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Have a kids alarm clock that requires 5v 500ma for power supply option (but can also run on 3x AAA batteries). I have a universal adapter that can supply 4.5 or 6v (with the correct polarity) but it rated at 300ma max. I've tried it and it seems to work fine. I understand that I may run into problems with the alarm clock funtionality / etc and you can't be totally sure - but is it "safe"? Do I risk things catching fire for example? If this is unsafe at all I will go spend the money!

user34854
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  • Fireunlikely but VERY slight possibility. Odds are it does not draw 500 mA though. Place 1 Ohm resistor in series with supply and measure voltage drop with VOLT meter. 1 Ohm drops 1V per amp so 500 mV at 500 mA, 100 mV at 100 mA etc. You can do this with plug and socket without wire cutting with a few bits of wire a meter a resistor and preferably two people. Twist 1 Ohm resistor onto meter probes. PSU power plug outer goes to clock power socket outer. PSU inner goes via resistor to clock socket inner. Use wire if needed. Suitable calisthenics MAY allow this to be done without wires. ... – Russell McMahon Dec 30 '13 at 08:51
  • ... 3 or 4 hands helps. Odds are clock draw is below 300 mA. – Russell McMahon Dec 30 '13 at 08:51

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Don't push your luck. You may get by with a 10% overload, but 67% is asking for trouble. Ratings are determined by the power handling capacity of the circuitry inside the adapter. Overload could lead to overheating and yes, as you suggested, a fire. Even if it works for a little while, it is likely to fail in short order. Besides, you don't want to put the child in danger of a home fire or a severe burn. It's worth the cost to buy the correctly rated unit.

user9717
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