Power supply problems - Page 1
Forum section: electronics / beginners

author: Fabiusp98
author_rank: Contributor
timestamp: January 03, 2014, 03:55:58 pm
content: I was building a power supply from a wall one. Nothing special: I soldered a switch and two banana plugs at the two wires. I built the thing and I found that the voltages are all +2v over the setting. Those are the measures:Setting | Reading1,5V | 3,2V3V | 5.03V6V | 8.39V7.5V | 10.24V9V | 11.95V12V | 15,21VThose measures are taken with a 220 ohm load resistor. I double checked with another multimeter. I also tried with another power suppy but same result.Multimeter: GBC KDM-500CF (I saw this multimeter rebadged over other brands)2nd Multimeter : DT-830B (Chinese crap super economic)Power supply: Palsonic MW691 (probably chinese)2nd power supply: Lafayette S-220A (veeeeery old)All two have 500mA max.Here in Italy the mains is 220V 50HzSome advices or ideas?Sorry for the bad english but I'm italian!I'm a beginner and I don't know so much, I only have this kithttp://www.adafruit.com/products/487and an Arduino.

author: XOIIO
author_rank: Super Contributor
timestamp: January 03, 2014, 03:59:56 pm
content: Edit: Disregard what I said, didn't read your post properly, unfortunately I can't find any info on your power supplies from a google search.I'd try and find an ATX power supply from a computer, those usually give nice stable and pretty close to accurate 3.3v, 5v, and 12v rails. You can then use an ebay cheapie (around $2) buck converter to drop the voltage to whatever you need, probably the cheapest method if budget is a concern.

author: Jon86
author_rank: Frequent Contributor
timestamp: January 03, 2014, 04:00:13 pm
content: Probably just a rubbish power supply, I've experienced this exact same thing with a similar unit myself.

author: Fabiusp98
author_rank: Contributor
timestamp: January 03, 2014, 04:13:27 pm
content: Here are the images.Thanks a lot for the support, 2 replies in 5min!XOIIO the Adafruit kit isn't a power supply, it's a starter kit.Until I manage to get this thing working I'll use the battery pack that is in the kit( the Adafruit page is wrong, there isn't a 5v regulator). Or I probably buy a kit and make a real bench power supply.

author: XOIIO
author_rank: Super Contributor
timestamp: January 03, 2014, 04:17:44 pm
content: Quote from: Fabiusp98 on January 03, 2014, 04:13:27 pmYeah thought it might come with some weird power supply bit in it or something, I have seen some that have stuff like that.You can get fairly nice 0-15v and 0-30v single and dual channel variable power supplies on ebay, dual are a fair bit more expensive but when I upgrade mine I'm going to spend the extra for the two channels just for the convenience.Anyways shop around kits are usually overpriced compared to what you can get pre-made with the same or more functionality, it just comes down to price and if you want to try making your own, if you want to I'd go for the kit depending on features and how much value you put on actually assembling it and learning about it.

author: rexxar
author_rank: Frequent Contributor
timestamp: January 03, 2014, 04:19:38 pm
content: I think those cheap power supplies just rectify different transformer taps to get your voltage, there's no regulation. I'd guess that they were designed for 220V nominal, and your mains voltage is a bit high.

author: Simon
author_rank: Unknown
timestamp: January 03, 2014, 04:25:06 pm
content: The italian mains is usually 220-230V, the problem is probably that you are not appliying enough load, you have an AC voltage that is being rectified (so you loose 1.4-2V) but what comes out has peak voltages aty 1.414x the RMS voltage that your meter measures so the capacitor will charge up to this voltage and it is only if you put enough load on that you get the RMS voltage, the transformer will also have an internal resistance so allow a larger voltage at low loads

author: Fabiusp98
author_rank: Contributor
timestamp: January 05, 2014, 02:41:25 pm
content: Thanks for the help,Exactly what load is required for making the output stable?

author: Simon
author_rank: Unknown
timestamp: January 05, 2014, 03:01:22 pm
content: A voltage regulator like one from the 78xx series or the LM317 if you want a voltage that is not available as a fixed.

author: mariush
author_rank: Super Contributor
timestamp: January 05, 2014, 03:56:18 pm
content: Like others said, those power supplies are very simple, just a transformer with several taps , a few diodes to rectify the AC voltage and a capacitor.It's not just the fact that you have 230v and the transformer was designed for 220v, it's also the fact that such small transformers usually have a +5-10% "error" at low loads from the start. A 9v 10VA transformer may output 10v AC at low loads, by design.A 220ohm is too low of a load to get the power supply to output what it should. At 3.3v, that's only about 15mA. Try something that puts about 100mA load on the transformer.By the way, those transformers are not really rated for 500mA, the transformer can probably barely do 300mA at 12v (or any voltage really) without overheating. I think it even says 5w somewhere which would work out to about 420mA.