Power supply load and accuracy - Page 1
Forum section: electronics / beginners

author: HoracioDos
author_rank: Frequent Contributor
timestamp: February 18, 2019, 07:04:33 pm
content: Hello!I was able to fix a cheap 220V/50Hz to 19V power supply by replacing two capacitors that were out of specs. The output was about 18V and now it is 20.25V. Wifi router needs 19V to work properly. I have two questions.a) How accurate are these cheap units? Which voltage difference +-V should I expect usually with a 500mA load? I forgot to connect DMM in series between the board and the output adapter.b) It seems that there is a resistor between positive and ground (420 Ohm according to multimeter) in the output that it is very discolored. I guess that's a load. How common is that? Should it regulate fine without any kind of load?Thanks in advance

author: Refrigerator
author_rank: Super Contributor
timestamp: February 18, 2019, 07:28:19 pm
content: SMPS power supplies can be a bit unstable without load, that's why the resistor is there.20.25V doesn't seem too far out of spec, i doubt your router needs exactly 19V but it might be some special router, i don't know.If it didn't work before on 18V it was most probably because of the ripple due to failing caps.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: February 18, 2019, 07:38:05 pm
content: Quote from: Refrigerator on February 18, 2019, 07:28:19 pmI don't know either but the specs are 19V@1.7A (Asus RT-AC66U B1). While I was fixing the power supply, I used a lab power supply to keep the router running and current was jumping between 200mA and 500mA and sometimes 700mA..

author: Refrigerator
author_rank: Super Contributor
timestamp: February 18, 2019, 09:57:51 pm
content: Quote from: HoracioDos on February 18, 2019, 07:38:05 pmThat's about what you'd expect, the router is doing it's thing processing data and that results in more or less current being drawn.My laptop has "19.5V 3.5A" written on it but that doesn't mean it pulls 3.5A all the time.Also on that note, almost all laptop power bricks have a 19V/3.5A rating, so if you don't trust your PSU, you could find a cheap secondhand laptop charger for your router.

author: xavier60
author_rank: Super Contributor
timestamp: February 19, 2019, 01:35:21 am
content: Sensed output voltage for the regulation loop is usually from the first capacitor after the rectifier diode.If there is too much ripple on this capacitor it can cause poor regulation.

author: xavier60
author_rank: Super Contributor
timestamp: February 19, 2019, 02:31:44 am
content: The voltage feedback divider resistors look to be 3K and 22K, which would cause the output voltage to be about 20.5V unloaded.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 19, 2019, 12:26:04 pm
content: R19 doesn't look like a 1-watt part so it could fail open. A 1-watt 470 ohm with a 0.5w 3K3 piggy-back should be easy to find.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: February 19, 2019, 02:29:08 pm
content: Quote from: xavier60 on February 19, 2019, 01:35:21 amGood to know. I'll try to check it just for fun.

author: HoracioDos
author_rank: Frequent Contributor
timestamp: February 19, 2019, 02:40:44 pm
content: Quote from: Cliff Matthews on February 19, 2019, 12:26:04 pmI had my doubts too about that because it is very discolored. It has the same size of R20 or R9. Thanks for the piggy-back resistor idea. I'm sure that I have these values somewhere.