Am I testing this power supply correctly? - Page 1
Forum section: electronics / beginners

author: gameaddict20
author_rank: Newbie
timestamp: August 09, 2018, 05:41:37 pm
content: Hi all,Recently been recapping an old Amiga CD32 that my parents found in their attic. As a bit of a side project I made up a constant current load out of one of those 150W Chinese LM324 four MOSFET kit from ebay to test the power brick that came with the CD32. I tested the load using my bench supply and a couple of multimeters and it seems to be working correctly. Today I tested the power bricks 5V rail (which has a max rating of 2A and the Amiga draws around 1A according to the internet). At a few 10s of mA the supply works fine and gives out 5V but as I get towards 500mA it has dropped to giving out 3V. I'm assuming this is massively out of tolerance and that the Amiga won't be happy with that.My first question is: Should I be testing like this, with only the ground and 5V line hooked up (12V line not hooked up), do power supplies tend to act differently when not all the lines are connected?My second question is: Is it common for power supplies to fail in this way, i.e. as they get older they fail to give out the correct voltage?My Final question is: Would the length and thickness of the cable between the plug on the amiga power supply and my current load affect the result?Thanks in advance for any help

author: tron9000
author_rank: Frequent Contributor
timestamp: August 10, 2018, 07:59:22 am
content: where are you measuring the voltage? off a DMM at the output of the power supply or on the load?If you're measuring off the loads meter then you need to take into account losses in your test cables. If that is the case and given the drop you are seeing, that's 4of resistance in your setup.Try measuring at the output of the power brick and see what it is.

author: helius
author_rank: Super Contributor
timestamp: August 10, 2018, 08:30:33 am
content: Quote from: gameaddict20 on August 09, 2018, 05:41:37 pmIt depends on the power supply's design. The simplest structure is for it to have independent linear regulators, fed by independent transformer taps. In this case, the output regulation is totally independent. You can test 5V and 12V separately and they do not interact at all.However, this design increases costs, since it requires more transformer windings.Another design would be to have a single winding on the transformer, with a 12V regulator, and then the 5V regulator runs off the 12. The outputs are mostly independent, but the 12V supply needs to be oversized, and it is also inefficient (dropping 12V to 5V is a lot of waste heat). This would be a strange choice but could be reasonable if the 5V load is small in comparison to the 12V load.A switching design is more efficient, and may be cheaper as well since a large transformer is not involved. But if they were pinching every penny, the supply may have been designed to only regulate one of the output rails, meaning that the other rails will not perform correctly if that primary rail is not sufficiently loaded.QuoteThe most common failure of linear regulators is a short or an open, i.e. they let full unregulated voltage through to the output, or they don't output anything at all. Switching regulators most often suffer damaged capacitors causing noisy output and reduced current ability.QuoteIf you are measuring the voltage at the load, then it could be that the difference is being lost in the cable. It would be more correct to measure voltage at the power supply output.In situations where the load is at the end of a long cable, power supplies sometimes have remote voltage sense facilities to compensate for cable losses. I haven't ever seen one like that used in a computer, though. In the absence of remote sense wires, the output voltage is regulated at the feedback node of the power supply, not at the load.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 11, 2018, 07:16:29 pm
content: Is yours a lineair or a switching design?(Re-read helius' post)Easiest would be to load boath outputs simultaneously with some decent current.Do you have some power resistors?A piece of lacqued ("enameled") wire in a bucket of water can serve nicely as a high power dummy load (See MikesElectricStuff on Youtube).An oscilloscope would be nice to see what's happening (maybe the power supply also has bad caps) but you should also measure both DC and AC voltage from the power supply, this will give an indication on the ripple voltage on the power supply.