A DIY power supply unit for all seasons
by the output of IC2. Depending on the input voltage applied to the LED, the R1 resistor value needs to be adjusted to give a reasonable brightness. Aiming for around 10 milliamperes (mA), which gives a nice brightness without stressing the LED, some suggested resistor values for different voltages are: 3.3 volts = 150 Ω, 5 volts = 330 Ω, 12 volts = 1 KΩ, 15 volts = 1.2 KΩ. Conclusion So, there you have it. This may not be a space-age Jetson-sque PSU with lots of bells and whistles, but it’s a nice little DIY workhorse board that can be used to satisfy the requirements of a lot of home projects. I’m planning on keeping a stash of these available, along with a collection of regulators, ready to leap into action for future projects. What say you? As always, I welcome your comments, questions, and suggestions.
the mounting tab connected to the center (ground) pin. As we previously discussed, the design presented here is intended to provide 5t and 3.3 volt outputs, but it can be easily modified to accommodate other voltage combos, such as 12 and 5 volts, as required. In this case, the regulator used for IC1 can be any 78xx style regulator (do NOT attempt to use a 79xx series regulator anywhere on this board as the pinout is different). There are two options for IC2. As we already discussed, the original design utilizes an LD1117V33 3.3 volt regulator. These regulators should use the IC2a pads as they have a different pinout compared with the more common 78xx series devices. If you wish to use a 78xx for the second regulator, then the pads labeled IC2b should be used.
directly from the output of the bridge rectifier and capacitor C1. By comparison, depending on your requirements, there are two possibilities for driving regulator IC2. If you wish, it can be fed directly from the bridge rectifier (fit jumper ‘c’ to ‘e’). Alternatively, it can be fed from the output of IC1 (fit jumper ‘d’ to ‘e’). This latter option is useful if the voltage output of IC1 is higher than the minimum input voltage required for IC2, as is the case with my implementation. This will reduce heat, but it assumes that regulator IC1 has sufficient capacity to provide power to your circuit as well as regulator IC2. If LED1’s current limiting resistor, R1, is connected between ‘f’ and ‘g’, the LED will be powered by the output of the bridge rectifier, BR1. If R1 is connected between ‘f’ and ‘h’, it will be powered by the output of IC1. And if R1 is connected between ‘f’ and ‘i’, it will be powered
Figure 2 : Using the component values, types, and placements indicated on the silkscreen (e.g., the regulator IC2 is shown connected to pads/vias IC2a), the PSU accepts 7 to 25 volts AC or DC as input, returning rock-solid 5 and 3.3 volt outputs. However, by swapping some of the components (e.g., connecting a different type of regulator to the pads/vias IC2b), a variety of other voltage combinations can be achieved. Image source: Joe Farr
(µF) electrolytic device that must have a voltage rating higher than the maximum expected board input voltage (we used a 35 volt part). In the case of capacitors C2 and C3, pretty much any 100 nanofarad (nF) capacitor with a working voltage greater than 35 volts can be used. With respect to capacitor C4, if IC2 is an LD1117V33 regulator, then a 10 µF/16 volt capacitor is ideal. However, if a 78xx regulator is used for IC2 (see also the discussions on the regulators below), this capacitor should be changed for another 100nF capacitor that is identical to C2 and C3. LED1 is any 5 or 3mm light-emitting diode (LED) with a forward voltage of around 2 volts. The value of the
current limiting resistor, R1, which should be rated at 0.25 watts, depends on what output voltage will be used to drive the LED (see the discussions below). Pertinent points to ponder A regulator works by taking the input voltage and lowering it to match its specified output voltage. In the case of the regulators used here, the difference between the input and output voltage is dissipated as heat, which means
the regulator can get extremely hot. To minimize the heat that needs to be dissipated, try to set the input voltage so that it’s around 3 volts higher than the output of whatever regulators are connected to it. Also, when running the board from DC, account for the bridge rectifier dropping the input voltage by around 1 volt. Some regulators – the LD1117V33 being a prime example – have the metal mounting tab connected to the device’s output pin. By comparison, 78xx regulators have
Regulator IC1 is always fed
Figure 3: The finished PSU. The jumper between ‘d’ and ‘e’ (center) means that the input to IC2 is driven by the output of IC1 (see notes below). Connecting the current limiting resistor (R1) between ‘f’ and ‘i’ means that LED1 is powered by the output from IC2, thereby indicating that all elements of the power chain (BR1, IC1, and IC2) are functioning. Image source: Joe Farr
we get technical
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