Running a power supply with no load - Page 1
Forum section: electronics / beginners

author: institches
author_rank: Newbie
timestamp: March 06, 2018, 11:43:51 pm
content: Hi,I was wondering if it's safe to run a power supply from an old computer (Amiga) with no load. I think it's linear? I'm assuming that because it uses transformers. Sorry, a bit new to this.It has +12V, +5V, -5V supplies.The main reason I ask is because I'm looking to replace the caps and I would like to measure the output noise before and after cap replacement out of curiosity.Should I be running it under load to get accurate results anyway? If not, is it safe to run any or all of the rails with no load?I've heard it is not good to run an PC ATX power supply with no load but I really was looking for confirmation.If anyone feels inclined, can you explain why a load is required.Thanks.

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 07, 2018, 12:02:23 am
content: For linear power supplies, a load is not required.

author: respoda
author_rank: Contributor
timestamp: March 07, 2018, 12:15:33 am
content: I thought pc psu's are switch-mode, not linear, and do need a dummy load ?I'm in the process of building one too, so I do believe you need some kind of a power resistor to act as a dummy load to keep the device running with no real load attached.edit: since it has a +12v, +5v and +3.3v lines, it is switch-mode, so the above 'should' be correct.

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 07, 2018, 01:16:52 am
content: Quote from: respoda on March 07, 2018, 12:15:33 amYesQuoteyesQuoteYou can easily make a linear power supply with these voltage outputs.

author: Circlotron
author_rank: Super Contributor
timestamp: March 07, 2018, 01:24:39 am
content: The Amiga supplying will be just fine without a load. You should measure the noise with various loads though. I used to have an Amiga back in the day and I remember the power supply box being fairly heavy so it was a linear supply, not like normal pc supplies.

author: institches
author_rank: Newbie
timestamp: March 07, 2018, 03:07:50 am
content: Thanks all.That clears things up.I'll be applying a load to my old Amiga (linear) power supply, not because it needs one but because it is recommended for accurately measuring noise.

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2018, 04:46:45 am
content: The Amiga 500 uses a linear power supply. The A2000, A3000 and A4000 use internal switchmode power supplies. In any case running without a load won't damage anything, it just won't give accurate measurements.

author: institches
author_rank: Newbie
timestamp: March 07, 2018, 07:54:57 am
content: Thanks @james_s It's an Amiga 2000.As an aside, is there an easy way to identify the two types of power supply?From what I've read, switch mode supplies will have smaller transformers than an equivalent linear supply.The one I'm looking to measure has two transformers.

author: xani
author_rank: Frequent Contributor
timestamp: March 07, 2018, 09:05:07 am
content: How it behaves will depend on power supply. Some with minimum current requirementwill notwork properly, the weirdest one was one honeywell power supply that would "cycle" (enable, then disable power) slowly when under no load (IIRC that was documented behaviour in datasheet) and generate loud high-pitched noise (switching freq) when below minimum current

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2018, 05:40:49 pm
content: Weight is a reasonably accurate way of identifying which is which. A linear power supply will have a big chunky iron transformer and will be quite heavy. A switchmode power supply will have a much smaller transformer, usually mounted directly to the PCB. If it's more than about 80W and you can hold it comfortably with one hand it's probably a switching PSU.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 08, 2018, 01:15:54 pm
content: Someswitchmode power supplies need a load, but others are quite happy to run into an open circuit.Early Philips TV SMPSUs would "hiccup" if used without a load, but all the Sony ones I ran into didn't care.

author: institches
author_rank: Newbie
timestamp: March 12, 2018, 10:44:03 pm
content: I have a PC ATX switch mode power supply I'm going to use as a test bed for applying loads and if we assume it needs a load to avoid damaging the supply, do I need to load down all available rails or just the one under test?e.g. If I want to test the 12V rail and load it down but don't load down the 5V, could that cause damage? If no damage will be done I could use a single DC electronic load to measure each rail individually.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2018, 03:37:11 am
content: Running it without a load won't damage it but it won't work properly either. You need some load on the main output or the voltages will all be too low. On those old ones it's normally the 5V output that needs a load.