License: Apache License, Version 2.0 Power Supplies Arduino ARM mbed
Now that we know what each component of the transformer and rectifier is supposed to do and we’ve selected the component values needed, we need to pick out some hardware if we ever want to build this circuit. We’ve got four things we need to pick out: a wall plug, a transformer, a full wave diode bridge rectifier, and an output capacitor.
The wall plug:
To plug into an outlet, we need a plug. Fortunately, NEMA has already picked one out for us, the NEMA 1-15P, the classic two-pronged AC plug you probably have dozens of for powering various devices. If you get DK part number Q114-ND, you can connect the two bare wire ends to the transformer.
The transformer:
We’ve been designing for a 12V output from wall voltage, which is sometimes referred to as 115V, so a 115V to 12V transformer will work for us. Next, we need to ensure it can output the current that the buck converter will draw- 0.15A under normal load, so our transformer should be able to output at least 0.3A for some margin. The transformer will be one of the largest components in our design, so we don’t want to excessively overrate it; if we do, the system will be unnecessarily large and more difficult to package. Let’s filter the Power Transformers section on Digikey.com until we get something that will work. As always, we’re looking for active products that are in stock, here with an input of 115V or 120V and an output of 12V. We can now sort by ascending price and look through the transformers for one with at least 0.3A output current. DK part number 237-1935-ND should work- it’s a 115/12V 4.2VA transformer that can output 12V at 0.35A. It has wire terminal leads that we’ll butt splice to the output of the power cord.
Connecting Wires:
In a schematic, you can directly connect two components with the stroke of a pen. In hardware, it’s a little more complicated and easily overlooked during design. Thankfully, there are a variety of easy-to-apply methods that let you make clean connections in hardware, most of which don’t involve any soldering! There are lots of good ways to connect circuit components, including ring and fork terminals that you can screw together, often using a barrier block, but a personal favorite is the butt splice. A butt splice is just a metal barrel that allows you to insert two wires you want to connect. You then use a crimp tool (often integrated into wire strippers) to crush the barrel around the wires, mechanically connecting them. If you strip the ends of the wires before inserting them into the butt splice, you get an electrical connection as well. Butt splices are usually covered with insulation, but good ones are covered in heat shrink insulation and have some hanging off the sides, which you can use to get clean insulation that covers both the butt splice and the wires it connects. The Molex 18-22AWG butt splice pictured below is a great example of this.
Selecting the Diode Bridge Rectifier:
We have a few requirements for the diode bridge rectifier: first, it must be a full-wave rectifier, so that we can get the 120Hz pulses we designed for, from 60Hz AC. Second, each diode must be able to block at least the peak voltage they’ll get from the output of the transformer- 16.9V. Ideally, they’ll be able to block more than that to give us some safety margin, so we’ll look for diodes that can block 25V when they aren’t conducting. Lastly, they’ll be able to conduct at least the 0.15A the buck converter will draw- ideally more, so as we did while selecting the transformer, we’ll look for an output current of at least 0.3A.
We’ll filter the bridge rectifiers section on Digikey.com to get something that will work. As before we want parts that are active and in stock. We’re rectifying single-phase, and we’re not doing power factor correction (PFC) or braking a single-phase motor (braking), so we’ll just select single-phase diodes. We want to block at least 25V, so we’ll filter for rectifiers with at least 25V of peak reverse voltage. Our output current must be at least 0.3A, which is the minimum average rectified current we’ll accept. We’ll build the prototype on a breadboard, so we’ll want through-hole mounting. The Diodes Incorporated 600V, 2A single-phase bridge rectifier should work to implement all four diodes in our schematic. The package has four pins, which we’ll stick into four different rows of the breadboard. We’ll then take the two secondary/low voltage leads of the transformer and put them into the rows with the AC pins of the diode bridge rectifier. We’ll then connect the DC leads of the rectifier to the output capacitor.
Selecting the Output Capacitor:
We know that the output capacitor has to be about 579μF and has to be able to withstand the peak output voltage that the rectifier will give it, 16.9V, ideally more than 25V to match the rectifier. We’ll need an electrolytic capacitor for that, with through hole leads we can stick into the breadboard. A 680μF Rubycon capacitor is a full 100μF more than we originally designed for, but is rated for 35V, has massive stock, can handle ripple currents 10x larger than DC, and is low cost. A larger capacitance means we’ll have a larger RC time constant (49ms, up from the 42ms we designed for) and a higher minimum voltage to give to the buck converter. This results in a smaller voltage ripple, which will reduce the output voltage ripple seen by the microcontroller.
Digi-Key offers a 570μF capacitor that is much closer in capacitance to what we designed for, but it’s more costly because it’s rated for 450V and 3.15A of ripple current. Plus, less capacitance would lead to higher than intended voltage ripple, which would probably be okay, but we wouldn’t want to pay extra for this design. In this case, the low-cost option is much better for our application.
That’s everything we needed to specify in this part of the design! Now, we can move on to designing the buck converter!
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