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I'm not sure if my multimeter is at fault here, but I made a partial circuit in which I soldered in a voltage regulator (7805CT) and hooked a 22uF capacitor at the input and output (I connected the other leg of each capacitor to ground). I also added another 0.1uF capacitor at the output as well.

I measured the voltage at the output leg of the regulator using the 20 DCV setting on my multimeter (model DT-830B) and it displayed 5.11 instead of 5.

Is my voltmeter being stupid or do voltage regulators have tolerances? I don't understand why the value should be 5.11 and not 5.

I don't think my tracks have much to do with it because I have everything connected within 3mm of each other and the track width is at least 0.5mm.

Also, my input is a DC wall adapter that outputs 6 VDC and 900mA.

Any ideas?

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    \$\begingroup\$ The LM7805CT datasheet shows the voltage can vary by up to 4%: mouser.com/ds/2/149/LM7805-1010961.pdf. So, you'd expect a range between 4.8 and 5.2 volts - therefore, your regulator is in spec and operating as intended. \$\endgroup\$
    – Synchrondyne
    Oct 14, 2016 at 3:33
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    \$\begingroup\$ It requires minimum load. Add a resistor to pull sbout 1 mA and the situation will improve. \$\endgroup\$
    – winny
    Oct 14, 2016 at 5:42
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    \$\begingroup\$ The 6V input is insufficient. See the data sheet. \$\endgroup\$
    – David Schwartz
    Oct 14, 2016 at 5:42
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    \$\begingroup\$ Read the datasheet (duh)! \$\endgroup\$
    – Olin Lathrop
    Oct 14, 2016 at 10:59
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    \$\begingroup\$ For persons with (a lot of ) experience it sounds simple to refer to a datasheet. However if you are in the learning phase a datasheet can be overwelming. So some simple guidance is not bad. \$\endgroup\$
    – Decapod
    Oct 14, 2016 at 12:08

2 Answers 2

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If you look in the datasheet you find that the output voltage of the 7805 can vary between 4.8 and 5.2 volt. This is the error margin of the output voltage.

Remember that in every design there is always an error. So the regulator can be off and/or your multimeter. Even the ruler to measure a distance is not 100% accurate. We can decrease an error but at the new level you find an error again.

As far as your DC input is concerned you need at least 7.5 V input to obtain a stable regulation. This again according to the datasheet.

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    \$\begingroup\$ Also, measure under a load. Some regulators need a minimum load for proper regulation. \$\endgroup\$
    – Passerby
    Oct 14, 2016 at 3:36
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    \$\begingroup\$ @Passerby. You are right. But for OP it was not clear that everything comes with a tolerance. So i focussed on that aspect only. \$\endgroup\$
    – Decapod
    Oct 14, 2016 at 3:43
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You should give the 7805 enough (2-3V) headroom to work. The adapter you have might be okay for light loads if it is unregulated, but 6V definitely is not.

As to why your voltage appears high- there is a tolerance on the output voltage of your 7805- it can be as high as 5.20V and still be within spec. Your meter also has a tolerance, and if the meter battery is dying it will typically read higher than the actual voltage, so make sure you have a fresh battery.

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