Beginners approach to DIY AC/DC bench power supply - Page 1
Forum section: electronics / projects

author: JHuston
author_rank: Newbie
timestamp: March 20, 2023, 11:08:06 pm
content: So I hope I don't show how new I am at some of this too awful bad... I am not a EE PE or even went to college. At some point in life I was interested in amplifiers (car audio repair) that led me to amplifiers (home amplifier repair) and now I find myself spending HOURS anytime I can reading and leaning. With that out of the way here is my plan for this and if someone has any help I would appreciate it. If I am out of my mind please let me know that as wellOk, so I want a AC/DC power supply that will be used to test,and repair amplifiers as well as other future electronics projects. The plan at this point is to start with a Isolation transformer that I have purchased. It is a 1:1 120VAC 2kW transformer. I was planning on using a 4U or 5U rack mount enclosure that I purchased many years ago. I would use EMI filter from mains plug, circuit breaker to isolation transformer. From here I was thinking a DPDT switch one side would feed 120V outlets on front panel that were just used as isolated outlets as needed. The other side of that DPDT switch would feed power to a Staco Variac. The variac I have my eye on is 120V in, 1-140v out 1kw (it's 10A rated) the Variac has a stepper motor and microcontroller. I will find technical info on existing controller or worst case scenario ditch it and use Arduino to control stepper motor with keypad for input and a display for output. I would like for the keypad to dialup the AC voltage thats wanted or the DC voltage that's wanted. If possible hopefully thru the Arduino I can create variable current for both AC and DC. It would also be nice to create a few 'scripts' to slowly ramp voltage/current for capacitors or slowly turning on new amplifiers or vintage equipment, etc. Again both AC voltage and current I would like to be controllable separate for CV or CC with both OVP and OCP. Now from the Variac I would like to use a contractor or relay to switch from AC out to DC out. Of course I would need to send my AC voltage thru rectification, CRC or CLC filter, for the sake of protecting the large capacitors that I already own I would like to limit the prerectification AC voltage to not exceed a chosen DC voltage. Again using I assume a Arduino again I want to use keypad to select a DC voltage/current along with CC/CV settings and OVP/OCP. I am not worried about extreme precision temperature coefficient nasa specs. I would like a bench supply that is a do it all unit that I can pass down to my son in some years. But for me now- I will essentially have a power supply to power any amplifier front end . That's nother issue that I need to figure out. Using a Variac there is no center tap for bipolar supply. How could I have the option for bipolar supply on/off. Or +/- DC voltageBasically and/or. Am I going at this the entirely wrong way? I also found and purchased a AC line regulator/conditioner. Hoping it had usable parts in it to help with this project. (Hope was to help maintain a clean 60hz/120VAC input/output at the isolation transformer.Thank you ANYONE for any and all of your comments should anyone choose to help me along.

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 21, 2023, 12:39:22 am
content: Your isolation transformer combined with a variac, is a great test setup for a variable AC supply for testing line powered equipment.But I wouldn't be using that to also make a DC power supply. For that, you'd want a dedicated unit with proper filtering, current limiting, and voltage regulation.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 21, 2023, 01:43:46 am
content: A motorized variac for amplifier service? Fugetaboutit, save your money.You need a decent dual secondary transformer to create a bipolar output supply. Put a standard variac in front of that transformer and you have a variable voltage AC supply. Add a bridge rectifier with filter caps and you’ve got a variable voltage DC supply. The voltage regulation will be a function of the quality of the components.At the voltage and currents desired, CV/CC/OVP/OCP are things found in only very expensive lab supplies. To achieve this in a home built supply would be a challenge and quite expensive even for an experienced individual and totally unnecessary for amplifier service and or design/experimentation.With some research, you’ll find that a few incandescent light bulbs, properly sized and applied along with proper fusing is all you’d ever need for supply protection.