"5VDC" Power Supply showing 5.41VDC but needs to be no more than 5.1 - Page 1
Forum section: electronics / beginners

author: MGaddict
author_rank: Contributor
timestamp: January 30, 2023, 06:21:14 pm
content: I am wanting to build a C64 power supply. I found an old wall plug to barrel jack adapter that says output is 5VDC at 2A which should, theoretically, be perfect. However, upon testing what the actual output was, I'm getting 5.41VDC. The C64 doesn't allow that much tolerance.I placed a load on the unit to see what the output was and no matter how much I drew, it didn't drop below 5.39VDC which is still WAY too high.I know, I should just find a better quality power source, but that wouldn't be any fun and I wouldn't learn anything.I've been trying to learn how these small switching power supplies work, and I keep finding it looks like the output voltage is controlled by resistors going to the PWM converter. However, the ST7203 on here doesn't appear to be controlled that way. As a matter of fact, there doesn't appear to be any way to control it at all..pdf]http://www.stcomponent.com/datasheets/ST7203-[V1.1].pdfI thought about adjusting the resistor going to the optical coupler to "trick" it into thinking it's overvoltage, but that feels like it would just effect the maximum amperage and not the voltage. So I guess the only way to lower it would be to replace the large diode.Admittedly, I don't have a whole lot of experience in modifying electronics. I've always just checked components and replaced with like parts. So this is kind of a level up for me and I'm enjoying it.The schematic on this PCB doesn't match what's in the pdf linked above for a typical application circuit which makes this a little more difficult. But that diagram is also for a 12V output.Honestly, I'm not afraid to blow this thing up, it's cheap. But I also don't want to do that for no reason. I have no clue what values I should try where. Idealy, I would love to install a trimmer pot someplace on here so that I could make fine adjustments but right now, it's just too high.This is a picture of the top of the board.https://photos.app.goo.gl/nVMsmqAvrc3PaVYa8And this is a reversed image of the back of the board.https://photos.app.goo.gl/FBE8TStB32sRkbqH7

author: wasedadoc
author_rank: Super Contributor
timestamp: January 30, 2023, 06:33:55 pm
content: Changing the large diode is not the way to alter the output voltage. The smaller glass diode, the Zener labelled ZD will.While the unit is powered up, VERY CAREFULLY measure the voltage across that diode. All the components on the other half of the board (other side of the transformer) are at high voltage, as much as 350 Volts!Then try replacing it with another diode with a slightly lower Zener voltage. Probably a 4.3 Volt Zener will get the closest to, but not exceed, 5 Volts at the output.And BTW changing the resistor does not affect the maximum current that can be drawn form the output.

author: MGaddict
author_rank: Contributor
timestamp: January 30, 2023, 07:49:48 pm
content: So it is 4.77V and I'd agree 4.3V would probably be right. This thing is tiny on here. Looks like a 500mW or 1W part. Would a 1W 4.3V diode work? I'm going to have to order 100 of them or get an assortment from Amazon or something.You know why I miss RadioShack? This is one of those times. Digikey takes about 2 weeks and charges like $8 to ship to me. Why wouldn't you say it was just a resistor or a rectifier diode? Geez. lol.I have assortments of 1/4W and 1W resistors, and hundreds of rectifier diodes, and probably 1000 different sized caps. But no, it's a zener I need. Figures. lol.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 30, 2023, 10:28:06 pm
content: Can't speak for the US but here in the UK I find ebay is a viable way to source small quantities of generic components, quickly and at competitive cost with big name distributors once minimum order value and shipping and handling are included.The power dissipation in the Zener is single digit milliwatts. I'd be tempted to get both 4.3 and 4.7 Volt ones as the actual Zener current will be somewhat lower than typically used to get the nominal voltage drop.

author: themadhippy
author_rank: Super Contributor
timestamp: January 30, 2023, 11:00:44 pm
content: Quoteleave the power suppy alone and try one of them in line with the output,it'll drop around 0.7v so maybe a bit much,but worth a try

author: tunk
author_rank: Super Contributor
timestamp: January 30, 2023, 11:10:49 pm
content: It's missing some output filtering (L1+C6), so it may be noisy.