Constant Voltage and Constant Current

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169117-2422 EST

Since it appears that a 9 V drycell battery of something like an Eveready A522 6LR61 has been brought up I decided to make a few simple measurements.

If using this battery you want a short time, constant voltage, source within a 1% voltage change, then the load current range needs to stay within about 0 to 23 mA, and the equivalent internal resistance is about 3.9 ohms.

Unexpectedly under shortcircuit conditions the current was about 6.4 A and the internal resistance calculates to 1.4 ohms. So it is appears to have about a 3 to 1 change in internal resistance as a function of load current.

For low currents you might consider the battery a constant voltsage source, but it is in no way a constant current source.

Connecting a Stanley EFR5366X LED across this 9 V battery I would expect it to explode. It did after a few seconds.

If in series with the LED a current limiting dropping resistor of about 750 ohms is provided, then you could vary the input voltage from 5 to 20 V and have very long LED life.

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Gar, your understanding of this is much more educated than mine and you obviously have a solid understanding of both electrical and electronics theories that I lack. That being said, the forum is full of inaccurate simplifications like the one I gave, with the "water through a pipe" example to explain current being one of the most infamous.

The challenge faced by myself and many other electricians at this time is to gain a simple understanding of how these electronic systems we're being paid to install work, at least at a basic level. Very few of us have any kind of electronics training or theory and the concepts we need now to understand the difference between a constant voltage vs. constant current driver or a leading edge vs. a trailing edge dimmer switch or even linear vs. non-linear loads can be tough to grasp, especially as many of us are no longer spring chickens with pliable brain cells.

The concept of a big battery vs. a small battery and the differences between them IS something that many of us will be able to grasp. Once that's done, it's a small step to understand that if you include a resistor between the big battery and the load you can limit the current it makes available to that load. I know it's not a perfect or technically correct illustration, but it works to get the point across to dummies like myself who never expected that we'd need to learn electronics and circuit design to do our jobs.

For most of us, all we needed to know was that if you had a low voltage load you needed an appropriately sized transformer with the correct voltage. At most, we needed to make sure we used a magnetic low voltage dimmer if needed, or a de-humming coil. Industry had our backs. We're now faced with a world where every job brings totally alien challenges that can potentially be way out of our sphere of understanding because there are no standards for all the solid state lighting we install. We may be asked to install one, two or three different LED systems on a single dimmer and then be forced to figure out why it doesn't work as planned. A simple lighting retrofit may entail deeper analysis than we ever had to do before, requiring knowledge we were never given with frustration levels we never thought we'd see after so many years in the industry.

So I apologize if my trenches examples bring nothing but noise to the thread, but in the din of battle, it's the best I can offer.
 
gar said:
169117-2422 EST
Unexpectedly under shortcircuit conditions the current was about 6.4 A and the internal resistance calculates to 1.4 ohms. So it is appears to have about a 3 to 1 change in internal resistance as a function of load current.
Click to expand...

Interesting.
9V batteries taken out of the packaging and left in a drawer with, say, a ring of keys, have been known to start fires that burned down houses. 1.5V batteries are apparently not so concerning in this respect.
 
160119-1448 EST

PetrosA:

Thank you for the response.

Electrical and electronic components being used in electrical systems today are creating many challenges. This is where trying to use basic electrical fundamentals to understand how the components and electrical systems interact is becoming very important. Following are some random comments.

A linear relationship is where variables when compared follow a straight line path.

A fixed invariant resistor has a linear relationship between voltage and current.

An incandescent light bulb is not a linear resistor. There is about a 14 times greater resistance in a tungsten filament incandescent when powered at nominal voltage compared to 0 volts, both at ambient room temperature.

An Ohmite power resistor is close to linear over its rated range. A 1500 W space heater is a little less linear over a range of 0 to 120 volts because of substantial wire heating (Nichrome), and the thermal coefficient of resistance. Nichrome probably has a resistance change that is more sensitive to temperature than the wire used in the Ohmite resistor, and the Ohmite wire is not raised to as high a temperaturre as is the space heater wire at its rated power level. The space heater has a resistance change of about 10% from cold to hot.

Some non-linear resistances are --- diode, triode, transistor, pentode, Transorb, MOV, zener diode, thermistor, SCR, etc. Many non-linear devices can perform a crude multiplication function which means that if two different frequencies are applied to the non-linear device that sum and difference frequencies can be produced. This is fundamental to superhetrodyne radio receivers.

Linear electrical circuits are often times reduced to their Thevenin or Norton equivalent circuits.

A Thevenin equivalent circuit replaces a complex circuit of multiple voltage sources and resistance networks with a single ideal constant voltage source and a single series resistance.

A Norton equivalent circuit replaces a complex circuit of multiple current sources and resistance networks with a single ideal constant current source and a single parallel resistance.

And you can equate a Norton to a Thevenin circuit. The voltage source in a Thevenin is equal to the open circuit voltage of a Norton. The series resistance of a Thevenin is equal to the shunt resistance of the Norton.

Many new devices are very complex non-linear circuits and do not interact well with each other. It is sort of hit and mis as to whether some work together. Some basic understanding can help.

A normal iron core transformer or inductor is not a linear device, but under some (many normal) conditions it can be considered linear. If you apply a balanced AC input with no DC component the transformer within its normal operating range can be considered linear. Add a DC component to the AC waveform and the core will be driven into saturation and likely the transformer primary will be burned out. There is a problem with many standard dimmers driving transformers because the dimmer output is not free of a DC component. To an incandescent light bulb this DC component is not a problem.

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gar said:
160119-1448 EST



A linear relationship is where variables when compared follow a straight line path.

A fixed invariant resistor has a linear relationship between voltage and current.

An incandescent light bulb is not a linear resistor. There is about a 14 times greater resistance in a tungsten filament incandescent when powered at nominal voltage compared to 0 volts, both at ambient room temperature.

An Ohmite power resistor is close to linear over its rated range. A 1500 W space heater is a little less linear over a range of 0 to 120 volts because of substantial wire heating (Nichrome), and the thermal coefficient of resistance. Nichrome probably has a resistance change that is more sensitive to temperature than the wire used in the Ohmite resistor, and the Ohmite wire is not raised to as high a temperaturre as is the space heater wire at its rated power level. The space heater has a resistance change of about 10% from cold to hot.

Some non-linear resistances are --- diode, triode, transistor, pentode, Transorb, MOV, zener diode, thermistor, SCR, etc. Many non-linear devices can perform a crude multiplication function which means that if two different frequencies are applied to the non-linear device that sum and difference frequencies can be produced. This is fundamental to superhetrodyne radio receivers.

Linear electrical circuits are often times reduced to their Thevenin or Norton equivalent circuits.

A Thevenin equivalent circuit replaces a complex circuit of multiple voltage sources and resistance networks with a single ideal constant voltage source and a single series resistance.

A Norton equivalent circuit replaces a complex circuit of multiple current sources and resistance networks with a single ideal constant current source and a single parallel resistance.

And you can equate a Norton to a Thevenin circuit. The voltage source in a Thevenin is equal to the open circuit voltage of a Norton. The series resistance of a Thevenin is equal to the shunt resistance of the Norton.

Many new devices are very complex non-linear circuits and do not interact well with each other. It is sort of hit and mis as to whether some work together. Some basic understanding can help.

A normal iron core transformer or inductor is not a linear device, but under some (many normal) conditions it can be considered linear. If you apply a balanced AC input with no DC component the transformer within its normal operating range can be considered linear. Add a DC component to the AC waveform and the core will be driven into saturation and likely the transformer primary will be burned out. There is a problem with many standard dimmers driving transformers because the dimmer output is not free of a DC component. To an incandescent light bulb this DC component is not a problem.

.
Click to expand...

Not one bit of that will or has helped me do my job of 30+ years.

God bless you and your knowledge Gar but I really don't think you understand what the majority of electricians do with their time. :)
 
jaggedben said:
Interesting.
9V batteries taken out of the packaging and left in a drawer with, say, a ring of keys, have been known to start fires that burned down houses. 1.5V batteries are apparently not so concerning in this respect.
Click to expand...
Many years ago on the way out the door to a band rehearsal I slipped an unprotected 9V battery into my pocket, then I got in my van and started driving. At some point after that as I was driving down the highway the battery terminals came in contact with a quarter in my pocket. By the time I got the van stopped and got out I had a battery shaped blister on my leg. Ouch.
 
ggunn said:
Many years ago on the way out the door to a band rehearsal I slipped an unprotected 9V battery into my pocket, then I got in my van and started driving. At some point after that as I was driving down the highway the battery terminals came in contact with a quarter in my pocket. By the time I got the van stopped and got out I had a battery shaped blister on my leg. Ouch.
Click to expand...
You are lucky it didn't blow. Someone told me they had a 9v battery floating around in the back of his truck. Apparently it came in contact with the side of the pickup. As he was driving he said it sounded like a shotgun blast. That's when he stopped and realized what had happened
 
160119-2352 EST

On a 9 V battery, IEC 6LR61 alkaline, starting fires or getting very hot, I believe is largely a result of its physical design. With both terminals being exposed on one end of the battery it is moderately easy to get a low resistance short across the battery with the battery in your pocket mixed with silver coins. For a short time there might be an internal power dissipation in the battery of 9 x 6 = 54 W. That is a lot of power in a small package.

AA, C, and D cells all should have higher short circuit currents than the above mentioned 9 V battery. Thus, potentially high internal power dissipation on a short circuit, but the package design makes it much harder to randomly produce a low rresistance short across the battery.

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iwire said:
Somewhat related and I think interesting.

What is inside a D cell. http://www.naturalnews.com/PhotoTour_Energizer_Batteries_2.html
Click to expand...

AFAIK, only Energizer and Duracell short change the D cells at 2500 and 3000 mAh.

Maha, which makes great batteries, has D cells in the 11,000 mAh range. Many other non name brands run in the area of 10,000 mAh.

For some reason, all AA - D sized Energizer re-chargeable batteries are 2500 mAh.

It's great that someone cut one open for buyers to see. I like that kind of stuff. If it's real, of course.
 
ggunn said:
Many years ago on the way out the door to a band rehearsal I slipped an unprotected 9V battery into my pocket, then I got in my van and started driving. At some point after that as I was driving down the highway the battery terminals came in contact with a quarter in my pocket. By the time I got the van stopped and got out I had a battery shaped blister on my leg. Ouch.
Click to expand...

You should know how to tune your guitar without a tuner, anyway. :p
 
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