volt and current measurement dongle from power supply shows 5.00 volts within 4% and current is less than 500 ma but lightning bolt showa undervoltage. WHY?
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2Need more information. Where is the measurement device - at the power supply or at the Pi? How long is the power cable? Which Pi? And what do you have attached to the pi? – NomadMaker Jun 25 '18 at 16:47
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Also the dongle may be broken (possibly by design). What voltage does it display anyway? Average, MIN, MAX, RMS? – Dmitry Grigoryev Jun 26 '18 at 09:29
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Because the voltage is dropping beneath 4.65V. Presumably there are periods when something kicks in and requires more current. You need a higher amperage power supply.
joan
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@Seamus I think you both mean higher current - what the OP needs is a supply that delivers the correct voltage @ the required current. – Milliways Jun 25 '18 at 23:53
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Either the voltage is too low AND/OR the resistance of the cable is too high - causing voltage to drop on current peaks.
The Pi is sensitive to very short transients (which are stretched to ~3 sec for display. See Raspberry Pi Power Limitations
Milliways
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