I am planning to light up 178 LEDs (3.4V-3.8V, 25MA) using a 18V/1A Adapter (DC radio shack transformer) and I need a little help.
Power Adapter says that is 18 volt but when I test it with volt meter it shows 22V. When I connect a string of 6 LEDs (plus small resistor) with it voltage is dropping to 21.4 (and shows 9.4ma current)
I don't have problem calculating my resistors (I am using
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but it seems my 18V (or 22V) Adapter is not really what is says (it drops).
My question is what should I put in that web site as my Source Voltage? (22V? 18V? or something else).
A fair approximation for your supply (based on both the rated voltage and your experimental results) would be a 22 volt source that includes a 4 ohm series resistor. When you connect an 18 ohm resistor across it, the 4 ohm internal resistance will drop 4 volts and the 18 ohm resistor resistor will drop 18 volts.
Include that internal 4 ohm resistance in your design calculations.
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Thanks but what do you mean by unregulated adapters? And also if the rating is the voltage at full load then how do we explain the Voltage Drop with 6 LEDs (that is far less than 1A for sure)?
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Some "wall warts" contain internal voltage regulators that keep the output voltage very close to the stated voltage on the adapter. If yours were regulated, it would deliver 18V, regardless of load, up to its rated current output of
1 ampere.
The unregulated variety have a non-negligible internal resistance that causes the output voltage to drop as the load increases. In a rare gesture of honesty, the manufacturer actually promises the rated voltage at full load.
Well, the greater the external load, the greater the voltage that will be developed across the internal resistance. This voltage will be subtracted from the output voltage.
The voltage drop follows Ohm's law, using the internal resistance and the load current.
John has calculated the internal resistance to be four ohms using (22V -
18V)/1A. That assumes a linear internal resistance but the 0.6V drop with a current of 0.0094A suggests an internal resistance of ~63 ohms which would never allow 18 volts output at 1 A. So maybe it is non-linear (at least over part of the output current range) or defective.
It can get complicated to work with an unregulated supply. For less than a dollar, you can add a simple IC voltage regulator and some capacitors and have your output voltage stay within 50mV of
18V at any load up to 1A.
Chuck
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