Constant Current Load

[PhaseShift]

Can someone please explain to me what a constant current load is, and what types of loads are considered "Constant Current"? I'm familiar with constant power and constant impedance loads but have not heard of a constant current load.

 

[steve66]

A constant current load would draw approx. the same amount of current despite wide varrations in the source voltage or other impedances in the circuit.

A 4-20ma temp. transducer would be a good example. Say it is designed to deliver 10 ma for 100 deg F. As the input voltage varies from 20 to 24 volts, the output might only change by a small %.

As the length of wiring changes, or as a PLC input resistance changes from 10 ohms ot 100 ohms, again the output might only change by a fraction of a %.

You might be thinking a transducer is more like a source than a load. But it really just depends on if you consider the voltage source part of the transmitter, or something separate.

Transistors are also frequently used as current sources, or current loads. Again, most people think of these as sources, but if you think of the voltage source as something separate, the transistor is more like a load.

 

[Jraef]

That's a good example from a signal standpoint, a power example would be something like Street Lighting where they use lamps that are all wired in series. They have a specialized "constant current transformer" feeding the lamp circuit. The lamps last longest if the current through them remains as constant as possible, so as voltage varies with the utility supply and the age of the lamps, the transformer changes the primary connection to maintain a constant current on the secondary.

 

[G._S._Ohm]

An incandescent lamp maintains a more or less constant current through it over a small range of supply voltages.

And a fluorescent ballast changes the 'voltage source' outlet voltage to a current source to supply the tube.

 

[wiigelec]

A 4-20ma temp. transducer would be a good example.

This type if device is not what I would consider a "constant current" device but in fact quite the opposite. An RTD (resistive temperature) works by changing resistance value based on temperature. Impress upon this resistance a known voltage and you will get a current proportional to the resistance and therefore the temperature. It is the changing current that makes this device useful.

 

[wiigelec]

Can someone please explain to me what a constant current load is, and what types of loads are considered "Constant Current"?

I am not to familiar with the details (yet) but constant voltage, impedance, and current loads are ways of modeling in order to analyze distribution networks and simulate faults. I am not sure if constant current has a "real world" equivalent...

 

[gar]

110315-2241 EST

wiigelec:

If you were referring to steve66's description, then you fail to understand how a process current loop works.

A resistive temperature sensor might be used in a 4-20 MA current loop sensor, but it is not its resistance directly in the current loop that determines the current, but rather its resistance value is processed to to produce a constant current output for a particular temperature.

The constant current device that is at the output of the 4-20 MA transducer has some finite voltage and current range over which it can provide the constant current output. Usually the transducer does not contain the voltage source that drives the current around the circuit. Rather the transducer is simply an electronic resistance that adjusts its resistance value as necessary to maintain the required loop current.

.

 

[wiigelec]

A resistive temperature sensor might be used in a 4-20 MA current loop sensor, but it is not its resistance directly in the current loop that determines the current, but rather its resistance value is processed to to produce a constant current output for a particular temperature.

Right, I forgot about the transmitter part, or the "hockey puck" as the technical term:grin:

 

[broadgage]

There are very few "real world" constant current loads that might be connected to a utility or similar supply.
The series street light lighting example given above is not much used these days, but anyway, it does NOT draw a constant current from the utility. It supplies a constant current to the lamps, regardless of changes in lamp volts or utility voltage. This means that the current drawn from the utility mains is variable.

Flourescent lamp ballasts "try" to supply a constant current to the lamps, but the supply current from the utility will vary according to line voltage and lamp type.

In years gone by, constant current loads were a bit more common.
For example an arc lamp might contain a manual or automatic adjustment of the series resistance in order to keep the arc current, and therefore the supply current, constant over a range of line voltages.

 

[iwire]

I am fairly certain FAA regulations require airport runway lighting to use constant current lighting circuits or perhaps it is just the most economical way of providing landing lights over such large areas.




http://ecmweb.com/mag/electric_series_lighting_circuitry/

 

[steve66]

I am not to familiar with the details (yet) but constant voltage, impedance, and current loads are ways of modeling in order to analyze distribution networks and simulate faults. I am not sure if constant current has a "real world" equivalent...

A real world equivalent of a constant current source is anything that has a very high impedence compared to the load impedance.

For DC circuits, we can simplify this and say a constant current source has a very high resistance compared to the load resistance.

Consider a 100V source with an internal resistance of 100 K. If you short the output, you get 1 mA of current flowing through the circuit. If your load is 100 ohms, you still get about 1 mA. If the load is 1000 ohms, still pretty close to 1 mA.

Steve

 

[gar]

110316-0822 EST

iwire:

Clearly a series circuit has great advantage in an airport type application for wiring over long distances with individual loads that are low voltage, 10 to 120 V would be low voltage. Also the lower the lamp voltage the more rugged is the filament.

Under on-off conditions lamp life should be increased because of the reduced filament stress on turn on with a constant current source.

However, a big disadvantage of a series string of lights is that if one burns out, then all in that string go out. Very little was said in the article on this subject. However, that does not happen in the airport application according to the article, but no explanation of how this is accomplished. Quite obviously this means the individual fixtures have built in sensors that detect lamp burn out and then short across the failed lamp when such failure occurs.


broadgage:

There are very few "real world" constant current loads that might be connected to a utility or similar supply.
The series street light lighting example given above is not much used these days, but anyway, it does NOT draw a constant current from the utility. It supplies a constant current to the lamps, regardless of changes in lamp volts or utility voltage. This means that the current drawn from the utility mains is variable.

If you use something like a Variac to maintain a constant current to a lamp load, then it is also a constant voltage source to the lamp under constant lamp conditions, and also a constant load power on the system. Thus, as the mains input voltage goes up the mains current goes down.

On the other hand if a large source voltage is used and an appropriate series inductor is used for current limiting, then the mains see an an approximately constant current load.

Fluorescent lamp ballasts "try" to supply a constant current to the lamps, but the supply current from the utility will vary according to line voltage and lamp type.

Following are measurements on two magnetic ballast fluorescent fixtures.

One 8' fixture
140 V 0.92 A 129 W
130 V 0.92 A 124 W
120 V 0.95 A 119 W
110 V 1.02 A 114 W
100 V 1.09 A 111 W
The current is considerably more constant than the voltage and thus approximates a constant current load.

Different 8' fixture
140 V 0.95 A 112 W
130 V 0.86 A 104 W
120 V 0.83 A 99 W
110 V 0.89 A 96 W
100 V 0.92 A 92 W
Again relatively constant.

A cheap electronic ballast unit, 4' instead of 8'.
140 V 0.83 A 79 W
130 V 0.81 A 73 W
120 V 0.72 A 66 W
110 V 0.62 A 58 W
100 V fails to light one bulb

A good electronic ballast should be able to provide constant light output, constant current thru the bulbs, and be a constant power load on the mains.

.

 

[wiigelec]

Read this:

http://www.manchesteruniversitypress.co.uk/uploads/docs/370384.pdf

"Constant impedance load model (constant Z). A static load model where the power varies with the square of the voltage magnitude. It is also referred to as constant admittance load model.

Constant current load model (constant I). A static load model where the power varies directly with voltage magnitude.

Constant power load model (constant P). A static load model where the power does not vary with changes in voltage magnitude. It is also known as constant MVA load model."

 

[iwire]

110316-0822 EST

iwire:

Clearly a series circuit has great advantage in an airport type application for wiring over long distances with individual loads that are low voltage, 10 to 120 V would be low voltage. Also the lower the lamp voltage the more rugged is the filament.

I understand that, my point was I am not sure if the FAA requires that type of circuit or if it is just used economic reasons.

One benefit is that the lighting will be all of the same brilliance anywhere on the circuit which might be important.

However, a big disadvantage of a series string of lights is that if one burns out, then all in that string go out. Very little was said in the article on this subject. However, that does not happen in the airport application according to the article, but no explanation of how this is accomplished. Quite obviously this means the individual fixtures have built in sensors that detect lamp burn out and then short across the failed lamp when such failure occurs.

If you go to the Crouse Hinds web site http://www.chalp.com/ I think you can find the fixtures and they do have an automatic shunt, I think some may even have two lamps so it switches over to the spare.

 

[Open Neutral]

110316-0822 EST

However, a big disadvantage of a series string of lights is that if one burns out, then all in that string go out. Very little was said in the article on this subject. However, that does not happen in the airport application according to the article, but no explanation of how this is accomplished. Quite obviously this means the individual fixtures have built in sensors that detect lamp burn out and then short across the failed lamp when such failure occurs.
.

When a lamp opens, the full open circuit voltage appears across it. It will have a reverse-fuse, a gap that arcs over and seals shorted. The same trick is used on Transatlantic Cable repeaters. There, +2500V is fed in from say the French/UK end, and -2500 on ours. Each repeater drops a few volts, and if they fail, they open...but not for long. {Remember, the original TAT's used vacuum tube repeaters..}

 

[tkb]

I am fairly certain FAA regulations require airport runway lighting to use constant current lighting circuits or perhaps it is just the most economical way of providing landing lights over such large areas.

http://ecmweb.com/mag/electric_series_lighting_circuitry/

I did work at the Worcester airport and they had constant current street lighting.

It had one 5kv unshielded wire running pole to pole in series.
I had to repair a failed wire underground.
It was buried in sand and where the wire failed, the sand turned to glass. It was surrounding the wire in about a 2"-3" diameter.

The system had large pole mount type transformers installed in the basement of the terminal behind a chain link fence.

I have never seen anything like it before and since.

 

[Electric-Light]

I did work at the Worcester airport and they had constant current street lighting.

It had one 5kv unshielded wire running pole to pole in series.
I had to repair a failed wire underground.
It was buried in sand and where the wire failed, the sand turned to glass. It was surrounding the wire in about a 2"-3" diameter.

The system had large pole mount type transformers installed in the basement of the terminal behind a chain link fence.

I have never seen anything like it before and since.

Seems extremely dangerous to me. If you remove a lamp, that's like a current transformer with the shunt removed. Imagine 5kV at the lamp socket

 

[LarryFine]

... +2500V is fed in from say the French/UK end, and -2500 on ours.

They'd have to use the earth to complete the circuit, a la SWER.

 

[Open Neutral]

They'd have to use the earth to complete the circuit, a la SWER.

There's a good ground available at each end.....
And SWER works on non-trivial runs such as the Pacific Intertie.

 

[dbuckley]

Airstrip lighting is constant current, the current is always 6.6A. The voltage across the series connected lamps is automatically adjusted (up to 5KV either side of ground) to maintain the current. Yep, 6.6A x 10KV is 66KW, there are airstrips that use a significant fraction of that power for lighting.

Many older systems literally have the lamps in series, with a 'thing' across the lamp that ruptures if the lamp fails to maintain continuity of the loop. More modern systems have a transformer for every lamp position, so even if the bulb fails, the transformer is still in circuit. And with the adoption of LEDs, the consumption is going down.

The reason that series lamps are usaed is so that all the lamps of the same wattage are of the same brightness. If you parallel fed the lamps the voltage drop would mean that lamps further away from the source would be dimmer, and the conductor sizes would get wild. Runways can be long. You also need more conductors for parallel, so twice as much copper. So the series system is much more economical.

Time was when streetlamps were done the same way, and I think the last place on the planet that still had series streetlamps was in LA, and its now gone as of a couple of years back.