Can someone explain to me how a 'constant current' source differs from PWM regulation? I understand that in flashlights PWM just generates pulses which average out to some specific brightness because our eyes are slow, thus the "digital" marketing scheme, it's all on or all off. I've been doing some research in another forum and I realized I don't know how a constant current regulator works. Does it operate a transistor (or two) in linear fashion like an amplifier? If so then the efficiency can't be too astounding because the transistor will dissipate heat.
Constant current source question
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MrAl
Flashlight Enthusiast
Hello there,
A constant current source puts out a current that is always the same.
If it is set for 300ma then it puts out 300ma always.
PWM regulation is when pulses are used to push current through a
device such as an LED where you want to regulate the current (or
voltage). If it is regulating the current, then it is also a constant
current source as the average value stays constant. Although there
is pulsing, many times the current is smoothed out by an inductor
anyway.
The transistor used for PWM does not operate in linear mode. If the
design is working properly, it is either fully on or fully off. This is how
many switchers get high efficiency. PWM is not, however, a switching
power supply. PWM is simply turning on and off the power getting to
a device, but while a switching power supply does the same it also
smooths out the current using an energy storage device like an inductor.
In practice however, the transistor is not perfect and still uses some power
but not nearly as much as a linear design in most cases.
A constant current source puts out a current that is always the same.
If it is set for 300ma then it puts out 300ma always.
PWM regulation is when pulses are used to push current through a
device such as an LED where you want to regulate the current (or
voltage). If it is regulating the current, then it is also a constant
current source as the average value stays constant. Although there
is pulsing, many times the current is smoothed out by an inductor
anyway.
The transistor used for PWM does not operate in linear mode. If the
design is working properly, it is either fully on or fully off. This is how
many switchers get high efficiency. PWM is not, however, a switching
power supply. PWM is simply turning on and off the power getting to
a device, but while a switching power supply does the same it also
smooths out the current using an energy storage device like an inductor.
In practice however, the transistor is not perfect and still uses some power
but not nearly as much as a linear design in most cases.
Last edited:
Mr Happy
Flashlight Enthusiast
Yes indeed, this is true. The simplest constant current source uses just a single transistor, but more accurate and sophisticated circuits use an IC voltage regulator. Since the transistor or voltage regulator has to drop the excess voltage to limit the current to the desired value it does dissipate heat, and depending on the values of current and voltage drop a heat sink may be required. Constant current sources like this are often used an battery charging applications where the poor efficiency doesn't matter so much.
