May 2014
While on vacation, I managed to lose the power supply wall wart to a cheap vintage portable no-name brand shortwave radio. On the back of the radio, the power jack says 9 VDC and has a C shaped circle and a dot in the center with a plus sign on the dot. I’m pretty sure this means nine volts; positive tip. However, what it doesn't state is the milliamp rating. If I use a power supply that has too high or low an amperage rating, am I in danger of damaging the radio? How can I determine the right supply?
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When choosing a wall wart you have to check three things and look out for a problem with wall warts.
The catch is with the voltage. Wall warts come in four different flavors.
The belief that the general public has about power supplies is that the supply will drive whatever current it is rated at.
This is WRONG in capitol letters. The device will draw whatever current it is rated at at the rated voltage. The power supply must be able to supply at least as much current as the device needs. If it can't the voltage will fall off. If it is rated at more current than necessary this won't hurt anything as long as the maximum voltage is not exceeded, and this is the catch.
This is why it’s important to check unregulated wall warts at their rated current. A 9 volt unregulated power supply may read as high as 18 volts with no load on it. So if an unregulated supply is rated at 1 amp at 9 volts and the device only needs 250mA or 1/4 amp the voltage output of the supply could be 12, 14, or even 18 volts. OOPS, here comes the smoke.
A regulated wall wart will put out the same voltage plus or minus some percentage loaded or unloaded. A switching wall wart will be very light compared to a transformer walwart of the same amperage rating.
Yes, the tip needs to be positive.
The symbol you see on the radio indicates a "coaxial" power adapter plug with "tip positive" polarity. Go to your local RadioShack and get a "Universal" wall wart that will deliver 9VDC @ 1500 (or more) mA DC (IMPORTANT: Take your radio to the store so you can get the proper "adaptaplug" to use with the wall wart!).
I suggest a 1500 mA (i.e., 1.5A) or larger current-rating to ensure you'll have enough power. DO NOT FEAR: a wart with a large current rating WILL NOT harm the radio - however, a smaller-than-needed current rating will quickly burn out the wall wart (or cause overheating and/or fire hazard).
Hope this helps and happy listening to your radio.
You are on the right track!
It will not harm your circuit to use a Wall Wart with a higher current (milliamp) rating. This is just like using a bigger battery; think D-cell vs. AA.
A Wall Wart with a lower current rating is like using weak battery. You may experience some distortion, lower volume, etc. This is not harmful to the circuit.
Just remember to use the correct voltage wall wart, this you cannot play around with!
As long as you use a 9 volt center pin positive adapter, you will be fine. Current will be limited by the resistance of the radio. There may be a problem if the current supplied by the wall wart is too low, as this may cause the voltage to drop. Using a 1 or 2 amp wall wart should be OK, and if the current of the wart is too high, it won't damage your radio (as long as the output is 9 volts).
OBTW, I am assuming this is a transistor radio, and not a real old "vintage" tube radio.
