Voltage Drop

[Ωhm]

Take a circuit with one (1) inline connector. That inline connector is designed to pass current unimpeded. If resistance increases between the contacts of the inline, voltage dropping will force current to flow. The resistance on the contact will start to act like "toaster wire". This inline was not designed to drop voltage.

 

Ohm's Law, only apply to circuits, with components designed to handle the wattage.

 

Keep in mind, this is how I justify to myself, how a voltage drop works on a connector designed to only pass current. So what I'm trying to say is "I don't know".

 

Time for CC forum members to chime in.

[Andy in Pa]

An inline connector is intended to pass current, voltage and power (watts) unimpeded.  If there is a poor contact, or for some reason a stranded wire is broken and only a few strands are still connected, the power draw from the device the connector feeds does not change.  It still requires 12 VDC at 3 watts.  The poor connection (or broken wire with only a few strands connected) cannot handle this power (watts) the device draws, so it melts the connector, or burns the stranded wire completely through.

 

Connectors have a multitude of failure modes.  The wire could be compromised inside the connector, a contact could be loose, or the contacts could have some oxidation on them causing a poor contact.

 

I'll give you an example--  I was trying to diagnose some electrical problems in my MJ recently.  Had power to the fuse block, fuses were NOT blown, no power coming out of the fuse block.  I replaced the fuse with a new one, and it worked.  The fuses had a buildup of this white oxidation on the connection tabs, and they were no longer making good contact with the contacts in the fuse block.

 

Hope this helps...  

[Manche757]

3 hours ago, Ωhm said:

 If resistance increases between the contacts of the inline, voltage dropping .........

I don't understand the question.  If resistance increases for any reason (corrosion, broken wire) there is an inverse relationship with voltage and current.  Can you rephrase your question?

 

At absolute cold (like a mathematical limit,  it can not be achieved but can be approached) is 0 degrees Kelvin or -459.67 Fahrenheit.  Resistance is still present even slightly above that.  Albeit pretty low.  

[Ωhm]

Question is, does a unwanted (between M/F terminals) voltage drop generate excessive heat or not?

[AZJeff]

Yes, a voltage drop WILL generate heat, assuming a constant current condition (which is usually the case).

[Manche757]

Electricity:  electrons and protons flow negative to positive and produce current that flows positive to negative.  When trying to determine a relationship in an equation, change only one variable at a time.  The relationship between resistance (the opposite of conductivity) and voltage  is inverse, i.e, when resistance goes up voltage goes down..  The resistance relationship to  current (amps)  is also inverse.  George Ohm, that you have named yourself after, came up with equations to measure the relationships; an Ohm being a measure of resistance.  As resistance increases it produces heat.  That can be good or bad. Intended resistance heats your oven or stove top burners or it creates resistance in the heat strips in the back up heat source in a heat pump.  Unintended resistance can start electrical files or melt the electrical connectors on your MJ.  Over simplification but hope that helps

 

 

 

 

[A-man930]

Keep in mind this is a topic that folks get doctorate degrees in, but this is how I guide my automotive students through low-voltage DC circuit basics:

 

-Ohm's law simplified: Volts push Amps (current) through Ohms (resistances).  

-Current (Amps) is the only thing that "flows."

-Current passing through resistances produces heat; higher heat = higher resistance...  

-Given a consistent voltage source (usually the case in automotive), the total resistance value of any given circuit  is what determines how much current will flow through that circuit.  Ideally, the only significant resistances in the circuit are intentional ones (motor, solenoid, light bulb, etc.)  However, an unintentional resistance added into a circuit will also contribute to the total resistance and reduce the amount of current that will flow; this is often why we see lights that aren't as bright as they should be, motors that spin slowly, etc. 

-Voltage drop is a phenomenon described in Krichoff's voltage law; voltage is "consumed" (drops) across resistances when current is flowing through the circuit, and ALL of the source voltage will be dropped somewhere in that circuit.  This is handy in diagnosis because we can count on seeing a voltage drop occurring at any point where there is a resistance, and the highest voltage drop value will be observed at the point where there's the most resistance.  In a healthy circuit, nearly all 12.whatever volts are dropped across the intentional resistors (motor, solenoid, light bulb, etc.)  However, any unintentional resistances in the circuit will also drop voltage...

 

So, when you attempt to operate your unidentified device (?), your hypothetical cruddy "toaster wire" connector will add resistance to that circuit, causing that load to perform poorly (or not at all), and you will be able to measure a correspondingly significant voltage drop across it (the connector).

 

[A-man930]

5 hours ago, Ωhm said:

Question is, does a unwanted (between M/F terminals) voltage drop generate excessive heat or not?

 Yes.

[A-man930]

14 hours ago, Andy in Pa said:

An inline connector is intended to pass current, voltage and power (watts) unimpeded.  If there is a poor contact, or for some reason a stranded wire is broken and only a few strands are still connected, the power draw from the device the connector feeds does not change.  It still requires 12 VDC at 3 watts.  The poor connection (or broken wire with only a few strands connected) cannot handle this power (watts) the device draws, so it melts the connector, or burns the stranded wire completely through.

 

Watts (power) is a way to express the brightness of a lamp, the torque of a motor, the output of a heater, etc.  Volts x Amps = Watts

I (personally) recommend keeping Watts out of the conversation as it's not really that useful in the diagnostic thought process.  It's mostly useful/necessary when designing circuits.  

 

In your wire example the wire burns through (or the connector body melts) because the resistance buildup at that point is causing heat, and the heat is further increasing the resistance in a vicious cycle, all the while the current flow is decreasing; it doesn't really matter what else is in the circuit...  If the wire/connector is too weak to handle the heat buildup then it will physically fall apart, otherwise it will simply exist, perhaps discolor/burn surrounding objects, and choke down the current flow through that circuit.  The circuit's intentional resistance (motor, lamp, solenoid, or whatever) must be designed to handle the heat it generates, otherwise it will suffer the same fate for the same reason.  

Edited October 3, 2019 by A-man930
clarity

[Ωhm]

11 hours ago, A-man930 said:

Keep in mind this is a topic that folks get doctorate degrees in

That's a great statement.

 

Spending available voltage to force current through a connector/wire that's designed to pass current will generate heat. Thanks to all for your responses.
 

[gogmorgo]

The current through the circuit is determined by the sum of the resistances. If you increase resistance without increasing voltage, it reduces the total current. 

As a rule resistance will generate heat (or some other form of work), however the conductor carrying the current will also dissipate the heat almost as effectively as it conductors electricity. In most cases by the time there's enough resistance to build enough heat quickly enough to damage components, the voltage drop will be already be obvious because the circuit has quit functioning as intended.

 

I've seen plenty of corroded connectors in my line of work (wrenching on highway plow/sanders) and a decent number of burnt or melted ones as well, but I can't recall ever seeing one that was both corroded and melted. Most of the melted connectors I've seen have been for switches with movable contacts and high currents, like fan speed switches, that rely on spring tension to maintain their connection, which lessens as the switch ages, and the heat has clearly been generated within the switch rather than the connector. No signs of corrosion.

I've seen a couple cases of melted connector pins randomly within a connector on an otherwise functioning circuit, but the heat is very localized, with the only evidence limited to the inside of the connector, just the area around the poor connection. The heat is conducted away pretty effectively by the wire. Again there's no obvious corrosion inside the connector, just melted plastic and burnt dielectric grease.

The few other cases I've seen of overheated connectors to the point of significant damage have been down to exceeding intended current, either through a short-circuit or if something was added to a circuit, like extra lights.

I've only encountered two cases of actual electrical fire. One was my MJ's headlight switch, again an aging movable contact switch under heavy load, a well-documented problem. The other was the main positive cable to a relay box controlling most of the functions of a vacuum truck, that rubbed the frame and shorted. Just because I find it interesting to point out, that last one did have circuit protection in the form of a type-2 (self resetting) circuit breaker which tested good, so it seems it didn't provide adequate cool-down time before resetting, and just pulse-width-modulated the accidental heating element until it caught fire. The cable wasn't switched so there's no telling how long it took to ignite; the truck had been parked for a few hours before someone noticed smoke and flames, although the operator said he didn't notice anything not working while it was running. I changed it out for a manual-reset breaker after replacing the burnt harness and relay box.