I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it be okay to use or should mA ratings match more closely?? Thanks.
Yes but you might have to add a regulator to the output. The output of your wall-wart might be poorly filtered as well. Use a volt meter and measure its unloaded output voltage. If it is higher than 5 volts, you need a regulator.
But a regulated supply is called for, and the above is only very crudely regulated, so you may need to buy one. The output current rating on a regulated 5V supply does not matter, as long as it is at least 220 mA. DCTX-512 for $3.50 from Allelectronics
Wall plug-in transformer adapters are a good economical source for transformers. Beware: most are only halfwave rectified but that is no problem if you output into a fullwave bridge. You can find them for little cost at swapmeets, garage sales, surplus stores etc. Some things to look for on the transformer case are voltage, current, and if it's DC or AC, and if it's a positive or negative ground -- usually indicated on the ouside ring of the plug symbol. AC is our first choice.
When measuring the output voltage with no load applied the voltage will always read as much as 40% higher than its rating. To determine the true voltage and current rating of the unit apply a resistance load at the ouput and measure the voltage.
For example, to find the resistance for a voltage rating of 12VDC @
300mA. Looking at Ohms Law with , E/I , E=12V, divided by I=.300A = .040k ohms or 40 ohms and the wattage of the resistor should be E X I or 12V X .300A = 3.6Watts. That's the wattage required for a permanent application, but for a fast voltage measurement a one watt resistor could be used. roma