Voltage Trips and Traps

[posplayr]

A question came up about why voltage appears on the end of a wire that has a bad connection. The answer is because there is no current flow.

The problem occurs like this; you use your trusty volt meter to measure the voltage at a point in a circuit. You see voltage so you think every thing is fine. Then you attach your electical device (like a light bulb) and it doesnt light. You figure the bulb is bad. You try another bulb and it doesnt work either. You test the bulb in another socket and it works. So what gives?

Basically you are doing an unloaded test, one that shows voltage but that is not actually pulling any current. This is a similar test to the open loop voltage test on the stator. You see voltage which makes it appear the stator is OK, but it is an inconclusive test, because when you try to pull current the voltage might drop if there is a near.

There are various ways to explain this all of which are equivalent. One way to think of it is if you are reading a voltage in the middle of a circuit, you may be reading the source voltage (i.e. the battery) through a resistance (for example a corroded connection). Since there is practically no current flowing into the meter (volt meters are high impedance), you see the voltage.

However, when you stick a device in the circuit that tries to pull current, if the resistance of the bad connector has a resistance equal to the resistance of the device you are putting in (12 ohms for a 12Watt bulb), then the voltage to the bulb is 1/2; This what is know as a voltage divider.


See the attached picture which using a coil to demonstrate the situation. If you have had the solenoid grounding dilemma, then this is the exact same situations.

Unless current is flowing there is no voltage drop (i.e. V=IR) that is what the equation means.

This is also the reason for doing the Stator tests at full load which is 5000 RPM. Otherwise you are not testing the resistance when there is maximum current flow. Exact same issue.

EDIT: In this analogy the coil is the corroded connector (something with finite resistance), and the switch is to indicate the open light socket (switch open) with and the bulb (closed switch).

[bwringer]

A plumbing analogy also works here -- if the pipe is almost blocked, you'll still read good pressure (voltage) but there won't be enough flow (current) to do anything useful.

[Steve]

A plumbing analogy also works here -- ...

I often try to use a plumbing analogy when trying to explain electrical principles to a newbie.
I usually helps a lot of them understand it better.

I also use that same blocked pipe analogy to explain that oil pressure does not mean that your bearings are getting lubricated.
It only means that your pump is working.

.

[posplayr]

Yep, it can be useful; there are at others in addition to hydraulic and thermal analogies to electrical. However there are limits.

In order to use the other analogies( hydraulic for example), you would have to view the switch as an infinite resistor when open and a infinitesimally small resistor when closed. Then perform the voltage divider which could be directly related to a pressure divider.

I thought it easier to realize when the switch is open there is no current flow and so there is no voltage drop across the coil.

Likewise when the switch is closed it is ground so there much be 12V across the coil. This is using sum of the voltages in any loop has to be zero which is the same as the hydraulic analogy for pressure drops.

Limits to the analogy
If taken too far, the water analogy can create misconceptions. For it to be useful, we must remain aware of the regions where electricity and water behave very differently.

http://en.wikipedia.org/wiki/Hydraulic_analogy

[Steve]

Gotta remember the KISS principle: Keep It Simple, Stupid.

Most of the people that need the analogy presented to them aren't going to understand those limits. It's easy enough to show them that they have a pump (battery/alternator) that pressurizes the system, and turning on a load (lights) is like opening a valve (kitchen sink), that reduces the pressure (voltage) in the whole system if the pump can't keep up with the demand.

.

[chef1366]

Being confused is in my confort zone. That's why Jim is my buddy.

[bwringer]

You can only take an analogy so far.

I'm still trying to think of a good plumbing analogy for what happens inside an ignition coil.

Maybe something about popping a water balloon, or a fat guy jumping in the deep end...

[posplayr]

You can only take an analogy so far.

I'm still trying to think of a good plumbing analogy for what happens inside an ignition coil.

Maybe something about popping a water balloon, or a fat guy jumping in the deep end...

If you look at that link, there is no analogy between an electrical inductor and an analogous hydraulic component. The author did draw a similarity to water hammer, but I don't think that is necessarily instructive.

Inductive spark Used in induction coils, similar to water hammer, caused by the inertia of water

[SAN]

Eletronics

My head hurts. You could also use biological functions.

[gs road racer]

Pressure is only the resistance to flow...the induction coil simply uses the collapsing magnetic field to create the spark voltage. It's basically a stationary generator it uses that field movement much the way a magnet in a generator does by turning the field around an axis cutting the generator coils.Control is by either points and condensor or solid state (cdi) electronics...

I often try to use a plumbing analogy when trying to explain electrical principles to a newbie.
I usually helps a lot of them understand it better.

I also use that same blocked pipe analogy to explain that oil pressure does not mean that your bearings are getting lubricated.
It only means that your pump is working.

.