Designing a Custom 3-Way Power Supply for an Old Consumer Device
Designing a custom 3-way power supply, especially one as specific as a 5VDC, 5VCD, -5VCD 120AC to 30-year-old consumer electronics, can be a challenging task. While there are valid reasons to attempt such a project, it is important to understand the risks and expertise required. This article provides guidance on how to approach such a design and the key considerations you need to take into account.
Why Isn't It Always Best to Buy?
There is little reason to attempt designing your own power supply (PSU) when you can buy a suitable one. The high-voltage side of a PSU is inherently dangerous, and attempting to design it without proper training and qualification is strongly discouraged. If you are an Electrical Engineer (EE), you likely already have the necessary skills and equipment to handle such a project, and the need to ask for help would indicate that you are not in that position.
Starting with an ATX Power Supply
A good starting point for a custom PSU design can be an ATX power supply. An ATX power supply already provides several necessary outputs, including 5V, 12V, and potentially -5V. If it doesn’t have the -5V output, you can use a L7905 voltage regulator to create an adapter. This is a common solution for devices that need a -5V supply.
Understanding Your Device Schematic
To proceed, you need a schematic for your 30-year-old device. In this case, the 5VDC, 5VCD, and -5VCD voltages are typically points in the circuit. They are likely connected to a single supply with a positive auxiliary (VCD), negative auxiliary (-VCD), and a zero volt-center tap. This configuration is fairly common and uses five terminals. Different voltages are used in various circuits, and understanding the schematic is crucial for designing a compatible PSU.
Current Requirements and Regulation
When designing a custom PSU, it’s essential to specify the current requirements for each rail. You need to know the minimum and maximum current, and whether the voltages need to be regulated. For example, if the voltages need to be regulated, you need to specify to what extent. Additionally, you need to ensure that the rails are isolated from each other. 120VAC is a lethal voltage, so the design must include proper insulation and isolation ratings to ensure safety.
Considering the Safety and Liability
If you plan to declare your design as safe, you must be aware of the potential liability involved. Even if the design is only for your use, it is your responsibility to ensure that it is safe. If it is for educational purposes and you are not compensated, designing your own PSU might be the most cost-effective solution. However, if you are planning to sell or distribute the design, you must be prepared for the legal and safety requirements.
Using Commercial Solutions
For a project like this, the sensible and cheapest answer is often to buy a commercially designed and warranted PSU. An example of such a PSU is a 120VAC to 5VDC transformer, which is extremely common and inexpensive. You can follow this with an inexpensive, isolated 5V input DC-DC converter to achieve the required -5V output.
Understanding the Terminology
The terms "VCD" and "5-pin connector" need clarification. VCD likely refers to a specific pin configuration or a positive auxiliary voltage, while the 5-pin connector is likely a connector type that the device uses. Knowing the current requirements for each rail is crucial for designing a power supply that meets the device's needs. It is also important to understand why you want to design something this trivial instead of buying a pre-built PSU with the appropriate connectors.
Open Source Aspects
Regarding the "open source project," a PSU design itself doesn’t require any code. The term “open source” in this context might be more relevant to the schematic or the documentation that describes how the PSU works. However, the actual design and construction of the PSU don’t involve any software or code. If you are considering an open-source approach, it could be to provide a detailed and accessible guide or schematic for others to build and modify.
Conclusion
Designing a custom 3-way power supply for an old consumer device is a complex task that requires significant expertise and careful consideration of safety and legal implications. While it might be tempting to try and build something from scratch, especially for educational or personal use, it is often more practical and safer to purchase a commercially available PSU and modify it to fit your needs.