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18V AC Adapter For 18V Drive Measuring At 24V?

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    18V AC Adapter For 18V Drive Measuring At 24V?

    I have an Toshiba AC adapter for an external Toshiba 5.25" drive. The adapter is almost certainly original to the drive. Both the adapter and the drive labels read:

    DC 18V 0.6A

    For output and input respectively. For input, the AC adapter label reads:

    AC 120V 60Hz 25W

    The adapter is receiving:

    AC 121V 60Hz

    However, the adapter output is just below DC 24V across my multimeter.

    Is this normal or safe for operation of the drive?
    Last edited by charnitz; April 11, 2021, 07:59 AM.

    As a follow up to this, I also have an apparently original Toshiba AC adapter for a Toshiba T1000. It is labeled as:

    AC 120V 60Hz 15W
    DC 9V 1.1A

    When receiving AC 121V 60hz, it measures 12.6V across my multimeter.

    Is it normal for AC adapters of this vintage or Toshiba adapters in particular to have a DC output voltage about a third over that labeled?


      Is it a simple transformer and rectifier type or a solid state switcher supply.
      The simple transformer units will often show much higher voltage without a load. Try using a resistor of about 30 ohms with at least 10 watts as a load. If it doesn't drop to 18V you may have an issue. 30 ohms is a little over 10 watts but for a short time to measure, it shouldn't be an issue.
      If it is a switcher supply, it has a problem, regardless of the input voltage.
      Also, check the AC voltage going to the device. At a previous place I worked at we had 145V AC. We had a hard time with the power company to get them to do anything about it. They accused our meters ( recently calibrated ) to be defective. The lady we talked to at first was saying we shouldn't complain as other were getting less. We finally convinced them that if we have an AC compressor go out that we would record the voltage and sue them for the repairs.
      That finally go them to check. It seems they had two line leveling auto transformers feeding the same lines. While one was trying to increase the voltage, the other was trying to decrease it. We were just upstream of the one that was trying to reduce it so our side got the boosted voltage. They realized the error and fixed it. It most likely even saved them some money as it was wasting a lot of power trying to compensate it self.


        This isn't uncommon--on most unregulated "wall warts" the no-load voltage will approach the peak-to-peak AC voltage. So 18 volts * 0.707 = 12.7V. Deduct rectifier bandgap and you get (surprise) 12 volts.

        Doubtless the drive unit itself has a separate 7805 (or similar) IC regulator to drop the 12V to +5. The unregulated 12V is good enough to run the spindle motor and positioner.


          Dwight Elvy and Chuck(G), thank you for your responses, the additional information, and the story of your lack of gratitude for the power company providing you 20% more voltage than you required.

          I connected the drive and it works.
          Last edited by charnitz; April 11, 2021, 11:45 AM.


            Originally posted by charnitz View Post
            However, the adapter output is just below DC 24V across my multimeter.
            I observe the same with my Toshiba 18V/0.6A adapter.
            See the 'AC adapters' section of [here].


              Just be careful because linear transformers can drift upward in voltage over time. The copper windings are wound so tight that over time the wafer thin varnish insulating the windings from each other can move around from the pressure, heat and vibration of the core. Once windings start shorting out, the voltage usually keeps going up. I've had to throw out dozens of old linear wall warts over time because they've gone so far out of spec that they were useless.


                Must be something that you're doing.

                I still have old Racal-Vadic wall warts from the early 70s that are fine.


                  Originally posted by Chuck(G) View Post
                  Must be something that you're doing.

                  I still have old Racal-Vadic wall warts from the early 70s that are fine.
                  Have you measured them recently though?


                    Yup, but they're regulated DC supplies. I don't have many unregulated ones--well, some AC "warts" but they're probably not old enough.