Connect with us

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

Discussion in 'Power Electronics' started by geratheg, Jul 16, 2014.

Scroll to continue with content
  1. geratheg

    geratheg

    42
    0
    Jul 12, 2014
    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!
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    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.
     
  3. geratheg

    geratheg

    42
    0
    Jul 12, 2014
    Thanks for the reply.

    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.
     
  4. davenn

    davenn Moderator

    13,653
    1,886
    Sep 5, 2009
    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
     
  5. geratheg

    geratheg

    42
    0
    Jul 12, 2014
    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: Jul 16, 2014
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    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.
    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.
    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
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    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.
     
  8. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    A charged car battery, current meter,thermometer, 12V heating element and a bucket of water is all you need to calibrate a multi meter. :)
    Adam
     
    Last edited: Jul 16, 2014
  9. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    This new learning amazes me, Sir Rouse. Explain again how sheeps' bladders may be employed to prevent earthquakes. ;)
     
  10. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    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. :)
     
  11. geratheg

    geratheg

    42
    0
    Jul 12, 2014
    Thanks for all the replies.
     
  12. geratheg

    geratheg

    42
    0
    Jul 12, 2014
    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.
     
  13. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    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.
    Adam
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-