Constant Voltage and Constant Current

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Dennis Alwon

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I have noticed that some LED drivers are marked constant current and/or constant voltage. These are DC output so what is actually meant by these terms. Is it constant voltage because it is DC and there is no sine wave or is it something else. Also what do they mean by the constant current
 

GoldDigger

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Underlying everything is the design of the DC supply to have a regulated output.
What is regulated can be the voltage or the current.
If the voltage is regulated, then for any load resistance (linear or not) within the allowed range the voltage on the supply output terminals will be constant. This is not ideal for a load which has a negative resistance characteristic or even a relatively steep current versus voltage characteristic.
A constant current supply, OTOH, will try to deliver the specified current into the load, varying the voltage as necessary to achieve that.
Since bare LED is a near constant voltage device it is best driven by a constant current source set to the correct current.
If the LED module itself contains a ballast resistor or a current regulator then it can be fed from a constant voltage supply.
 

Dennis Alwon

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If I have a transformer that supplies 12 v then how is that not constant. Are you saying that the voltage may fluctuate???? Why would you want that..
 

gar

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Dennis:

I don't know what is being implied in your case.

A constant voltage supply, whether DC or AC, is ideally a source where the output voltage is constant independent of load. This means zero equivalent internal impedance. Never really achieved, but with electronic circuit control can be very good. It may be possible to build a 10 V DC 10 A supply where the voltage changes less than 0.001 V from zero load to 10 A load. This is a voltage drop of 0.01%.

For a residence a 5% change might be considered constant voltage.

A Sola constant voltage transformer I have is rated 95 to 130 V about 100 VA (1960 nominal voltage was somewhat lower than today) has the following results:

Load change --- With 115 V input the output changes from 123.3 V at 0 load to 120.5 V with 100 W load. These are not designed for load regulation.

Constant 100 W load and regulation relative to input voltage ---
095 121.1
105 120.9 diff 0.2
110 120.8 diff 0.1
115 120.5 diff 0.3
120 120.2 diff 0.3
125 119.8 diff 0.4
130 119.4 diff 0.4
135 118.8 diff 0.6
In the mid area the regulation is about 0.25 % per 5 V change. Over the range 95 to 130 it is about (1.7/120)*100 = 1.4 % per 35 V change.


A constant current source is one where the current is constant independent of load. Quite obviously there is a physical limit here. We can not go to a large resistance load without the voltage becoming excessive. For example 1 A into a load of 1 megohm. But, within reason, we can do pretty good with electronic control.

Consider an LED diode itself. The voltage drop across the diode in the forward direction may be in the range of one to several volts, and for a given current will vary with temperature. Brightness is a function of current. Suppose we applied a constant voltage to this device, then its brightness would be a function of temperature, and interchanging different LEDs, because of manufacturing differences, might cause differences in brightness. Thus, the LED chips themselves must be supplied by a moderately constant current source. If you have a 12 V source and one LED, then a series dropping resistor is usually sufficient to provide an approximately constant current. But, in high power applications this is power wasting. Electronic circuitry is used to solve this problem.

I have DC power supplies that are both constant voltage and constant current sources, but not simultaneously. Generally these are constant voltage supplies with current limiting above some current level. Some are fold back on the current limiting.

The manual on your power supply may tell us what they mean by constant voltage/constant current, or experiments on the supply may be needed.

.
 

iwire

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If I have a transformer that supplies 12 v then how is that not constant. Are you saying that the voltage may fluctuate???? Why would you want that..

A standard transformer supplies a constant voltage.

On the other hand a typical ballast or constant current power supply varies the output voltage to maintain a constant current output.
 

Dennis Alwon

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Okay thanks all. I think I am seeing what is happening. It seems that a constant current unit has to vary the voltage to obtain that setting. I was assuming that a constant voltage/constant current unit did so simultaneously but as Gar stated that is not the case-- at least with his unit and I suspect they cannot be both simultaneously.

Part of my confusion was I had not noticed tranies that stated constant voltage or constant current- not the ones we have used anyway. I appreciate the time to explain it to me.
 

GoldDigger

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...
...

Part of my confusion was I had not noticed tranies that stated constant voltage or constant current- not the ones we have used anyway. I appreciate the time to explain it to me.
Not unreasonable, given that transformers are usually passive linear components to a good approximation, while even the simplest DC supply is likely these days to include active circuitry instead of just diodes and capacitors.
:)
 

Carultch

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Okay thanks all. I think I am seeing what is happening. It seems that a constant current unit has to vary the voltage to obtain that setting. I was assuming that a constant voltage/constant current unit did so simultaneously but as Gar stated that is not the case-- at least with his unit and I suspect they cannot be both simultaneously.

Part of my confusion was I had not noticed tranies that stated constant voltage or constant current- not the ones we have used anyway. I appreciate the time to explain it to me.

A constant voltage/constant current specification cannot simultaneously exist on a DC power supply, because it is impossible to force all loads to have an identical relationship between voltage and current.

A constant voltage power supply will supply a fixed voltage (assuming it isn't overloaded), and provide a current that can be sustained within the load at that voltage.
A constant current power supply will supply a voltage which can adjust as necessary, in order to provide the specified current to the load.

For a linear Ohmic resistive load, it doesn't make much difference which one of these you use. Because we all know Ohm's law will set the relationship between voltage and current. The fixed voltage power supply is the most likely one you will use. It will supply a fixed voltage, and the resistive load will respond by drawing the corresponding current.

For a load like an LED, it doesn't obey Ohm's law, and the current-to-voltage function is a lot more interesting. In particular, it is very steep. Usually resembling an exponential function. Once it reaches its turn-on voltage, the change in current is extremely sensitive to small increases in voltage.
http://www.electronics-tutorials.ws/diode/diode12.gif?81223b
 

PetrosA

Senior Member
The reason it's important for LEDs is that the diode itself acts as a current sink - IOW, it will keep getting brighter and brighter the more current you feed it (unlike an incandescent light bulb which will burn the same whether you have it hooked up to a 10A circuit or a 100A circuit). Depending on the circuit topology, some LEDs need to have that current limited by the transformer, usually the DC LED strips are like that. AC LED strips will have resistors on the tape limiting current to small groups of diodes.
 

gar

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160115-2126 EST

PetrosA:

Your statement
The reason it's important for LEDs is that the diode itself acts as a current sink - IOW, it will keep getting brighter and brighter the more current you feed it (unlike an incandescent light bulb which will burn the same whether you have it hooked up to a 10A circuit or a 100A circuit).
is not correct.

The problem is that by saying "a 10A circuit" in this sentence you do not really mean a constant current 10 A circuit, but I believe you are referring to an approximately constant voltage circuit that is rated for up to a 10 A load, and that rating may not not even limit at 10 A. And you are comparing this with another circuit, your 100 A, that has approximately the same constant voltage value as the 10 A circuit. For this discussion you have to define these as constant voltage sources.

If you did have two current sources, one 10 A and the other 100 A, then if the bulb being tested at 100 A did not burn out, then when fed 10 A it would burn much dimmer.

If I take an LM317 voltage regulator and wire it as a constant current supply to produce 10.09 mA, then from an applied voltage change from 3 V to 25 V DC the output current reading did not deviate from the 10.09 mA. Why did I use 10.09? Because that is what the particular current setpoint resistor produced.

.
 

PetrosA

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160115-2126 EST

PetrosA:

Your statement is not correct.


.

I know it's not technically accurate, but I think it gets to the heart of the confusion for electricians who think of current as a rating in a circuit. LEDs are a totally different beast from an incandescent light bulb which you can't overload with current because it's not a current sink.

Another way that might help non-electronics types understand the difference is thinking about replacing your 18V cordless drill battery with two 9V batteries in series. Both batteries are supplying 18V, but the tool battery has a lot more current available and can run the tool faster than the two 9V batteries in series can. Conversely, if you were to test some 12V LEDs using the same two battery setups, you would probably be okay using the two 9V batteries together, but might burn out the LEDs using the tool battery pack.
 

gar

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PetrosA:

I have to continue to disagree.

The current rating of a circuit does not determine how much current flows. The intent of Dennis Alwon's original post was to learn about the relationship of the terms "constant voltage" and "constant current".

Both LEDs and incandescent bulbs can be considered as current sinks (they both consume power).

Another way that might help non-electronics types understand the difference is thinking about replacing your 18V cordless drill battery with two 9V batteries in series.
What is the difference? They both constitute an 18 V voltage source.

Both batteries are supplying 18V, but the tool battery has a lot more current available and can run the tool faster than the two 9V batteries in series can.
If the motor is essentialy a permanent magnet or shunt wound field type, then the source voltage defines the motor speed. Available current is determined by the voltage source internal impedance. Other than voltage you have not defined the battery characteristics. Why does the tool battery have more current available?

Conversely, if you were to test some 12V LEDs using the same two battery setups, you would probably be okay using the two 9V batteries together, but might burn out the LEDs using the tool battery pack.
This makes no sense assuming that you were connecting the 9 V batteries in series as above.

.
 

GoldDigger

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160115-2434 EST

PetrosA:

I have to continue to disagree.

The current rating of a circuit does not determine how much current flows. The intent of Dennis Alwon's original post was to learn about the relationship of the terms "constant voltage" and "constant current".

Both LEDs and incandescent bulbs can be considered as current sinks (they both consume power).

What is the difference? They both constitute an 18 V voltage source.

If the motor is essentialy a permanent magnet or shunt wound field type, then the source voltage defines the motor speed. Available current is determined by the voltage source internal impedance. Other than voltage you have not defined the battery characteristics. Why does the tool battery have more current available?

This makes no sense assuming that you were connecting the 9 V batteries in series as above.

.
I suspect that he is talking about the common small 9V batteries used in electronics. The maximum current they can supply is relatively limited compared to the tool battery.
 

gar

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GoldDigger:

I agree that PetrosA probably is talking about a 9 V battery such as an Eveready A522 6LR61, but he has not defined the 9 V battery.

This thread was about constant voltage and constant current and PetrosA's discussion added nothing but noise to the discussion. His argument needs to be based on basic electrical theory, and he needs to do comparisons of comparable items. And the items being discussed need to be defined. When the discussion is about constant voltage, then you do not use the comparison of an 18 V low internal impedance battery with the series connection of two high internal impedance 9 V batteries, unless the purpose of the comparison is to show the effect of battery internal impedance.

Those that are not familiar with basic electrical circuit theory need a correct education not some mixed up mess that creates total confusion.

A constant voltage source is one with an ideal voltage source in series with a low internal series impedance. A constant current source is an ideal current source shunted with a very high internal impedance.

At the terminals of such sources the constantness of the source is dependent upon on the ratio of the load impedance to the internal impedance of the source.

.
 

jaggedben

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... This thread was about constant voltage and constant current and PetrosA's discussion added nothing but noise to the discussion. ...
.

I daresay that's equally true of your posts. Petros gave a real world example that I think was intelligible to just about anyone.

Why does the tool battery have more current available?

For the non-engineer, it's enough to just observe that it does. Anyone tinkering will find out that at a higher current the voltage doesn't hold up with two 9V Energizers as it does with Milwaukee Fuel. I think that was the point. But thanks anyway for explaining the theory behind it. :D
 

gar

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jaggedben:

If you don't understand my posts, then ask for clarification.

A 9 V battery defines nothing except that its nominal open circuit voltage is about 9 V.

The subject of the thread was constant voltage and constant currernt sources, not the internal impedances of grossly different types and sizes of batteries.

How does the discussion of a cordless drill powered by two different undefined batteries have much to do with the subject of this thread?

Do you understand what is the meaning of a constant current source? And of a constant voltage source?

.
 

mivey

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Gar,

I agree with your erudite response to Peter. Unfortunately, it might be to much for some who would prefer a much simpler version, even if the simpler version is incorrect. Such is life.
 

iwire

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Gar,

I agree with your erudite response to Peter. Unfortunately, it might be to much for some who would prefer a much simpler version, even if the simpler version is incorrect. Such is life.

If the message cannot be understood it is of no more use than a partially correct one.
 

jaggedben

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jaggedben:

If you don't understand my posts, then ask for clarification.
.

I believe I understand the entirety of your posts just fine. Also Petros' posts. I found his to be actually helpful in knowing what to look for if specifying power supplies for LEDs.
 
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