Calculating Resistance With Just Voltages? Basic DC Circuit Design

[Tee]

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?

 

[dorke]

Welcome to EP.
You should supply each element it's specific working voltage.
So only elements working at the same voltage can be paralleled.
In your case the two 5v motors only, and they should be supplied with 5V .

Is for school homework?
It isn't a very practical thing to build with those specific voltages (5v and 10v).

Using series resistors for dropping voltages to meaningful loads isn't practical either.
For doing that you need to know the currents!

 

[AnalogKid]

Without knowing the operating current of each device, you cannot calculate the resistor values.

Separate from that, you approach will not work. None of those devices draws a purely constant current; the load current varies with the actions of the device. Motor startup current can be many times the normal running current. Same for the controller as it performs different computing tasks or idles between them.

Each of those devices is designed assuming that it is powered by a constant voltage source with a very low output impedance. You need some kind of voltage regulator for each of the three required voltages. These regulators can be simple or complex, power-wasting, or very efficient, expensive or cheap. There are a lot of ways to get where you want to go, but all of them require more parts than just resistors, and a better understanding of how electrical power works.

ak

 

[BobK]

And you still need to know the current required.

Bob

 

[Tee]

Without knowing the operating current of each device, you cannot calculate the resistor values.

Separate from that, you approach will not work. None of those devices draws a purely constant current; the load current varies with the actions of the device. Motor startup current can be many times the normal running current. Same for the controller as it performs different computing tasks or idles between them.

Each of those devices is designed assuming that it is powered by a constant voltage source with a very low output impedance. You need some kind of voltage regulator for each of the three required voltages. These regulators can be simple or complex, power-wasting, or very efficient, expensive or cheap. There are a lot of ways to get where you want to go, but all of them require more parts than just resistors, and a better understanding of how electrical power works.

ak

Thanks for your advice mate, it is really appreciated, I have taken it onboard as below and am wondering what your thoughts are on my approach.

Ok, so I have found the operating currents and calculated the resistances to suit these, will supplying the components with the operating current be sufficient enough to start the motors, or do I need to supply the motors with a greater current? if I supply with a greater current, will I need to find a way to drop this current down as soon as the motor has started, or is it a case where the motor will draw whatever varying current it needs and the excess current is turned to heat or something?

I have designed my circuit to include voltage dividers that supply the components with a constant voltage source as you have suggested, I have also dropped the supply voltage to a single 24 volt battery as not all components will draw voltage at the same time and this is the maximum amount that will be drawn at any one point.

please see new circuit attached and let me know your thought's.

 

[kellys_eye]

If you are using voltage regulators you won't need the resistors.

Now you have to consider the power dissipation in each regulator - this will equal the (volts in minus the volts out) multiplied by the current being drawn. Answer is in 'watts' and must fall within the capability of the chosen regulator device, whatever that is.

 

[dorke]

You do not need 24V to operate this .
Dropping 24v to 5V is very wasteful of energy, and will create unwanted heat in the regulators.
12V should be enough.


1. You can connect the 12v directly to the pump.

2. 10V controller? sounds like a strange animal...Have any datasheet?
Are you sure it will not operate at 12V?
What is the current needed by the controller?

 

[davenn]

please see new circuit attached and let me know your thought's.

you SHOULD NOT be using resistors in series with your motors, it will stop the motors working correctly
get rid of the resistors

 

[Tee]

Noted thanks

 

[Tee]

My aim is to be able to feed all these components from the same power source, the reason I chose the 24v power supply is because the worst case scenario has the controller working at 10v and the Pump working at 12.

The controller is the Arduino Uno, It can operate at 12v at a maximum and the maximum current drawn from the Arduino is 1amp, which leads me to ask, if I design to allow the controller to receive 1amp, what happens when the controller doesn't need all that current? will it just dump the rest as heat or what?

Thanks for your advice mate

You do not need 24V to operate this .
Dropping 24v to 5V is very wasteful of energy, and will create unwanted heat in the regulators.
12V should be enough.


1. You can connect the 12v directly to the pump.

2. 10V controller? sounds like a strange animal...Have any datasheet?
Are you sure it will not operate at 12V?
What is the current needed by the controller?

 

[Tee]

you SHOULD NOT be using resistors in series with your motors, it will stop the motors working correctly
get rid of the resistors

Thanks mate, do you recon you could elaborate on why? I am using the reulators to get the voltage required, but what other than a resistor can I implement In the circuit to ensure the motors get a specific current and a specific voltage?

 

[Tee]

Thanks All,

I suppose I should describe the aim of this circuit better. What I am really after is a way to power the motor, the pumps and the controller, all from one power source, ensuring everything gets the correct amount of voltage and current. The motors are 28BYJ-48 DC5V stepper motors with 4 phase 5-wires and ULN2003 driver board TE759, the controller is an Arduino UNO, the pump is a DC12V 3m 240L/H Ultra Quiet Brushless Motor Submersible Pool Water Pump.

 

[dorke]

The purpose of a voltage regulator is to provide a fixed and stable voltage at it's output.
Putting a series resistor with the load will oppose this:
the voltage to the load will be dependent on the load current.

 

[kellys_eye]

All devices will only draw (take) the current they are designed to - you can fit a 1000A power supply to your Arduino and it will only 'take' what it needs i.e. 1A. Same applies to everything else you've got connected.

Your power supply should be able to deliver a voltage that is equal to the highest voltage device you want to power and with sufficient current to power them all. You add regulators to drop the highest voltage to the lower ones you need elsewhere.

If the motor draws 1A, the Arduino draws 1A, the pump draws 2A you need a supply capable of delivering AT LEAST 1+1+2 = 4A. Doesn't matter if it supplies MORE - as I said, the devices connected will only take what they need.

 

[dorke]

The Arduino UNO can be easily operated from 12V,hence you don't need the 10v regulator.
Drive the all thing from a 12v dc source.

Drop the voltage to the motors with a 5v regulator.
In fact the 28BYJ-48 motors can be operated at 12V as well !

So you can power the all thing from a single 12V source directly!!!

 

[davenn]

Thanks mate, do you recon you could elaborate on why? I am using the reulators to get the voltage required, but what other than a resistor can I implement In the circuit to ensure the motors get a specific current and a specific voltage?

The purpose of a voltage regulator is to provide a fixed and stable voltage at it's output.
Putting a series resistor with the load will oppose this:
the voltage to the load will be dependent on the load current.

As Dorke said, you already have a voltage regulator to give the required voltage.
the addition of a resistor is going to cause current limiting which will cause the motor not to get enough current to operate properly

Motors require a large startup current before reaching nominal speed the resistors will severely limit that required startup current

In fact I will go as far as saying, that depending on the voltage regulator you use, you may still run into a lack of startup current
as they are not good at giving bursts of high peaks of current

I have seen that problem a number of times over the years

just noticed this comment from, you and it goes against your circuit diag
that had shown just one wire to the motor

The motors are 28BYJ-48 DC5V stepper motors with 4 phase 5-wires and ULN2003 driver board TE759

steppers are quite current hungry

now knowing that, you really should use DC-DC switching buck regulators

 

[(*steve*)]

The Arduino runs from 5V internally. Sure, it has a regulator which allows it to run from a higher voltage but there's no way that you can draw 1A from it without letting the smoke out.

In fact, the maximum input voltage to an Arduino is generally limited by the power dissipation of the regulator.

If you're not getting anything more significant than a few LEDs from the Arduino power supply you'll be fine. Running your motors from their own 5V supply (as you're doing) is the right thing.

It is worth asking why you need two 5V regulators. In most cases one will suffice. If you want to switch power to the motors, a switch after the regulator will probably work just as well.

The regulators will require a third connection (to ground), and to eliminate problems with ground currents may need to be wired carefully (depending on the magnitude of the currents involved)