Risk of damage using an unregulated power supply

Discussion in 'General Electronics Chat' started by alvescarlos, Jul 4, 2015.

  1. alvescarlos

    Thread Starter New Member

    Jul 4, 2015
    1
    0
    Hello to all,

    I have an unregulated Power Supply with output specs of 1.5-3-4.5-6-7.5-9-12V an 1000mA 12VA (max). As with all unregulated power supplies, when I measure its output at 9V setting, I get about 14V. I understand that with this power supply when the device consums about the 1000mA, then I would get the 9V output.
    What I would like to know if it's Ok to use this power supply to power a device that needs to be powered at 9V, with a consumption of no more than 80mA, or am I having the risk of damagind the device? Should I use the power supply at a lower output voltage so I can get something more around the needed 9V?

    Thank you very much in advance.

    Cheers!
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    Under full load, you should get 1.414*9=12.7V.
    It depends on whether the device will tolerate voltages higher than 9V, or lower.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,659
    632
    If your power supply peaks are within are under 10 volts there is little chance of damage.

    A good solution would be to follow the 12 volt output with a 9 volt regulator such as an LM7809.

    [​IMG]
     
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  4. crutschow

    Expert

    Mar 14, 2008
    13,044
    3,244
    You might also consider using one of the lower voltage outputs whose open circuit voltage is closer to 9V when loaded with 80mA.
     
  5. Ancel UnfetteredOne

    Member

    Jul 3, 2015
    36
    14
    If you know the load is 80mA, then a dropping resistor of R= V/.080 where V is the desired voltage drop (5V?).

    Using 5Vdrop I calc about 62Ω required. Power = .08x.08x62 = 400mW. You can squeak by with a 1/2watt rated resistor with decent air flow. 1 watt is better.
     
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  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    The regulation on those old iron-cored transformer PSUs is pretty dire, you can sometimes find better ones with an LM317 behind the voltage selector switch.

    If you don't mind investing a bit of cash in not blowing stuff up - tight regulated SMPSU versions of the switchable voltage PSUs are becoming more common.
     
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  7. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I have the same power supply, and that is exactly what I do with it.
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,803
    1,237
    Amen!:D
     
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