John Mortensen's Electrical Primer

[NobleHops]

John Mortensen just posted this on the Brewer list, and I thought it a nice addition to our recent discussions on electrical system theory and nomenclature. Here is is, reposted with his permission:

I am going to avoid talking about analog D'Arsonval meters. If you own one you know how they work.

Today’s DMMs (digital Multi Meters) have a digital numeric display so understanding how to read it is not a problem for most.

A DMM will have several selections to choose from. It will have two wire leads a red lead and a black lead. Red to the positive potential black to the negative or ground potential. In DC volts, DC current, and diode modes the color of the lead wires and where they are connect is important to understanding the potential differences and what the meter is trying to tell you. In AC volts and AC current polarity is not important since AC modes are alternating both positive and negative.

AC volts - Used to measure an alternating voltage. (Usually house voltages 120 volts AC, 240 volts AC, 24 volts AC) There is little need for AC volts on a bike. If anything you might use this to measure the output of a running generator/alternator.

DC Volts – Used to measures DC voltages (batteries, power supplies and such)

Ohms (continuity) – Used to measure whether or not a continuous connection from one point to another. This scale setting is used to measure the resistance to current flow. 0 ohms or close to 0 means there is an electrical connection form point A to B. Ohms setting should be made with a dead circuit (not under power). Turn off anything if you are trying to measure continuity. The DMM will apply a small potential difference between the meter leads and measure the current flow between the leads. It will calculate the resistance using Ohms Law and display it as a resistance measurement.

DC volts and Ohms are the most common uses for a meter around a vehicle.

Then there is usually a “diode” setting – Used to verify a diode junction polarity and voltage breakdown. A diode will allow current to flow in only one direction generally with a 0.6 volt drop across the junction. Diodes are used to convert AC voltages to DC voltages using a diode bridge. A 4 diode in a bridge configuration will convert AC voltages into the DC voltages. Almost everything converts the AC voltage to a DC voltage. DC voltages do most work. AC voltages are used in AC motors and lighting but generally not much more.


You will also have an AMP setting both AC and DC. Using a meter in a current mode means you would need to open a circuit and insert the meter leads, placing the meter in series with the load. This will measure the total amount of current flowing in a circuit. This usually means that you need to change where on the meter the lead wires are attached. Current modes use separate connection points on a meter. There generally is a need to understand what the current “should” be. Since all of the current flowing in a circuit will flow THROUGH the meter and the meter has a fuse to protect it from too much current flow understanding what SHOULD be flowing is important. You can blow the fuse in you meter if you don’t know what SHOULD be flowing. My meter has a 10 amp MAXIMUM so I am usually careful when using this mode. I don’t use this mode very often and usually only when I am building something electronic to verify the current draw so I can properly fuse the circuit like with the custom LED lighting that I do for people. The fuse in the current mode is NOT used in AC or DC voltage mode so if you happen to blow the fuse in the current modes it won’t affect the OHMS mode or Voltage modes of a meter…

Then there is usually a Milliamp AC and Milliamp DC current setting (also fused). These setting are for more accurate current measurements in very small current modes. Mostly used in circuit design and troubleshooting to the component level. These modes are not very useful for the garage mechanic (usually).

I hope some of this helps. Anyone with specific questions I can attempt to answer. I am sure I forgot to mention some things and can address specific questions as best I can. I am also sure I have confused many! J The best thing I can say is get yourself a DMM of some kind and use it. Put the meter in AC volts modes and push the leads into an outlet in your house and see it show you the AC volts. Now put your meter in DC volts and place the leads across a battery. It will show you the potential across the batteries terminals. If you forget to change from AC volts to DC volts or vise versa don’t worry you won’t hurt anything. You won’t get a reading at all.

There are DMMs that have other function also but I won’t speak to these. As seen in the DMM link the meter shown in that Wiki link has many other function not mentioned here.

As positive reinforcement… I have shown many people how to use a meter. These people are not trained in electronics but some basic principles and understanding how to use a meter sure can help.

John

[Ringnalda]

A useful example:


You have a 65 watt high beam light. You want to know what the current draw
might be. There are several ways of doing so. To know exactly you would
need the Meter we have been talking about. With a meter you can make basic
measurements and then do the exact calculations. In some circuits you could
use a meter to make direct measurements of currents but only if you know
approximately what would be expected. (Read my previous e-mail about
current measurements and their fuses!)



But let us assume some things. We have a 12 volt system that in actuality
is somewhere around 13.8 volts regulated voltage while running. According
to Ohms Law, POWER equals VOLTS multiplied by the AMPERES (current). We
know two things, the volts and the wattage. Using a calculator we divide
the watts by the voltage 65/13.8=4.71 amps of current flow.



Knowing 4.71 amps if flowing we can now find the filament resistance using
Ohms Law. Using the resistance (Ohms) formulas the resistance of the
filament after the basic math is 2.93 Ohms.



Using these most basic formulas you can essentially calculate anything you
might need to know about basic electronics



One last thing about my above calculations. If you measure the resistance
of a 65 watt high beam light bulb with your DMM you will find that my
calculations seem incorrect. It will measure much higher out of circuit
(cold). That is the key. A filament has a negative temperature
coefficient. As it heats up the resistance drops. When cold you will
measure continuity in Ohms of resistance with you meter of 10 ohms or so.
As the filament heats up the resistance drops to the low 2.9 ohm calculated
above. This is why Ohms Law is a good thing to know. You could not
directly measure the filament resistance when it is on but you can
accurately calculate it!



Now that you have calculated that a 65 watt headlight draws approximately
4.7 amps you now know that you could actually use the DMM in SERIES with the
lamp and directly measure the current since you know you DMM has a 10 amp
fuse in it's DC current measurement mode! But you measure 5 amps of
current.??? HMMMM???? We now can take the 13.8 volts we measured with or
DMM when the bike is running and recalculate the wattage.. These actual
measurements and calculation yield the fact that we have a 69 watt high
beam!



All this electronics talk has me thinking about POWER in Kilowatts. How
many Kilowatts of power will it take to start my 5 HP lawn mower and
actually accomplish something today? BTW 1 horsepower is equal to 745
watts. And 1 horsepower equals 178 calories/second. A human can maintain
0.1hp indefinitely (according to science). I have about a half acre of
grass to mow. A 21" mower with 1" overlap. How many Kilowatts of power
expended by the mower and how many calories burned?



One other tip. Josh Madison
<http://joshmadison.com/software/convert-for-windows/> 's Convert is one of
the best little programs you can have on you PC. It is a safe download and
very useful to answer a lot of conversions. You can find it
<http://joshmadison.com/software/convert-for-windows/> here.
http://joshmadison.com/software/convert-for-windows/





John Mortensen