BASICS of ELECTRICITY

Thanks to Noel Hunt for this one.

SOME BASICS OF ELECTRICITY: 

Watts, Volts, Ohms, Amps, and Milliamp hours.  What the heck are all these electrical terms and what do they mean to me?  How do I use them?  One of the easiest ways to understand all this is to compare these terms to a water tower: 

Definition Stuff: 

The higher a water tower tank is off the ground, the more pressure we will have available to us.  This is like VOLTAGE. 

But if we only provide a 1/8 inch pipe for the water to flow down, we have placed a lot of RESISTANCE to the flow and we will get very little flow or CURRENT.   A larger diameter pipe offers less RESISTANCE, and allows more flow, or CURRENT. 

The volume of water flowing in a given time will be affected by the height of the tower and the diameter of the pipe.  This volume per minute can be thought of as the POWER generated. 

How long the water will flow, depends on the CAPACITY of the tank on the tower.  A bigger diameter tank will provide POWER longer than a smaller diameter tank.  This is like a battery’s CAPACITY. 

What are the units for measuring these?  For modeling purposes, we use the following: 

VOLTAGE (V) is measured of course in VOLTS (V)
RESISTANCE (R) is measured in OHMS (Ω)
CURRENT (I) is measured in AMPS (A)
POWER (P) is measured in WATTS (W)
CAPACITY is measured in MILLIAMP HOURS (mAh) 

As you can see from the water tower example, all of these are affected by each other.  If we don’t change the water tower height, but increase the pipe diameter to ¼ inch, we reduce the RESISTANCE and will get more CURRENT flow and therefore more flow per minute, or POWER; But the tank CAPACITY will be used faster.
Or we could get increased POWER from the little 1/8 inch pipe by increasing the height of the tower (VOLTAGE). 

Formula Stuff: 

Some deep-thinking dudes (like Mr. Ohm) figured out how these things are related: 

          For example:  P = V I  (or POWER equals VOLTAGE times CURRENT) 

So if you want 12 Watts, you can use a 12 volt battery and a circuit that draws 1 amp (12V x1A = 12W)

Or you can use a 6 volt battery and a circuit that draws 2 amps (6V x 2A = 12W) 

Also:  V = R I  (or VOLTAGE = RESISTANCE times CURRENT) 

So in the 12 volt example above, if the circuit is drawing 1 amp, the resistance of that circuit must be 12 Ohms.  (12V = 12 ohms x 1A).  In the 6 volt example, the resistance must be 3 ohms  (6V = 3 ohms x 2A) 

All electrical circuits have some inefficiency.  In our water tower example, this is like comparing a 1/8 inch pipe that ends only 10 feet from the tank, with a pipe that ends 100 foot from the tank.  The longer pipe is less efficient at letting the water flow.  An electric motor might be 75% efficient.  12 watts going into the motor would only generate 9 watts at the prop.  (75% of 12W = 9W) 

Real Stuff: 

1. Receiver Batteries:  Let’s look first at battery packs:  Many receiver battery packs that come with our RC systems, consist of 4 NiCd cells and have a VOLTAGE of 4.8V.  Their CAPACITY is 600mAh.  That means these packs can provide 600 milliamps at 4.8 volts for one hour.  (600 mA is the same as 0.6A.)  Then the VOLTAGE drops quickly.  But a typical trainer aircraft (receiver and 4 standard servos) has an electrical circuit RESISTANCE that draws a CURRENT of about 360 mA 

At 360 mA, a new 600 mAh pack will last about 1 hour 40 minutes!  (600mAh / 360mA) = 1.7 hours).  Note: It is not a good idea to test this calculation on your plane in the air! 

And we can figure out the RESISTANCE of the receiver/servo combination:   4.8V = R x.360A (from V = R I).  So R = 13.3 Ohms. 

If we added a cell and used a 5-cell pack, would the pack duration go up?  Let’s figure it out – again, not with your plane in the air!  The circuit RESISTANCE did not change (there might be a slight change due to changes in efficiency, but we will ignore this)

From V = R I, if the VOLTAGE went up and the RESISTANCE stayed the same, then the CURRENT had to also go up.  So for the 5-cell pack: 

6V = 13.3 Ohms x I  (V = R I)
I = .451A or 451 mA 

Therefore pack duration is 1 hour 20 minutes (600mAh / 451mA = 1.33 hours) 

So changing from a 4-cell to a 5-cell pack, our pack duration decreases!  Some guys like to install 5-cell (6 volt) packs in their planes for faster and stronger servo response, but be aware that pack duration decreases.  And use caution:  Some servos can’t take 6 volts.  

2.  Power Supply Load Resistor:   In the article on the 12V Power Supply, I mention a 10 Ohm, 10 W resistor.  Why not use a cheaper 10 ohm 1/8 W resistor?
On the power supply, we connected the 10 Ohm resistor across the Common and +5V leads.   So the CURRENT will be 0.5 A   (V = R x I or 5V = 10 Ohm x I)  And we can then figure out that the power that the resistor must be capable of handling is 2.5 Watts ( P = V x I  or  P = 5V x 0.5A)
So the 1/8^th W resistor would overheat and the next readily available size is 10W.
Are you getting the idea of how this stuff it used? 

Disclaimer Stuff: 

This basic explanation of electrical concepts is not intended to make electrical engineers out of anyone, but may help you understand some concepts, and perhaps help you make informed component choices.

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