A DIY power supply unit for all seasons Written by Clive “Max” Maxfield
Joe told me that he never uses USB to power his creations because “You cannot be sure of a solid 5 volts.” He also explained that, ever since one lied to him, he doesn’t trust power supplies that say they have +Ve on the inner connector and 0 V (ground) on the outer connector. To be honest, this reminded me of a pair of rather tasty computer speakers I acquired that went up in a puff of smoke due to a somewhat similar occurrence. All of this led Joe to create his own circuit that accepts 7 to 25 volts as input, where this can be AC or DC (the DC can be either polarity). In turn, this supply provides rock-solid 5 and 3.3 volt DC values to power his gadgets and gizmos. This struck me as a jolly good idea. So good that I asked Joe if he could spin this off as a separate board that could be used by other enthusiasts (like yours truly) for their own hobby projects. You’ve probably already guessed that the very next morning I found this design sitting in my email Inbox, along with Joe’s permission to make it available to anyone interested.
to be easily disconnected from the rest of the project. However, you can use any five-pin connector with a standard 0.1” (2.54mm) pitch, or you can solder header pins, or even solder wires directly to the PC board. When it comes to BR1, any bridge rectifier with a working voltage of at least 50 volts and a current rating of a minimum of 1 ampere (A) (always slightly more than the combined maximum load of the board) can be used. With respect to our particular implementation of this design, we require both a 5 volt output, which is generated by IC1, and a 3.3 volt output, which is generated by IC2. The component we used for IC1 was a 7805 that was lying around in Joe’s treasure chest of spare parts (the part we employed was similar to the MC7805ACTG from onsemi). In the case of IC2, the original design, which is reflected on the silkscreen, is specified as using an LD1117V33 LDO (low dropout) regulator, such as the LD1117V33 from STMicroelectronics . This is shown on the right side of Figure 3, which shows the finished PSU.
Feel the power The circuit diagram for Joe’s power supply unit (PSU) is shown in Figure 1. At first glance, you may think that this is nothing revolutionary. The deeper you dive, however, the more you realize that a lot of thought has gone into this. Such as the fact the pc board is single-sided, thereby allowing enthusiasts to make their own at home if they so desire (Joe has also kindly supplied the design files for those who would rather have their boards fabricated professionally). To discuss the components, we also need to consider the layout as shown in Figure 2. Let’s start with power connector SK1, which can be any pc board barrel-type connector with the correct footprint. Based on my existing wall wart supplies, I would typically use a 2.1 millimeter (mm) version, such as the 54-00166 from Tensility International Corp. As an alternative, you can simply solder the wires from the power source to pads ‘a’ and ‘b’ on the PC board. In the case of the output connector, SK2, a five-pin version from Molex is ideal as it allows for the board
1P12T rotary switches, probe terminals for my oscilloscope, and… the list goes on (if you are really unlucky, this board may be featured in a future blog). Amongst his many other talents, Joe is a PIC guru (he may even hold a black belt in PIC technology), so we set up a Zoom call for me to ask his advice. We spent a happy hour or so bouncing ideas back and forth and then broke for the day. You can only imagine my surprise and delight the following morning to discover that Joe had decided to create the PC board design ‘just because’.
living in an idyllic country hamlet just outside Cambridge, where he telecommutes to his full-time (day) job as a senior software and hardware engineer with the London Ambulance Service. In the evenings (often through the nights), Joe creates the most amazing thingamajigs and thingamabobs you’ve never seen. The reason I’m waffling on about this here is that in the same way Robyn calls on Harry when she has a problem, I call on Joe. A couple of weeks ago, for example, an interesting project landed on my desk. It involved the use of old- school 8-bit PIC microcontrollers, like the PIC16F18346-I/P from Microchip Technology . I may regale you with tales of my adventures with PICs in a future blog. For the moment, however, we have other fish to fry. My problem is that, in addition to programming these little rascals, I also need to subject them to a suite of tests. In turn, this requires me to create a custom board with a 20-pin DIL zero insertion force (ZIF) socket, such as the 222-3343-00-0602J from 3M , surrounded by a bunch of other things like DIP switches,
I have a friend (stop laughing; it’s true!). We will call him Joe (because that’s his name). To be honest, I no longer recall how we met, although I do know it was via the Internet. Joe and I were email chatting about this earlier today as I pen these words. Joe says he has emails dating from April 2006, but these already reflect an ongoing discussion, so we’ve decided to set the date we were introduced as April 1, 2005. That means, in just three years, we will have known each other for two decades (break out the party hats). Apart from anything else, this made me realize that I’ve met a large proportion of the people I now count amongst my closest friends via the internet, which certainly gives one food for thought. Have you seen the TV series The Equalizer starring Queen Latifah as Robyn McCall? One of her friends is Harry Keshegian (played by Adam Goldberg). Harry is a master hacker who lives underground in an unused subway station, which – since I was brought up on Batman and Robin – I always think of as being his ‘Batcave’. Well, Joe is the English equivalent of Harry, rarely emerging into the light of day,
And in this corner…
While Joe was walking me through the pc board schematic and layout (once again, all thanks to the wonders of Zoom), he made a throwaway comment along the lines of, “And this area in this corner is my standard power circuit.” When I asked for more details, Joe explained that he had decided it would be a good idea to test the PICs at both 5 and 3.3 volts, so he threw in a tried-and-true circuit he uses for many of his designs.
Capacitor C1 is a 470 microfarad
Figure 1: The circuit diagram for the PSU looks straightforward at first glance, but a lot of thought has gone into it, such as the fact the pc board design file is single-sided, allowing enthusiasts to make their own at home. Image source: Joe Farr
we get technical
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