Please help: Building a bench power supply  - Page 1
Forum section: electronics / projects

author: Lado
author_rank: Contributor
timestamp: March 29, 2022, 06:25:55 pm
content: Hi guys!Initially I planned to build a variable power supply from an old PC PSU. Because the case was too small and for various other reasons I decided to use an old APC backup UPS.I assumed that the big transformer in the UPS charges the battery at 12V and then a buck boost converter turns it into 220 V AC when there's no power. I thought I would just take the motherboard out, connect the transformer to 220V AC, put a switch in-between and I would get 12V DC on the other side of the transformer that I could connect to the buck boost converter in the picture below.I've measured 0V on the side of the transformer with two wires and around 14V on the side with five wires.I didn't plan to run it for longer periods of time but for safety I planned to add a pair of fans to keep the electronics cool.Could someone please help me with the wiring?Do I need to ad a capacitor between the transformer and buck converter?https://ibb.co/gFKPC9vhttps://ibb.co/7r4486Qhttps://ibb.co/vcLyY99

author: mariush
author_rank: Super Contributor
timestamp: March 29, 2022, 06:47:23 pm
content: If the transformer outputs some voltage to charge the battery, it will be AC voltage, and will probably be more than 12v AC.Transformer outputs AC voltage, which needs to be rectified using diodes (a bridge rectifier) to DC, and then that waveform must be smoothed out with a capacitor.Rectification produces a DC voltage with the peak voltage equal to approximately sqrt(2) x Vac - 2 x [voltage drop on diode inside rectifier] so for a 12v AC output transformer, the peak DC voltage will be 1.4142 x 12 - 2 x 0.7v = ~ 15.5vThe lead acid battery's peak voltage will be around 13.8v, you charge a lead acid battery at that voltage. The UPS will use the battery until the voltage of the battery discharges down to around 10.8v and then stops, because below that threshold a lead acid battery can be damaged.So most charge controllers will require a voltage slightly higher than 13.8v in order to charge the lead acid battery with 13.8v ( edit: it's actually 2.15v..2.35v per cell, there's 6 cells in a battery, so you can use 12.9v+ to charge, higher voltage charges faster etc)One more note ... the transformers are not really... let's say designed for long use at high load... It's rare the battery will be fully discharged, most of the time there's gonna be short seconds or maybe 1 minute long power failures, so the battery won't fully discharge, so most of the time the battery will be charged at slow rate, basically in a "top up battery" mode and transformer won't be pushed hard.They're supposed to be used for only short periods like a few hours at full load, when charging completely empty batteries and then as the battery charges up the amount of power pulled from transformer lowers.So in some designs you'll find undersized transformers, often helped stay cool with the help of some fan when running at full load, so they stay at 100c or less ... they make them just good enough that they'll be able to handle the duty cycle ( fully charge maybe once a month, partially charge maybe once or twice a week on short power failures)So... yeah... I wouldn't use it to build a power supply.... as for how much power the transformer could do, I'd say maybe 25-50 VA max(You can estimate ... if it's 12v AC and 25VA, that means max AC current would be 25VA/12V = 2.08A ... if you rectify 12v AC you'll get around 15.5v and the peak DC current can be estimated to around 0.62 x Iac = 0.62 x 2.08 = 1.3A)edit : the transformer may have multiple secondary windings, like for example 2 15v AC secondary windings of low current, that are only used to produce +/- 9-12v for comparators/opamps/whatever on the circuit board, maybe 5v for microcontrollers (if any) etc ...

author: Lado
author_rank: Contributor
timestamp: March 29, 2022, 07:15:12 pm
content: Thank you for the explanation.Like mentioned, I wouldn't use the power supply for longer periods with high loads. The buck boost converter is rated for 80W and that's probably a bold claim.The transformer is pretty beefy. I don't understand the markings besides the class 180 sign.https://ibb.co/SBWC8GyI checked the output on the transformer and it's 13.45V DC on multiple wires. I didn't measure any AC voltage.The outlets are at 240V AC.

author: H713
author_rank: Regular Contributor
timestamp: March 31, 2022, 04:42:49 am
content: The markings are more or less useless, aside from the insulation class rating. If you actually manage to find something googling a transformer part number, you should probably buy a lottery ticket.The transformer will output 13.8 V (ROUGHLY!) AC. You will need to rectify and filter that in order for it to be useful.That said, UPS transformers aren't the most useful things for a bench power supply. They can take a lot of current, but about 13 - 14 V is about all you'll get out of one.I used a toroidal transformer from AnTek for the last lab PS I built. Good quality transformers for a very reasonable price. The best part? You have some (small) amount of manufacturer data to work with, and they have a nice, wide range of output ratings.

author: Lado
author_rank: Contributor
timestamp: March 31, 2022, 11:47:57 am
content: Buy why I'm I measuring DC output? :')I planned to get a buck boost converter to upscale the voltage from 13V.I'm planning to use it for voltage injection, maybe testing LEDs and stuff like that. Nothing more demanding and for long periods of time.