# Constant voltage, variable current

• HarrisC
In summary: Variac" adjustable transformerfind an old 250 watt soldering gun (thrift shop)find a wire with a good core (preferably copper) rated at at least 20 amps @ 120 voltsac.
HarrisC
Will my described method allow me to achieve my objective? If so, could you recommend a rheostat that is rated for at least 20A @120VAC that will work for my application? If not, could you describe an alternate method?

Objective:
Determine the ampacity of various wires at 120VAC

Method:
Wire a rheostat in series to the wire to be tested. Attach this circuit to a power cord and plug it into the wall (120VAC power source). Use a multimeter to measure the current across the wire and adjust the dial of the rheostat until the wire fails.

This thread will probably get shut down - this is too dangerous using 120VAC.

Note -- Ampacity has nothing to do with the voltage - 120VAC?, you could do this with a car battery and be safer ( still not totally safe)

A better test would be to attach a thermal sensor to the wire - and measure the time it takes to get to some critical temperature, or if it achieves steady state below that temp. There are many factors for this type of investigation - insulation type, ambient temp, airflow. Is the wire suspended or on a surface - etc.

As a thought experiment, or the plot for a short Daffy Duck comic book, that sounds like a good attention-getting start. But there are some serious safety breaches in your method were it to be applied in real life.

Is this a school exam question? Or how has it arisen?

Can you identify some problems that might arise? How do you think we might be able to improve the procedure?

Not a school question, this a proposal for an actual experiment. It seems I should be using a much lower voltage. Is windadct correct in implying that resistive heating is not effected by voltage, only current?

Those are answers you'll be able to investigate by experimentation, providing you survive.

Yes, only low voltage experimentation up to around 15 volts would be allowed. Using a car battery is not a safe procedure, either. Your school should be able to supply you with an overload protected variable power supply of 12 or 15 volts maximum, and which complies with your country's safety standards. Otherwise, you cannot perform experiments safely.

Resistive heating depends on..

a) The current flowing in the wire (P=I2R) and/or
b) The voltage drop down the wire (P=V2/R)

The voltage at one end of the wire makes no difference.

PS: I share the safety concerns that others have raised. Voltages over about 40V can kill. Even using lower voltages the wire may get red hot and an inappropriate power supply might be damaged. Car batteries can produce hydrogen (=Boooom). Lots of potential hazards. Hopefully your proposed experiment will be vetted/approved before you do it.

So you won't be testing samples at 120 volts. You can investigate their current capacity at lower voltages, the results will be the same.

The appearance of smoke is not a good measure of amp capacity. Smoke indicates destructive testing, and after that the cable is not fit for further use.

Windadct's suggestion of a thermal sensor sounds like a better indicator, because you should be testing for gentle warming only. You may even be able to borrow an IR temperature probe, you point it at something and it reveals where heating is occurring.

Thanks! Here is my revised method based on your feedback:

Wire a rheostat in series to the wire to be tested. Attach this circuit to 12V constant voltage LED driver. Use a multimeter with a thermocouple to monitor the temperature of the wire. Use a second multimeter to measure the current across the wire and adjust the dial of the rheostat until the temperature rises to an unacceptable level, while making sure to stay at least 10% below the maximum current output of the LED driver.

Any suggestions?

Thats better. Typically it is the temperature that limits how much current a wire can carry because high temperature affects the life of the insulation.

Depending on the application the voltage drop may also limit the max current that the wire can carry.

HarrisC said:
Attach this circuit to 12V constant voltage LED driver.
No standard power supply is going to appreciate having its output shorted. You need a power source that is specifically designed to put out lots of current at a low voltage into a short circuit or heavy load.

Why don't you just look up the ampacity in wire tables?

berkeman said:
Why don't you just look up the ampacity in wire tables?

You should also do this to work out the how much current the experiment is likely to require.

berkeman
Have you figured how much power you are asking that Rheostat to handle? It's going to be an expensive industrial sized unit.

An alternate , safer approach........
... would be to

find an old 250 watt soldering gun (thrift shop)
find a clamp-around ammeter

The soldering gun steps 120 volts down to just a few volts yet is capable of several tens of amps. Connect a short length of wire to be tested in place of the soldering gun's heating element. Plug the soldering gun into the Variac.
The Variac will give you fine control of the current.
To measure small currents, wrap several turns around the ammeter jaws and divide reading by Number of turns .

j

dlgoff

## 1. What is constant voltage, variable current?

Constant voltage, variable current is a type of electrical circuit in which the voltage remains constant while the current fluctuates. This means that the voltage source provides a steady amount of energy, but the amount of current flowing through the circuit can change depending on the resistance of the components.

## 2. How does constant voltage, variable current work?

In a constant voltage, variable current circuit, the voltage source maintains a steady output while the current changes. This is achieved by using components such as resistors, which can adjust the amount of current flowing through the circuit. The voltage remains constant due to the properties of the source, such as a battery or power supply.

## 3. What are the advantages of using constant voltage, variable current?

Constant voltage, variable current circuits are commonly used in various electronic devices because they offer several advantages. These include stable and reliable operation, efficient use of energy, and the ability to power different types of components without causing damage.

## 4. What types of applications use constant voltage, variable current?

Constant voltage, variable current circuits are used in a wide range of applications, including regulating power supplies, LED lighting, and electronic devices such as laptops and smartphones. They are also commonly used in industrial and laboratory equipment to power and control various processes and experiments.

## 5. What are the potential drawbacks of constant voltage, variable current?

One of the main drawbacks of constant voltage, variable current circuits is that they can be more complex to design and implement compared to other types of circuits. Additionally, if the voltage source is not properly regulated, it can cause fluctuations in the output current, which can affect the performance of the circuit. In some cases, this may also lead to overheating or damage to the components.

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