I have a new telephone instrument which comes with adopter 110 acv to 7.8 dcv with 450 mA as parameters. I wish to use it for 220 acv. I purchased an available 220 acv to 9 vdc with 1 Amp parameters. If I want to use this for the instrument can I use a resistor to reduce voltage to 7.8 dcv? I did some calculations below with my very little knowledge(?) of Physics. I would be using non conventional terminology; wrong terminology - however the problem is real.
Vo= Voltage parameter of adoptor:Io = Current parameter of adopter
Therefore Ro = Resistance of adopter(imaginary?) = Vo/Io
Add a resistance R1 to the connecting wire of adopter to instrument
Inew = Current value after the resister = Vo/(Ro + R1)
Therefore the voltage across the resistor =(R1/Ro+R1) Vo
Does the above make sense?

 

Your calculations would be corrrect but you cannot do it this way as your instrument does not take 450mA it is likely to take different amounts of current at different times and the voltage across it will vary. If you instrument is really simple like a light bulb it would be ok. And are the adapters regulated?
Use a voltage regulator circuit - see LM317.

 

The LM317 will likely not work as it's drop-out voltage is too great. But a couple of diodes in series, dropping 1.2V to 1.4V would probably get you where you want to be.

 

Thanks for your thoughts and suggestion. According to another expert your suggestion may not of help.
After I put the question, I did carry out a experiment. I connected telephone instrument in series with a resistance that I can change value.So the circuit had only two components in the power circuit. I assume that the power source is needed to charge the battery and transmit the telephone conversation to cordless.By knowing the resistance value, I could calculate the resistance of the instrument. Surprisingly for five different values of this resistance I got five different values of the resistances of the instrument!These vary from 87 to 133. I am using a breadboard for the experiment.
I am just a new student( 73 year old) fascinated by the subject. I am unable to come up with any plausible explanation for such a variation, when the instrument was not used for any other purposes. As a statistician by profession( in the past) I would use the average of this resistance to calculate the required resistance to reduce the actual voltage of about 10 vac to about required 7.8 vac and rest depend on the robustness of the instrument.
Can I use diode in an ac circuit? The instrument requires ac input for charging the battery and/or transmitting conversation to cordless.
Thank for your input.
sureshparanjape