# Calculating Resistance With Just Voltages? Basic DC Circuit Design

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#### Tee1

##### New Member
I am trying to design a simple DC circuit that powers 2 x 5v motors, 1 x 12v Pump and 1 x 10v Microcontroller using 4 X 9v Batteries. All I have to work with is voltages, I do not have any values for the currents required by these components.

I have wired the components in parallel and the batteries in series as to put the entire 36v (4 X 9v Batteries) across each component. As can be seen in the picture attached.

My questions is, how do I calculate the resistances required to give all the components the appropriate amount of voltage? Can anyone explain this or recommend any decent books or literature I can read to learn this?

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#### ericgibbs

##### Well-Known Member
hi Tee,
Using batteries and resistors in this way is VERY inefficient.
Consider tapping into the batteries in series in order to select the voltage closest to the required voltage for the device it is driving.
E
A very rough example showing another option.

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#### Tee1

##### New Member
I can explore tapping into the batteries in series to increase efficiency, but I still need to know how I can calculate the resistances required to give all the components the appropriate amount of voltage?

#### ericgibbs

##### Well-Known Member
I can explore tapping into the batteries in series to increase efficiency, but I still need to know how I can calculate the resistances required to give all the components the appropriate amount of voltage?
hi,
OK, first you have to know the current each load will require in order to operate.
Say your motor is 500mA at 10V and you have a voltage source of 18v, this means you have to drop [18v-10v]/0.5A = 8v/-0.5A = 16 Ohms resistance.
The important point is that the load current of the motor will be higher than 500mA when its starting up or running under a load.
So the voltage drop across the resistor will increase and the motor will be starved of current and may not start or even stall.
This same problem is common to all loads which do not draw a constant current from the source thru the resistor.

E

#### JonSea

##### Well-Known Member
If each load always draws the same current, you can use Ohm's Law to find the needed resistance to drop a given amount of voltage:

V = I ×R where

V = voltage across the resistor in volts

I = current through the resistor in amos

R = resistance in Ohms

For this to work, the current must be a constant. As Eric explained, the current drawn by the motor won't be constant, so your idea won't work.

You could use voltage regulators for each load.

#### Nigel Goodwin

##### Super Moderator
As others have mentioned, you don't calculate the resistances - because you don't (and can't) use resistances for doing this.

You first need to decide exactly what you're wanting to do, what the current requirements are for the different parts, and what battery life you expect to get.

Assuming you're talking about 4 x 9V PP3 size batteries, then those are far too small for feeding motors (and probably the pump?), but one should be fine for feeding a micro-controller - although why do you think it's 10V?, or is it a board (such as an Arduino?).

#### flatfootskier

##### Member
If you outline what you are trying to achieve, rather than jumping at a (poor) solution you will probably get some good help from here. As it stands though, it's not really viable.

#### flatfootskier

##### Member
My first 'dislike' I didn't even realise it was available!
Thank you. I hope you learn from your experiences

#### crutschow

##### Well-Known Member
You added a Dislike to one who gave you good advice, so don't expect any help from me.
Ignorance I can tolerate.
Stubborn stupidity I can't.

#### schmitt trigger

##### Well-Known Member
That has happened to me before also.
Got a thumbs down for mentioning that the proposal he was advancing was conceptually flawed.

My lesson learned: is that I will now only respond to questions which show that the OP has a basic understanding of electric circuits.

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