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[Timo2]

   Hi All (Electronic Wizards)

  How do you drop the voltage down    20v to 15v and keep power output the same e.g.  :- “4 amps”

  Can I fit something in line on a cable ?

   Timo2

[roycv]

Hi Timo2, You quote 4 amps as the 'Power'.  This is the current flow.

If you mean starting with 4 amps at 20 volts then the power is 20 x 4 = 80 watts.
Watts are units of power.

To maintain 80 watts from 15 volts the current must increase.  That is the resistance in the circuit must go down so that the current can increase to 5.33 amps (15 x 5.33 = 80).

If the item being driven is an electric motor then it has to do more work say by increasing the propeller size so that the current increases.

Look up Ohms law on google.
regards Roy

[Timo2]

Hi

  It,s not a motor , but a power supply output 20v @ 4amps ,   looking for only 15v max Please KISS

  only a simple old blacksmith not very good with wires and things

   Timo2

[malcolmfrary]

To lose 5 volts at 4 amps is simple enough, you just insert a 1.25 ohm resistor in series with the supply, of at least 20 watts rating.  However, and this is important, if the demand of the load varies (i.e. is ever less than 4 amps) the voltage out will increase, and whatever is being supplied will probably fry.  There are plenty of circuits around to do the job and keep the voltage regulated, most using a zener diode as a voltage reference, and a power transistor to be a self adjusting resistor.  These usually need a quite hefty heatsink to get rid of the unwanted heat.