# What's a good voltage tolerance for a small power supply?

#### geratheg

Jul 12, 2014
42
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.

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!

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
There's no definitive answer to your question. In practice any circuit that's designed to work from 9.0V will have no problem with 9.36V; a brand new recently manufactured 9V PP3-type battery could easily measure 9.36V with no load on it. But a 0.28V error at 3V is 9.3% which seems a bit high to me.

You may be able to reduce the voltage by adding a load resistor across the adapter output. A resistor of 680 ohms will dissipate about 0.23W at the 12V setting; if you use a resistor rated for 0.5W it will get pretty warm but won't be damaged. If that doesn't drag it down much, try a 5W 120 ohm resistor such as this one: http://www.digikey.com/product-detail/en/SQP500JB-120R/120W-5-ND/18681

If you're really worried, it will be possible to modify the voltage-setting components inside the adapter, if there's room. You could also add some components to smooth the output; if it's a cheap product, the output is probably pretty noisy, as well as being the wrong voltage. If you're that keen, open it up and upload some photos.

In practice, the voltage error is unlikely to be a problem, and the noise will probably only be a problem if you use it to power audio or AM radio circuitry.

#### geratheg

Jul 12, 2014
42

I don't think I'll be using it for anything sensitive anytime soon. I bought this for use with the Make: Electronics book, which does use some logic chips, which I hope aren't sensitive to a 0.3V voltage increase.
In this case, is the 3V setting being 3.28V a problem?

I don't really know what kinds of electronics are sensitive.

#### davenn

Moderator
Sep 5, 2009
14,150
And the other thing you may not have considered is ... how accurate is your meter ?

How much of that 0.28 or 0.36 is meter inaccuracy ?

maybe all of it ? maybe only 0.1V ??

cheers
Dave

#### geratheg

Jul 12, 2014
42
You may be right. I thought so myself since these readings are consistently the same increase in voltage.
Is there a way to check the accuracy of my multimeter?

Edit: I'm not sure if this is an accurate test, but I measured several brand new batteries and got these readings:
Duracell AAA 1.61 V
Kirkland AA 1.57 V
Energizer C4 1.59 V
Duracell 9V 9.58 V
Also measured an 18.5 V laptop charger with a reading of 19.54 V

Last edited:

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
I don't think I'll be using it for anything sensitive anytime soon. I bought this for use with the Make: Electronics book, which does use some logic chips, which I hope aren't sensitive to a 0.3V voltage increase. In this case, is the 3V setting being 3.28V a problem?
Very unlikely to be a problem. Most general logic ICs that are designed to run at 3.0V or 3.3V are specified with a maximum supply voltage of 3.6V. If the circuit has any large or expensive ICs, check the manufacturer's data sheet.
I don't really know what kinds of electronics are sensitive.
If the circuit is designed to run from batteries, don't worry. If the documentation doesn't mention using a regulated power supply, or any special type of power supply requirements, don't worry.
(re voltmeter accuracy) You may be right. I thought so myself since these readings are consistently the same increase in voltage.
Is there a way to check the accuracy of my multimeter?
Ah, I meant to suggest this possibility too. It's unlikely, unless your multimeter is a really cheap piece of rubbish. Check it against a known accurate voltage source, or more likely, a known accurate multimeter. Do you have a friend with one? Do you know someone who works at a technical school? Bring your multimeter and a few voltage sources and get someone to compare the readings. Or build a reference voltage generator using one of these:

Texas Instruments LM4040AIZ-2.5 2.5V ±0.1% ±100ppm/°C: http://www.digikey.com/product-detail/en/LM4040AIZ-2.5/NOPB/LM4040AIZ-2.5/NOPB-ND/182334 USD 1.71

Analog Devices REF192GPZ 2.5V ±0.04% ±25ppm/°C: http://www.digikey.com/product-detail/en/REF192GPZ/REF192GPZ-ND/997116 USD 3.25

Maxim MAX6250BEPA+ 5V ±0.1% ±7ppm/°C: http://www.digikey.com/product-detail/en/MAX6250BEPA+/MAX6250BEPA+-ND/1099024 USD 4.86

Maxim MAX6225ACPA+ 2.5V ±0.04% ±2ppm/°C: http://www.digikey.com/product-detail/en/MAX6225ACPA+/MAX6225ACPA+-ND/1512527 USD 7.05

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
The battery voltages you got seem to be in the range of what I'd expect for new batteries. Your meter is not likely to be substantially inaccurate.

#### Arouse1973

Dec 18, 2013
5,178
A charged car battery, current meter,thermometer, 12V heating element and a bucket of water is all you need to calibrate a multi meter.

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Aug 13, 2011
1,114
This new learning amazes me, Sir Rouse. Explain again how sheeps' bladders may be employed to prevent earthquakes.

#### Arouse1973

Dec 18, 2013
5,178
LOL. Sorry that's a new one on me sheeps bladders. But I suppose stretching them over the tectonic plates would have an effect of some kind.

#### geratheg

Jul 12, 2014
42
Thanks for all the replies.

#### geratheg

Jul 12, 2014
42
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. #### Arouse1973 ##### Adam Dec 18, 2013 5,178 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.

Yes most likely, but I wouldn't consider that a very good power supply. The voltage would only be stable for a given load, not really regulating the voltage very well by the sounds of it.

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