How to read a wall power supply - Page 1
Forum section: electronics / beginners

author: mrwebb
author_rank: Contributor
timestamp: July 10, 2012, 02:45:51 am
content: Just curious what the output values on my wall power supply.It says:OUTPUT: 5VDC 500mAI realize that this must mean that the output from the unit will be 5VDC, but where does the 500mA number come from? Wont the 500mA (which I assume is the current_ be dependent on how much resistance there is?

author: Rerouter
author_rank: Super Contributor
timestamp: July 10, 2012, 02:49:46 am
content: it means it should be safe to supply 500mA before things start to melt / explode, depending on its origin,fakes will melt at 2/3rd this rating usuallygenuine (point quality genuines) should fall out of regulation at that point, e.g. your 5v becomes 4.8

author: mrwebb
author_rank: Contributor
timestamp: July 10, 2012, 05:54:56 am
content: Thanks Rerouter, so basically it is saying that unless you have at least 10 ohms of resistance in your device then you are going to be in trouble?

author: LEECH666
author_rank: Frequent Contributor
timestamp: July 10, 2012, 06:31:55 am
content: Let's assume it's a power supply of decent build quality and not Chinese crap. So I wouldn't worry about melting for now.The current rating tells you how much current you can draw from the PSU at the given Voltage before the voltage starts to drop down.And yes the current depends on your load. Let's say you hook up an ohmic load that would draw 700 mA at 5V. It might be able to supply the current but the voltage will probably drop to a considerably lower voltage than the rated 5V. You're overloading the PSU in that case and that isn't a good thing usually. You want to avoid that condition.Cheers,Florian

author: jimmc
author_rank: Frequent Contributor
timestamp: July 10, 2012, 09:43:23 am
content: Most of the older (non switchmode) wall power supplies are unstabilised so that when supplying less than their rated output current, the output voltage will be higher than stated.Your 5v 500mA unit might output as much as 9 or 10v when lightly loaded.Jim

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: July 10, 2012, 11:29:23 am
content: This link may help.AC-DC Power Supplies - Using Wall Warts

author: Mint.
author_rank: Frequent Contributor
timestamp: July 10, 2012, 12:35:11 pm
content: Will the wall power have any safety circuits into them, to actually limit the current to the max current before the voltage starts to drop?

author: Unknown
author_rank: Unknown
timestamp: July 10, 2012, 02:02:02 pm
content: Quote from: Mint. on July 10, 2012, 12:35:11 pmThat's like asking "Do cars have anti-lock brakes?". The answer is "some will!". You (at times) get what you pay for. Don't expect much if any protection in budget plug-packs, apart from there own inability. As a sweeping generalisation switch mode supplies will be better protected (and regulated) than transformer based ones. However many are not, these things are built for a (generally low) price.

author: digsys
author_rank: Unknown
timestamp: July 10, 2012, 02:51:06 pm
content: Even if they have a fuse, which is rare because it becomes unserviceable, a good plug-pack MUST beable to handle a short circuit. At this point the transformer (in a linear unit) will start to heat up andit MUST be protected by a thermal fuse. That gives you a reasonable amount of time to remove theshort before it becomes unserviceable. In a S/Mode, the short cct detecter SHOULD turn the unit OFF.There's a few shoulds in there. There are NO "typical" characteristics, trust nothing :-)

author: Mint.
author_rank: Frequent Contributor
timestamp: July 10, 2012, 09:54:00 pm
content: I've short circuited such power supplies and some give me a high pitched noise coming from the black box they are in, while others just give me sparks between the two points shorted and nothing else.

author: digsys
author_rank: Unknown
timestamp: July 10, 2012, 11:57:36 pm
content: Quote from: Mint. on July 10, 2012, 09:54:00 pmSo that tells you - the ones with the high pitched noise are S/Mode, and the ones giving sparks are Linear. And so far, if they've recovered,they were likely designed properly.

author: Mint.
author_rank: Frequent Contributor
timestamp: July 11, 2012, 12:03:51 am
content: Quote from: digsys on July 10, 2012, 11:57:36 pmAwesome! Thanks for pointing that out for me, now I know a bit more about power supplies

author: Unknown
author_rank: Unknown
timestamp: July 11, 2012, 12:06:34 am
content: Quote from: digsys on July 10, 2012, 11:57:36 pmAnd the ones the magic smoke comes out of are the unprotected ones. OP should note it's never a good idea to short out anything to see what it will do! That is a habit to get out of quick smart!

author: Mint.
author_rank: Frequent Contributor
timestamp: July 11, 2012, 12:26:54 am
content: Quote from: Uncle Vernon on July 11, 2012, 12:06:34 amWell sometimes I use wall power supplies for project testing and usually Iaccidentallyshort the supply. Whats a method for testing the power supplies if they are short circuit protected without actually harming the supply?

author: Unknown
author_rank: Unknown
timestamp: July 11, 2012, 12:40:08 am
content: Quote from: Mint. on July 11, 2012, 12:26:54 amAbout now you should be getting well across ohms law, power equations etc. So a smart fellow like yourself would be reading the nameplate ratings,calculating an appropriate resistive load, applying that load and measuring the results with that fancy dandy multimeter thingo. compare the loaded and unloaded voltage and current.

author: digsys
author_rank: Unknown
timestamp: July 11, 2012, 12:57:25 am
content: QuoteThat's not an easy question, even for engineers who've been around a while !!! I'd say it depends on what the Plugpack, Power adapteris for. If it's for equipment ie modem, hub, phones etc DON"T TRY IT !! It's NOT worth it !! There's nothing to be gained !!!! Seriously.If you're using it for projects and you really want to know how it will perform, then you still have decisions to make.1/ Can I repair it if I blow the crap outta it?YES - fine, go for it.NO - Do a load test at 100% current, and a SHORT load test at say 150% current. Watch the voltage with a Meter. If it STILL holds oronly drops a little after a few secs, END OF TEST !!Unless you just want to see if it'll blow up ... purely in the interests of science, of course. Just BE CAREFUL !!