What's a good voltage tolerance from small a power supply - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: July 16, 2014, 05:20:00 am
content: Hey guys!I'm gearing up to get started learning about electronics and bought a Dynex 1200mah variable AC to DC power supply, and measured the voltages with a multimeter.For each 3V, 4.5V, 6V, 7.5V, 9V, and 12V setting the reading was about 0.28V higher than it says. So 3V was 3.28V and so on.The 9V setting had a voltage about 0.36V higher, it read 9.36V.Are these considered big inaccuracies or small inaccuracies?Should I worry about these inaccuracies?What's a good tolerance when it comes to voltages?Are these values within a good tolerance?Thank you!

author: nerdyHippy
author_rank: Contributor
timestamp: July 16, 2014, 05:23:58 am
content: Of course it all depends on what you intend to do with those voltages. I reckon that if you needed an accurate voltage supply you'd know it, so the supply you've got should be alright.As an aside, are you certain that your multimeter is accurate?

author: geratheg
author_rank: Regular Contributor
timestamp: July 16, 2014, 06:32:48 am
content: I'm not sure, is there a way to check multimeter accuracy?I bought it for use with the Make: Electronics book to learn about electronics.There are a couple of logic chips to play with, which I hope aren't too sensitive.Edit: I'm not sure if this is an accurate test, but I measured several brand new batteries and got these readings:Duracell AAA 1.61VKirkland AA 1.57VEnergizer C4 1.59VDuracell 9V 9.58VAlso measured an 18.5 V laptop charger with a reading of 19.54 V

author: Fsck
author_rank: Super Contributor
timestamp: July 16, 2014, 06:57:40 am
content: ironically, the only way to check a multimeter is with a known reference voltage.

author: johansen
author_rank: Super Contributor
timestamp: July 16, 2014, 07:41:39 am
content: most of the time voltages are really not that critical. +/-10% is fine for most projects. but the question is why do we care.if a 12 volt regulator is supplying 11.5, then something else is wrong, its not that the voltage is too low, its the reason why its too low. insufficient filtering, too much current, etc, etc.I would bet you a beer your volt meter is between 1% and3% high. its really not at all important.

author: geratheg
author_rank: Regular Contributor
timestamp: July 16, 2014, 06:01:10 pm
content: Thanks for all the help. I think it's probably my multimeter.

author: idpromnut
author_rank: Unknown
timestamp: July 16, 2014, 06:53:35 pm
content: Quote from: geratheg on July 16, 2014, 06:01:10 pmThe other possibility is that the voltages you have been measuring are all "open" circuit, i.e. have no load and so will be a bit high anyways (although you shouldn't see this with the batteries, or not enough to notice perhaps). I would re-measure your power supply with a load on it and see if you get a measurement closer to the specified output voltage.

author: Zbig
author_rank: Frequent Contributor
timestamp: July 16, 2014, 07:38:11 pm
content: From my limited experience it seems that PSUs for consumer equipment ("power bricks") tend to output voltages a little higher, rather than lower, than the rated ones. I think manufacturers try to roughly compensate for the cable losses when loaded at or near their rated currents.EDIT: I'm talking switched mode type ones (SMPSs) or regulated transformer ones. For unregulated transformer type it's normal for the voltage to be all over the place and vary wildly with varying load (output current).

author: Rick Law
author_rank: Super Contributor
timestamp: July 18, 2014, 02:48:58 am
content: Quote from: geratheg on July 16, 2014, 06:32:48 amBattery voltage will vary by a lot even with new ones; and same with chargers/wall warts. You are better off checking the USB output of your laptop instead of the output of the charger. The USB output should be within 5% of 5V and likely better.My experience is, best way to test a multimeter is with DMMCheck and DMMCheck Plus (about $70) from Voltage Standard. You have 4 resistance standard, current, and both AC+DC voltage.http://www.voltagestandard.com/DMMCheck_Plus.htmlBest yet, you can send it back to recalibration. Bit expensive, but I am glad I have one to do sanity check at will.Good luck, check your USB power out lines from the laptop. See what that reads.Rick

author: geratheg
author_rank: Regular Contributor
timestamp: July 18, 2014, 03:43:29 am
content: Measured USB output within 1% with a $10 multimeter. It showed 5.05 V.I guess the power supply has slightly greater voltage to compensate for losses under load.

author: johansen
author_rank: Super Contributor
timestamp: July 18, 2014, 04:12:39 am
content: Quote from: geratheg on July 18, 2014, 03:43:29 amno, it has a greater voltage so they can get away with 50 cents worth of parts to make up the feedback loop.

author: Rick Law
author_rank: Super Contributor
timestamp: July 18, 2014, 04:28:35 am
content: Quote from: geratheg on July 18, 2014, 03:43:29 amGreat. You can now use your laptop USB as a reference of some sort. Until you have something better like a real voltage reference or DMM Check/DMM Check Plus...When you wonder you did something and mess up your DMM, you can use this same laptop to check your DMM. As reference, best you do it again and write down the exact reading, the date, which port you use (and all other port empty), and also make sure you boot into BIOS and on battery. (Running program may change load thus voltage).Your own 1-2% reference until you can get something better... (EDIT: well, more like 5% to be conservative)...

author: 6502nop
author_rank: Regular Contributor
timestamp: July 18, 2014, 10:56:18 am
content: Okay, I'm going to go out on a limb, here...QuoteYou haven't set your location, but here in the States, Dynex is the house brand of Best Buy. Overall, it's mostly cheapo import crap. Still, some of it works okay.When you say "variable", then list a set of common voltages that battery-powered devices use, that sends an alarm to my brain that what you have isn't variable - it'sswitchable. Biiigg difference!Therefore, based on text and no picture evidence, I'd say what you have is an AC-DC switchable voltage adapter (also known here as a "battery eliminator"). It probably looks like this:http://www.radioshack.com/product/index.jsp?productId=3875403AmIright?In which case, everything's fine. It works by either switching between taps on the transformer, or by switching in different sense resistors for an LM317 or somesuch regulator. The fact that you're within .28V or so, I'm guessing the latter, as a 317 can do 1200mA, and transformer taps wouldn't be near that accurate, figuring loading and all. It'll work just fine for most any Make project, but get into the habit of triple-checking the voltage setting before powering up!nopP.S. - Does it really read "1200mAH"? That means milli-Amp-Hours, which is a measurement for batteries, not power supplies.

author: idpromnut
author_rank: Unknown
timestamp: July 18, 2014, 12:14:05 pm
content: Quote from: geratheg on July 18, 2014, 03:43:29 amYou mean you measured it as being within 1% of the 5V spec; that is different than it actually being 1% from the 5V spec