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# Finding appropriate source for the electrical load

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Have four circuits. One takes 9Vdc, two 5Vdc, and last one takes 12Vdc. Have variable transformer for source. How do I calculate or choose the voltage needed to run all four circuits with out blowing them all up, and at the same time run each circuit by themselves or two at a time or three, etc. How do I know how much current the source needs and where do I place leads to measure current and voltage for each individual circuit?

## SOLUTION This solution is FREE courtesy of BrainMass!

Find the resistance of each of the loads by connecting a multimeter to the leads of the eqipment. Now find V/R for each device, which gives the value of the current each of them needs.

To run all the circuits at the same time, the source must be able to give a current equal to (or above; fine) the sum of the currents we found above.

We must connect a current limiting resistor before each circuit to prevent it from blowing up. If you know the output voltage of the source transformer, find the value of the resistance as follows

Our circuit needs 5 volts 1 A, and the source is giving 12V 5A. Thus we must drop a 12-5 = 7volt across the Current limiting resistor.

Now calculate R from the equation, 12/(R+RL) = 7

we know RL (resistance of the circuit)

Find the value of the resistance for other circuits also. Then connect them as we connected 5v circuit as shown in the figure. Since R will drop 7volt across it, our circuit will get 5 V.

Please see the attached figures and contact me if more help is needed.. 102509

This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!