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The Load affects the Process. (Electrical Theory and More)

A Simple Linear Flow Diagram

This started out as another post about electrical theory, but the more I thought about it the more I decided to make it also a more general post. This will get into specifics about electricity, but the general idea applies to almost everything we do and that includes much more than making things. However, I will leave the general philosophy to the guru’s and will stick to physics and physical things.

In general when we think of machines or processes we think that we feed inputs to the process and it creates an output that may feed another process. Finally, after possibly many processes, a “finished product” is released into the world. This is the part shown in the blue text and blue arrows in the first picture on this post. However, there is a second, reactionary process almost always occurring and very often must be considered.  This process is shown in the red arrows and red text below the processes. The load almost always affects the process and the machinery. For example, as trucks are loaded they must be designed to handle the larger load. The “ride” of the truck is affected and the fuel consumption is also affected.   Very few things operate in a vacuum and have no effect on other things.  Also they are almost always affected by other things.

Unit Prefixes

Before we jump head first into this post, there are a few things we need to put behind us. First, I will be referring to an actual meter and have put the datasheet for this meter for your download here. Please download the Simpson 260-8 datasheet pdf. The second thing I need to talk about is prefixes for units.  Probably you already know these terms but I need to make sure we are all talking the same terms.   Each of the units prefixes commonly used are multiples of 1000 and these are shown in the second picture.

The generic circuit for this post.

In the third picture, I show the circuit used for the rest of this post.  I call it generic because I call the resistors R1 and R2 because we will be using different values.   This circuit uses an Ideal Voltage Source of 1 Volt and the current will change depending upon our choice of resistors.  For the moment we will assume we also have an ideal voltmeter.  It is ideal because it does not draw any current, but simply measures the voltage across it.  Using those assumptions, various values of resistors, and Ohms law to calculate the current we can get the following values.

Ideal Meter Calculations

Now it is time to move into the real world. First I need to explain how an analog meter operates.  The actual meter movement is a small coil of very fine wire.  This coil is attached to the meter arm and both are on an axle to allow the meter to swing.  This assembly is inside a set of magnets and as current flows through the coil a magnetic field is created around the coil.  The coil magnetic field and the magnetic field outside the coil interact to create a force causing the meter hand to move.  Some small spiral springs apply an increasing force on this movement as it moves and the arm reaches equilibrium when the magnetic and spring forces cancel each other.   A picture of this meter movement can be found here.

If you read very closely you noticed the words “current flows through the coil”, but our assumption with the ideal meter was “no current flows through the meter”.  The question now is: How much current flows through the meter?  The answer can be found on the Simpson datasheet.  If you look on the DC voltage section you will see the meter has a sensitivity of 20KΩ per Volt.  We are on the 250 mV scale so the meter has a resistance of 5KΩ.  The term for this is this meter has an Input impedance of 5KΩ on the 250 mV scale.  In DC electricity impedance is always the same as resistance.  However, in the future when we deal with AC and signals Impedance includes some other terms. This is a measure of how much the meter “loads” the circuit.

Voltage with the meter present.

Now we have a new set of calculations.  We now have a 5KΩ resistor in parallel to R2 and this will affect the value read on the meter as shown in the final picture.  To show how bad it can get I added one more set of calculations with R1 and R2 ten times higher than the previous set of calculations.  The highest set of values for R1 & R2 are actually more typical of those found in electronic circuits.

The Simpson meter is a very fine meter and I chose the very worse example to prove the point.  However, this example is a very normal situation in electronic circuits.  This meter just would not be a good choice for electronics work.  However, at higher voltage ranges this meter has a very high impedance.  Be aware that all meters have an effect on the circuit they are measuring.

Now to take it back to the more general case, all meters have some effect on the measured device.   If I had a racing type of bicycle with the very very thin profile tires and used a typical tire gauge to measure air pressure, each time I used the tire gauge I would consume a small amount of the pressurised air.  If I repeated these measurements eventually the tire would become flat.

Summary:

Input resistance is always important, this includes both the loads and any measuring devices. We will be concerned with the input impedance on circuits we design.
And:…

Just about everything affects everything else if you look close enough at the interactions

Gary

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