k0ua
Epic Contributor
Now that we have the problem figured out, and some of us have maybe drunk several beers. Lets analyze a few things to try to have a true understanding of the relationship of voltage, current and resistance. Yes they not only have a relationship, they are forever wedded. A threesome if you will. They are defined by the equation E=IR This is Ohms law. He made this law almost 200 years ago. The E stands for Electromotive force, we measure that in Volts in honor of a fella named Volta. The I is current and we measure that in Amperes or we just say Amps. yeah another old dead guy. The R is for Resistance, or the opposition to the flow of current. If you have a flow of water coming out of a garden hose, and then you step on that hose with your foot and make a restriction in the hose, you are opposing the flow of the water. The gallons per minute that it will fill a bucket will be less, but the water pressure it can develop at the end will be the same (the Voltage)
The original poster found a "bad ground". A corroded connection. OK but WHAT does that mean? WHY can I still get good "juice" (voltage? not sure what juice is) reading with a bad ground. Why doesn't the voltage show low thru a "bad ground" (is that anything like a "bad dog?")
Well, a "bad ground" is nothing more than a resistor. Whats a resistor you ask?. Well a resistor, resists the flow of electrical current(very much like you stomping that hose shut with your foot). But a resistor doesn't prevent you from reading full voltage thru that resistor right up to the point when you try to pull some current thru that resistor. Then when you attempt to pull current thru it is when the voltage will drop.
Example: Lets take a 12 volt car battery, a 100,000 ohm resistor, and a automotive tail lamp bulb. Lets hook them all up in series., and just before we make the last connection to the tail lamp bulb, lets take our volt meter and place one its probes from one of the battery posts, then place one end of the resistor on the other battery post and the other probe of the voltmeter on the other end of the resistor. So we measure 12 volts thru the resistor right? So we should be able to hook up our taillamp thru this resistor right? We have 12 volts, why won't the taillamp light up? It should right?.. Nope it never will.
Why? Someone said you need current to make a starter spin. Yep, you sure do, BUT you also need voltage too. In essence you need POWER, which is delivered by both Voltage (water pressure) AND current (water flow) you need BOTH to deliver POWER to the starter, or in our example the tail light bulb.
Delivering electrical power is a simple formula too. P=EI or Power equals E (Electromotive force) times I (current measured in amps)
In our above battery, resistor, taillamp example we had the voltage coming thru the resistor, but we could not deliver the current in amps. Lets do the math, it is not that difficult.
E=IR So lets use that "useless" algebra we all learned about 50 years ago, and substitute the values we know and calculate the values we don't know. This is what they mean when the say "solve for the unknown" And yes, you remember in algebra, or every day life it is perfectly fine and dandy to multiply and divide with letters of the alphabet that are placeholders for real numbers. So here is what we know. The battery is 12 volts, so that is the value of "E" . We know the value of the resistor (the R) because I told you what it was. What we don't know is the value of the I (or what will the current be when the tail lamp is hooked up. Well lets see now, The wattage of the tail lamp is about 25 watts, or so they say, and the specs say that maybe the resistance of the lamp is about 6 ohms. So if we put it in series with the 100,000 ohm resistor, we now have 100,006 ohms of resistance in our series circuit. So the lamp is not changing the resistance in the circuit much.
Lets do the math 12=I x 100,006 so lets divide through both sides of the equation by the 100,006 to get the I, the value we don't know by itself on one side of the equation, (following the algebraic rules here). we have 12 divided by 100,006 =I. so .0011 amps of current will flow in the circuit. That is a very small amount of current, and the power delivered into the lamp is is P=EI so Power is equal to 12 x .0011 amps so about .0014 watts. so how bright is a .0014 watt light bulb? Uh not very bright. like as in not visible. So the resistor has "resisted" the flow of the needed current that the bulb tried to draw, to light up just like the corroded high resistance connection shut off the flow of current the starter tried to draw to spin the engine.
Still with me? I bet a lot of you said "to heck with this" and left already. BUT in the off chance some are still reading:
So here is what we know. We can deliver all the voltage we want to a starter terminal, but until the LOAD of the starter is engaged, and the electrons begin to flow and that flow is unimpeded by a high resistance then and only then will we deliver POWER to the starter motor. Power is defined as P=EI or power is volts times amps. Our current path of our starter circuit MUST be unimpeded and have LOW resistance through the path for large amounts of power to be delivered. This is often why we can see dash lights light up, and then the instant we call for real POWER to be delivered to the starter circuit this voltage drops to nothing and the dash lamps go out. The dash lamps drew so little power that the high resistance of the "bad ground, bad cable, corroded connection, etc" was able to provide enough voltage and current (POWER) to let them work. Often these defective high resistance joints actually change resistance dynamically as more loads are put upon them.
OK, school is out for the day. We can answer question if you have any later...
Tomorrows lesson is using geometry in everyday life...
The original poster found a "bad ground". A corroded connection. OK but WHAT does that mean? WHY can I still get good "juice" (voltage? not sure what juice is) reading with a bad ground. Why doesn't the voltage show low thru a "bad ground" (is that anything like a "bad dog?")
Well, a "bad ground" is nothing more than a resistor. Whats a resistor you ask?. Well a resistor, resists the flow of electrical current(very much like you stomping that hose shut with your foot). But a resistor doesn't prevent you from reading full voltage thru that resistor right up to the point when you try to pull some current thru that resistor. Then when you attempt to pull current thru it is when the voltage will drop.
Example: Lets take a 12 volt car battery, a 100,000 ohm resistor, and a automotive tail lamp bulb. Lets hook them all up in series., and just before we make the last connection to the tail lamp bulb, lets take our volt meter and place one its probes from one of the battery posts, then place one end of the resistor on the other battery post and the other probe of the voltmeter on the other end of the resistor. So we measure 12 volts thru the resistor right? So we should be able to hook up our taillamp thru this resistor right? We have 12 volts, why won't the taillamp light up? It should right?.. Nope it never will.
Why? Someone said you need current to make a starter spin. Yep, you sure do, BUT you also need voltage too. In essence you need POWER, which is delivered by both Voltage (water pressure) AND current (water flow) you need BOTH to deliver POWER to the starter, or in our example the tail light bulb.
Delivering electrical power is a simple formula too. P=EI or Power equals E (Electromotive force) times I (current measured in amps)
In our above battery, resistor, taillamp example we had the voltage coming thru the resistor, but we could not deliver the current in amps. Lets do the math, it is not that difficult.
E=IR So lets use that "useless" algebra we all learned about 50 years ago, and substitute the values we know and calculate the values we don't know. This is what they mean when the say "solve for the unknown" And yes, you remember in algebra, or every day life it is perfectly fine and dandy to multiply and divide with letters of the alphabet that are placeholders for real numbers. So here is what we know. The battery is 12 volts, so that is the value of "E" . We know the value of the resistor (the R) because I told you what it was. What we don't know is the value of the I (or what will the current be when the tail lamp is hooked up. Well lets see now, The wattage of the tail lamp is about 25 watts, or so they say, and the specs say that maybe the resistance of the lamp is about 6 ohms. So if we put it in series with the 100,000 ohm resistor, we now have 100,006 ohms of resistance in our series circuit. So the lamp is not changing the resistance in the circuit much.
Lets do the math 12=I x 100,006 so lets divide through both sides of the equation by the 100,006 to get the I, the value we don't know by itself on one side of the equation, (following the algebraic rules here). we have 12 divided by 100,006 =I. so .0011 amps of current will flow in the circuit. That is a very small amount of current, and the power delivered into the lamp is is P=EI so Power is equal to 12 x .0011 amps so about .0014 watts. so how bright is a .0014 watt light bulb? Uh not very bright. like as in not visible. So the resistor has "resisted" the flow of the needed current that the bulb tried to draw, to light up just like the corroded high resistance connection shut off the flow of current the starter tried to draw to spin the engine.
Still with me? I bet a lot of you said "to heck with this" and left already. BUT in the off chance some are still reading:
So here is what we know. We can deliver all the voltage we want to a starter terminal, but until the LOAD of the starter is engaged, and the electrons begin to flow and that flow is unimpeded by a high resistance then and only then will we deliver POWER to the starter motor. Power is defined as P=EI or power is volts times amps. Our current path of our starter circuit MUST be unimpeded and have LOW resistance through the path for large amounts of power to be delivered. This is often why we can see dash lights light up, and then the instant we call for real POWER to be delivered to the starter circuit this voltage drops to nothing and the dash lamps go out. The dash lamps drew so little power that the high resistance of the "bad ground, bad cable, corroded connection, etc" was able to provide enough voltage and current (POWER) to let them work. Often these defective high resistance joints actually change resistance dynamically as more loads are put upon them.
OK, school is out for the day. We can answer question if you have any later...
Tomorrows lesson is using geometry in everyday life...
