# Physics anomally...this makes no sense..a wall wart that puts out constant d.c. voltage 24 hours after unplugged?

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#### peplovick

##### New Member
I have a wall wart rated at 12vdc output that I measured with a quality dvm before using on a model railroad project but after unplugging from the AC source I (for absolutely no logical reason).. measured again unplugged and still read 12.4 vdc output I of course questioned my meter and measured it again with a quality Hewlett Packard. dvm and got the same solid measurement? Huh????....How is that possible? I measured it unplugged again 24 hours later and now measure 8.2vdc solid without ever plugging the transformer in again?? It basically acts like a slowly degrading battery. I thought of stored capacitance but that makes no sense since loading it with the meter should fully disipate that immediately. This defys any logic I can come up with?? Any viable theory out there?

#### JimB

##### Super Moderator
I thought of stored capacitance but that makes no sense since loading it with the meter should fully disipate that immediately.
Think again.
A high impedance meter will present very little load to the capacitor, the current drain will be very low and the capacitor will not be discharged immediately.

As an experiment, try loading the wall wart with say a 1k resistor and watch the voltage reduce.

JimB

#### crutschow

##### Well-Known Member
A DMM typically has an input resistance of 10 megohms, which is a rather small load.
So if the capacitance was 100uF (not unusual for a power supply filter) the decay time-constant would be 10M * 100u = 1000 seconds.

jjw

#### MikeMl

##### Well-Known Member
A DMM typically has an input resistance of 10 megohms, which is a rather small load.
So if the capacitance was 100uF (not unusual for a power supply filter) the decay time-constant would be 10M * 100u = 1000 seconds.
But without the meter connected, only the leakage of the capacitor (and possibly reverse leakage through a rectifier) is discharging the cap.

If there is an internal "bleeder" resistor across the charger's output, then that capacitor would be discharged with a few seconds. They probably left that out so as to prevent discharging the battery (slowly) if the charger is left connected to the battery but unplugged from the AC.

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