# PWM vs Constant Current



## D-Dog (Dec 23, 2008)

As I understand it, PWM seems to be better at everything... It has these advantages, just to name a few...

*Better runtime (less energy lost as heat at lower levels)
*Can achieve lower "low"
*Can have an almost infinite number of output levels

Why the heck would you ever want a light with constant current regulation? I have heard the electronics needed are cheaper... but is that it? Especially with the PWM frequencies being high enough now so that you can't even notice any flickering.


----------



## Yoda4561 (Dec 23, 2008)

While that may seem true on paper, real world tests have shown that the difference is usually quite negligible, and sometimes actually favors constant current. I believe pwm is the only choice for super low ouput though. There was a big discussion here with graphs and everything, and I recall that being the general conclusion.

edit: heres a few links

https://www.candlepowerforums.com/threads/186520

https://www.candlepowerforums.com/threads/179765


----------



## D-Dog (Dec 23, 2008)

Just found what you may be referring to:

http://www.molalla.net/~leeper/pwm_cc.htm


----------



## Kiessling (Dec 23, 2008)

This was an extensive topic here some years back. Basically, the conclusions were, IIRC:

- PWM is less efficient than constant current dimming at lower light levels and about as good at higher levels

- PWM has no color shift but constant current has

- PWM can flicker, constant current not

- PWM uses a different driver, giving you different options and restrictions for you UI

Now ... usually constant current can achieve a lower low as with a lot of PWM drivers you see flickering then, think Eternalight or Photon Freedom. The Titan has the lowest low ever, and it is current regulated.
Also, both can have virtually an infinite number of levels, not that this was needed anyway. 

So ... one is not better than the others in general, both have their merits and flaws. It is a matter of choice and good R+D which to use in your light.

bernie


----------



## Kiessling (Dec 23, 2008)

D-Dog said:


> Just found what you may be referring to:
> 
> http://www.molalla.net/~leeper/pwm_cc.htm




Yes, that is a summary by Newbie which covers it quite well. Good link.


----------



## Mr Happy (Dec 23, 2008)

PWM has all sorts of disadvantages, including more complex and expensive circuitry, more difficulty achieving regulation, and no boost capability. In practice, a standard buck or boost circuit is a better engineering solution in almost every way.


----------



## SemiMan (Dec 27, 2008)

Mr Happy said:


> PWM has all sorts of disadvantages, including more complex and expensive circuitry, more difficulty achieving regulation, and no boost capability. In practice, a standard buck or boost circuit is a better engineering solution in almost every way.



??? This is not accurate. PWM and buck/boost, linear, etc. are not mutually exclusive concepts. PWM is not necessarily more complex or even that much more expensive. In fact it can be quite simple.

Newbie's detailed analysis pretty much sums it up.

For those without as much electronics experience, you should know that PWM is a techniqute that is used for DC\DC conversion (buck, boost, etc.) and should not be confused with PWM for brightness control which turns the light on and off very fast essentially. Often these two techniques will be combined.

For a flashlight, perfect color control is rarely needed, so constant current dimming (likely with a switch-mode power supply) will be the most efficient down to a point ... you can see it in the graph where the efficiency of the LED peaks. It is pointless to reduce the current much below this. If you want dimmer, at this point, you are better off to start PWMing. This will give you the best overall run-time. 

Like any design, there are tradeoffs to be made depending on operating current, input voltage, etc. that will determine the absolute best circuit under any circumstance.

Semiman


----------



## SemiMan (Dec 27, 2008)

Kiessling said:


> - PWM can flicker, constant current not



Should add that this is a matter of the PWM implementation. With low low on PWM, the on-time is quite low. Depending on how you are controlling current (if at all), then it may not be able to respond as fast hence the flicker. Straight PWM modulation is the worst at this. There are other methods, pulse density, etc. that offer much better "lows" while still working with the same concept.

Semiman


----------



## 2xTrinity (Dec 27, 2008)

D-Dog said:


> As I understand it, PWM seems to be better at everything... It has these advantages, just to name a few...
> 
> *Better runtime (less energy lost as heat at lower levels)
> *Can achieve lower "low"
> *Can have an almost infinite number of output levels




Constant current is more efficient in all but _extremely_ low brightness settings
(eg <1% of max rated output).
one could argue that low efficiency isn't a problem at low levels if it means the difference between 2 week runtime and 3 week runtime on a flashlight -- both are effectively "unlimited"

Both methods can have arbitrarily low "low" levels.
Both methods can be infinitely variable.



> Why the heck would you ever want a light with constant current regulation? I have heard the electronics needed are cheaper... but is that it? Especially with the PWM frequencies being high enough now so that you can't even notice any flickering.


The only difference between a high frequency PWM filter, and a switching mode constant current regulator is a simple filter that "smooths out" the PWM pulses into a constant current. Oftentimes very small lights, such as AAA sized lights (ie the LF2x) will omit this filter due to size constraints, but the cost difference (from manufacturing perspective) is pretty negligible.


----------



## Mr Happy (Dec 27, 2008)

SemiMan said:


> ??? This is not accurate. PWM and buck/boost, linear, etc. are not mutually exclusive concepts. PWM is not necessarily more complex or even that much more expensive. In fact it can be quite simple.


Well, OK, but what about regulation? With a standard buck or boost circuit, a regulated output voltage can be achieved quite simply with just a few off-the-shelf components. But if you attempt voltage regulation with just a plain PWM controller, how do you obtain the feedback signal for accurate control? There is a thread here involving a lot of complex design with a programmable controller. Is there a simpler way? (Just asking, since a PWM regulator without the extra L and C of a buck circuit would presumably be even simpler, yet they don't seem to be common in lights.)


----------



## Resqueline (Dec 28, 2008)

You can control current with just a resistor, or with a linear constant current circuit (adjustable or fixed). Both methods wastes power.
Then you have efficient switchmode drivers (buck/ boost etc.) that may use PWM (or other schemes) for their regulation. They should also provide a more or less constant current to the LED, pretty much regardless of battery voltage. These may also be adjustable or fixed.
Then you may put a PWM circuit in front (or back) of any of these circuits if you want less light than the LED's efficiency peak dictates you can get.
This last kind of PWM regulation has no L-C output filter, and is usually "dumb/blind" in that it does not sense it's own output. Regulation is simply "commanded" %wise (like the throttle of a car). If there's a need for sensing then the feedback can simply be filtered (R-C) before being used by the PWM regulating circuit.


----------



## SemiMan (Dec 28, 2008)

Mr Happy said:


> Well, OK, but what about regulation? With a standard buck or boost circuit, a regulated output voltage can be achieved quite simply with just a few off-the-shelf components. But if you attempt voltage regulation with just a plain PWM controller, how do you obtain the feedback signal for accurate control? There is a thread here involving a lot of complex design with a programmable controller. Is there a simpler way? (Just asking, since a PWM regulator without the extra L and C of a buck circuit would presumably be even simpler, yet they don't seem to be common in lights.)



Mr. Happy, you are confusing the concepts of PWM for brightness control with current regulation (as has been done a few times in this thread).

PWM for brightness control is simply turning off the LED rapidly (generally greater than a few hundred Hz) and varying the on-time in order to control the brightness. All that takes is a 555 timer or a 40 cent micro.

That PWM circuit may simple drive a FET that is direct driving an LED via a resistor from the battery, or it may be turning a constant current source on and off. That constant current source could be linear (generally inefficient but simple) or it could be driving some sort of switch mode regulator (buck, boost, buck-boost, sepic, etc.).

The issues with flicker with PWM circuits, which is very real, is that the constant current regulator that is often after the PWM has a limited response speed. When you try to get really low output from a PWM circuit, that requires very short on times .... which could be too fast for the current control circuit to respond to properly. As a way of getting around this, people use methods other than simple PWM which is a fixed off/on time and use forms of pulse density control, etc. that do not have fixed off/on times. 

Past the efficiency peak for an LED, in general, varying the current to the LED is the most efficient form of dimming. Once you drop the current down to the efficiency peak, then using PWM to achieve any further reduction in brightness would be the most efficient.

To clarify, you do not need any feedback around the PWM for brightness control. The feedback will be in the current regulation.

Not sure what was meant by "The only difference between a high frequency PWM filter, and a switching mode constant current regulator is a simple filter that "smooths out" the PWM pulses into a constant current. Oftentimes very small lights, such as AAA sized lights (ie the LF2x) will omit this filter due to size constraints, but the cost difference (from manufacturing perspective) is pretty negligible.".... since the LF2x has a full and proper buck-boost switch-mode....that is the only way to get from 0.9V in to the voltage required by the LED. There is more to a switch-mode power supply than a simple "filter" in many cases. For a simple buck regulator there is some truth to this statement, but not for other topologies where you are charging an inductor and using switches to essentially move the inductor to another part of the circuit and transfer the energy and generally change the voltage level.

Semiman


----------



## mpf (Dec 30, 2008)

This thread https://www.candlepowerforums.com/threads/201383
has a very simple and fully explained multi level current control using pwm (well variable pulses anyway) with full construction details and programming. The tutorial is designed for people just starting out in uC Led control and does not use any SMD devices which greatly simplifies the construction.

It is an example of a "constant current regulator is a simple filter that "smooths out" the PWM pulses into a constant current."

However the constant current setpoint is variable to give you varing brightness.  

matthew


----------



## SemiMan (Dec 30, 2008)

mpf said:


> This thread https://www.candlepowerforums.com/threads/201383
> has a very simple and fully explained multi level current control using pwm (well variable pulses anyway) with full construction details and programming. The tutorial is designed for people just starting out in uC Led control and does not use any SMD devices which greatly simplifies the construction.
> 
> It is an example of a "constant current regulator is a simple filter that "smooths out" the PWM pulses into a constant current."
> ...




This is not really a PWM brightness controller and it is not a switchmode controller either. Neither does it use filtering to smooth out the PWM to the LED. It is a linear constant current regulator that uses digital feedback to regulate the current. What it does use filtering for is to smooth out the pulses to the gate of the FET so that they FET does not turn off and on rapidly (which is what would happen in a classic PWM), but stays at a somewhat constant value.

Semiman


----------



## 2xTrinity (Dec 30, 2008)

SemiMan said:


> This is not really a PWM brightness controller and it is not a switchmode controller either. Neither does it use filtering to smooth out the PWM to the LED. It is a linear constant current regulator that uses digital feedback to regulate the current. What it does use filtering for is to smooth out the pulses to the gate of the FET so that they FET does not turn off and on rapidly (which is what would happen in a classic PWM), but stays at a somewhat constant value.
> 
> Semiman


I would simply call that "PWM-controlled-linear regulator" a digitally controlled linear-regulator:

The "PWM" is actually just the digital output signal from a microcontroller. The filter is used for digital-to-analog conversion. The analog signal is used to bias the gate of the MOSFET in its "linear" operating region. That is, the MOSFET is being biased to act like a variable resistor, and not like an on-off switch.

IMO mentioning PWM at all for that device only adds confusion -- as PWM usually implies the LOAD is being pulsed off an on.


----------



## mpf (Dec 31, 2008)

mpf said:


> It is an example of a "constant current regulator is a simple filter that "smooths out" the PWM pulses into a constant current."



Actually I think this is a fairly accurate description of the circuit. If you remove the FET gate filter capacitor you will get PWM pulses of current in the led. The 'average' current will be about right but without the filter capacitor the current in the LED will be very pulsey.

I started with just PWM, using TaskLed's D2Dim controller, but did not like the over current supplied to the leds from fully charged batteries. So I fed the PWM into a constant current circuit. As mentioned below you need a fast response constant current circuit for this to work. (see http://www.forward.com.au/20WTorch/20WTorch.html) 

I prefer the approach I am using now as illustrated by the tutorial. Few components, easy to program, high efficiency (see above link), good control from 100% to 1.5% of current and scaleable to high wattage, 60W plus, torches.

As stated above this is basically a 'constant current' regulator that uses PWM as part of its control, so I am definately in the 'constant current' camp.

matthew


----------



## 2xTrinity (Dec 31, 2008)

mpf said:


> Actually I think this is a fairly accurate description of the circuit. If you remove the FET gate filter capacitor you will get PWM pulses of current in the led. The 'average' current will be about right but without the filter capacitor the current in the LED will be very pulsey
> 
> I started with just PWM, using TaskLed's D2Dim controller, but did not like the over current supplied to the leds from fully charged batteries. So I fed the PWM into a constant current circuit. As mentioned below you need a fast response constant current circuit for this to work. (see http://www.forward.com.au/20WTorch/20WTorch.html)
> 
> ...


I understand. PWM is great for loads that are approximately linear, or loads like incandescent lamps where the thermal intertia of the filament acts like a "filter" of sorts (why "dimmed" incan lamps don't flicker but simply settle at a lower steady-state termperature when driven with PWM).

With LEDs however, as you acknowledge from your own experience, depending on the voltage of the battery, when the MOSFET is fully "on", it could be possible for a true PWM to heavily overdrive or even destroy the LED, whereas a linear regulator (which will introduce resistance in series) would keep the current limited to some acceptable range. As LED current is strongly non-linear, 50% PWM duty cycle may not ncessarily coincide with operating a MOSFET at some intermediate bias point.



> Well, OK, but what about regulation? With a standard buck or boost circuit, a regulated output voltage can be achieved quite simply with just a few off-the-shelf components.



I recall from the discussion of trying to make a regulated incan driver that the feedback in that case is not trivial to figure out. Because an unfiltered PWM output to the lamp is actually desired, the driver must sample the PWM output, then perform a calculation to determine the RMS voltage of the pulse wave, rather than simply feeding back the (constant) output voltage to an opamp or something.

A pure PWM driver like this would not be suitable at all for LEDs. It counts on the fact that the incan takes a fairly long time to "warm up" (significantly longer than the pulse period). This way, you can (for example) run a 6V lamp from a 12V battery, by supplying it with a 25% duty cycle PWM input, as long as frequency is a couple hundred hertz or higher. Supply an LED with double its rated voltage for even a microsecond and it's toast. 

So to answer the question to how to "regulate" a non-constant-current PWM driver, as far as I know nobody has attempted it with LEDs. Most LED drivers only use PWM to vary the brightness, and they simply toggle a regulator off and on. The PWM itself is only a tool for dimming, thus feedback is not necssary.


----------



## Mr Happy (Dec 31, 2008)

2xTrinity said:


> I recall from the discussion of trying to make a regulated incan driver that the feedback in that case is not trivial to figure out. Because an unfiltered PWM output to the lamp is actually desired, the driver must sample the PWM output, then perform a calculation to determine the RMS voltage of the pulse wave, rather than simply feeding back the (constant) output voltage to an opamp or something.
> 
> A pure PWM driver like this would not be suitable at all for LEDs. It counts on the fact that the incan takes a fairly long time to "warm up" (significantly longer than the pulse period). This way, you can (for example) run a 6V lamp from a 12V battery, by supplying it with a 25% duty cycle PWM input, as long as frequency is a couple hundred hertz or higher. Supply an LED with double its rated voltage for even a microsecond and it's toast.
> 
> So to answer the question to how to "regulate" a non-constant-current PWM driver, as far as I know nobody has attempted it with LEDs. Most LED drivers only use PWM to vary the brightness, and they simply toggle a regulator off and on. The PWM itself is only a tool for dimming, thus feedback is not necssary.


This is what I was trying to get at before.

With LEDs, regulation is highly desirable to avoid driving them too hard.

With a pure PWM circuit by itself, good regulation is hard to arrange and so you typically need PWM for dimming plus a constant current circuit for regulation => extra complexity.

On the other hand, with a constant current circuit alone you can achieve regulation, and you can also achieve dimming by varying the feedback voltage. (Maybe not with best efficiency, and with some limitations on dimming range.)

Overall, if it should be suggested that a PWM circuit might replace a constant current circuit of whatever kind (buck/boost/linear), this is not often likely to be a useful choice or simplification.


----------



## SemiMan (Jan 3, 2009)

Mr Happy said:


> On the other hand, with a constant current circuit alone you can achieve regulation, and you can also achieve dimming by varying the feedback voltage. (Maybe not with best efficiency, and with some limitations on dimming range.).



From a purely practical standpoint, I can probably add a $0.40 micro or 555 timer that turns a regulator on and off just as easy as varying feedback voltage especially if I am trying to achieve a low feedback voltage to increase efficiency.

Semiman


----------



## MrAl (Jan 3, 2009)

Hi,


I thought i would take a minute to talk about the biggest misconception of
all when it comes to PWM vs Constant Current.

The biggest misconception is that PWM is more efficient because the switch
(transistor) is not always on, when actually, the switch 'on' period has nothing to
do with lowering the efficiency.
This is because although the switch is not 'on' all the time, when it is 'on' there
is more current flowing than when you use a constant current. This is because
to match the brightness of a constant current circuit the average current level
has to remain the same in a PWM as the CC circuit, and to do that the current
peak has to be higher. For a 50 percent duty cycle for example, the peak 
current has to be two times the constant current level, which is significant.
This leads to higher inefficiencies in the LED itself, due to the higher drive 
current to get the same light output.
The net efficiency gain for PWM over CC then is less than 1, so it's actually
LOWER than constant current.

To prove this, i presented all the math and the numbers right here on CPF
a few years back. The thread is most likely still here somewhere.


----------



## SemiMan (Jan 4, 2009)

.... and in the process kept promoting yet another misconception which was explained earlier in the thread.

PWM FOR DIMMING and CONSTANT CURRENT ARE NOT MUTUALLY EXCLUSIVE! They refer to two completely different concepts that may be used together (or not).

What you stated below has been stated numerous times in the thread already.....



MrAl said:


> Hi,
> 
> 
> I thought i would take a minute to talk about the biggest misconception of
> ...


----------



## iaboyeah (May 7, 2014)

Hello, I understand that pwm drives an LED in its least efficient range, but I am assuming PMM delivers power more efficiently. Therefore I question why a capacitor is not paralleled to the LED to smooth out the pulses effectively driving the LED more efficiently in the lower modes therefore extending run time in the lower modes. I assume constant current introduces resistance in the lower modes. Can some one help me here?

Thanks

Ed


----------



## SemiMan (May 7, 2014)

Constant current would normally be done with a switch mode regulator for high efficiency. Capacitors are effectively short circuits for pwm. They charge instantly or as much as the power supply can dump out. Pwm into a cap may just end up at full output but with higher draw.


----------



## iaboyeah (May 7, 2014)

SemiMan said:


> Constant current would normally be done with a switch mode regulator for high efficiency. Capacitors are effectively short circuits for pwm. They charge instantly or as much as the power supply can dump out. Pwm into a cap may just end up at full output but with higher draw.



A simple circuit of a battery, a capacitor and a switch in series, at 100% duty cycle (switch on) current flows momentarily, no current flows after the capacitor is charged. If an LED were in parallel with the capacitor only the LED would draw current. When switched off the capacitor discharges through the LED, as soon as the LED reaches zero current the capacitor remains charged at that voltage. The capacitor would not have to be totally recharged to the desired led voltage every pulse. The capacitive reactance would only occur between the wave form peak and trough, with the capacitor in the circuit the wave would no longer be square, it would never drop to zero while PWM is opperating, This is my theory. 

Ed


----------



## SemiMan (May 7, 2014)

It's a wrong theory. Every time the switch turned on it would completely charge the cap. If the pwm frequency is high, the cap never discharges much. You can play around with values and duty cycle but this is the most inefficient solution possible.


----------



## iaboyeah (May 7, 2014)

SemiMan said:


> It's a wrong theory. Every time the switch turned on it would completely charge the cap. If the pwm frequency is high, the cap never discharges much. You can play around with values and duty cycle but this is the most inefficient solution possible.



Not every time the switch was turned on. Only initially, as below the point where an LED junction begins to draw current, approximately 2.5V the capacitor will remain at 2.5v between pulses, Each pulse of the PWm will charge above 2.5V but it was already at 2.5 volt. what ever energy the cap uses it transfers to the LED between pulses. Reducing the peak current of the LED like filtering in a power supply. I have the feeling this will be more efficient way to drive an LED at low levels with PWM.


----------



## SemiMan (May 7, 2014)

Nope. The cap is a short circuit every time the switch is on presenting a short to the battery increasing losses.


----------



## iaboyeah (May 7, 2014)

The cap is not a short, it is capacitive reactance.


----------



## SemiMan (May 7, 2014)

Its for all intents a short when first connected with the current only limited by battery resistance and capacitor ESR. Its the most inefficient way to charge a cap from a battery.


----------



## iaboyeah (May 7, 2014)

Your saying the capacitor completely discharges to zero between each pulse?


----------



## SemiMan (May 7, 2014)

Does not matter if it does, it discharges significantly especially from an energy standpoint and it still behaves like a short.


----------



## iaboyeah (May 7, 2014)

Significantly? Where does the charge go?


----------



## SemiMan (May 7, 2014)

Into the LED but you are missing the point. Pwm right into cap from battery = bad idea.


----------



## iaboyeah (May 7, 2014)

What I am missing is how the capacitor would require the energy to charge it from 0 to 2.5 volts on every pulse, if it was standing at 2.5V after initial charging- a few pulses maybe - depends on the pulse width, sure this initial energy wil be lost in the source, Are you saying the same loss occurs on every pulse?


----------



## jtr1962 (May 7, 2014)

The problem here is the capacitor won't do what you think it will. Maybe for certain duty cycles and frequencies it'll work somewhat but it's a poor solution. If the cap is too large, then you won't get appreciable dimming even with a very low duty cycle. If it's too small, then it won't filter out flicker. In order to smooth the pulses from PWM, and have a steady current delivered to the LED, you need to have an inductor in between the PWM output and the LED. Once you do this, guess what you have? A traditional step-down switching regulator. The point of using PWM is simplicity, and also to prevent color shift when dimming. If you PWM in the kilohertz range, you're not going to see flicker.


----------



## iaboyeah (May 7, 2014)

jtr1962 said:


> The problem here is the capacitor won't do what you think it will. Maybe for certain duty cycles and frequencies it'll work somewhat but it's a poor solution. If the cap is too large, then you won't get appreciable dimming even with a very low duty cycle. If it's too small, then it won't filter out flicker. In order to smooth the pulses from PWM, and have a steady current delivered to the LED, you need to have an inductor in between the PWM output and the LED. Once you do this, guess what you have? A traditional step-down switching regulator. The point of using PWM is simplicity, and also to prevent color shift when dimming. If you PWM in the kilohertz range, you're not going to see flicker.



Things don't often do what I think they will do but they often do what I know they will do till they fail. The problem is I don't know what they will do in tis case.


----------



## jtr1962 (May 7, 2014)

iaboyeah said:


> Things don't often do what I think they will do but they often do what I know they will do till they fail. The problem is I don't know what they will do in tis case.


Well, I can tell you as an electronics engineer that a capacitor will only work for a limited number of duty cycles and frequencies. Of course, in theory you could shunt in several different capacitors to achieve filtering at more frequencies/duty cycles. However, this adds to the cost/complexity, defeating the purpose of PWM in the first place. In truth, I was never a fan of PWM, at least until we were able to PWM in the kilohertz range. At that frequency I just don't notice flicker or strobing, even moving the light very rapidly back and forth.


----------



## iaboyeah (May 7, 2014)

If I understand right PWM is less efficient al low levels. I suppose there would be hysteresis loss in the caps. Hadn't considered that for one thing. About constant current, is that like having a resister in series I wonder, except for regulation, do you know? I am from the old school of discrete components, looking at todays schematics mean little to me.


----------



## jtr1962 (May 7, 2014)

iaboyeah said:


> If I understand right PWM is less efficient al low levels. I suppose there would be hysteresis loss in the caps. Hadn't considered that for one thing. About constant current, is that like having a resister in series I wonder, except for regulation, do you know? I am from the old school of discrete components, looking at todays schematics mean little to me.


PWM is less efficient at low levels because you're basically pulsing the LED at a higher current to get less light, as opposed to driving it steady at a lower current. LEDs are more efficient at steady low drive currents rather than pulsed high currents. Losses in the capacitor really aren't significant here.

Yes, the constant current is similar to having a resistor in series except that switching regulators are able to drive at constant current more efficiently than resistors.


----------



## iaboyeah (May 7, 2014)

Thanks for the feedback

Ed


----------



## videoman (May 8, 2014)

The problem we videographers have with led lighting is the flickering and banding ( horizontal lines) that appear on the video when PWM dimming is used. It happens at certain dim levels and at certain camera shutter speeds/ frame rates, usually when the camera is set for slow motion playback like 60,120 frames per second or higher. The normal 30 frames per second ( actually 29.97 in NTSC) don't seem to be affected. You would not see the effect with your eyes but when put on the monitor, it is too late as there is now way to correct the problem in post production/editing. Problem is that the dimming scheme is never mentioned in the light unit's specs on the spec sheets. Some cheaper on-camera led units do not seem to have this problem and then there are the expensive ones that do. Is there any way to find out if a led light uses the PWM or CC variety, if not indicated on specs. ? This problem is discussed many times in the broadcast/ video sectors.


----------



## SemiMan (May 8, 2014)

Not without seeing it. Often you can tell pointing your camera phone at it. Need an optical detector.


----------



## iaboyeah (May 11, 2014)

SemiMan said:


> Into the LED but you are missing the point. Pwm right into cap from battery = bad idea.



So the energy discharged by the cap is not lost, merely transferred to the LED. I believe the battery would never see the transient current with a cap in parallel with it also.


----------



## reppans (May 11, 2014)

Most of my current regulated lights are a lot more efficient than most of my PWM lights on low lows, but one PWM light (a Malkoff MDC AA) blew away all my current regulated lights on efficiency (lumen-hours), and by a factor of 2x or more on sub-lumen - this includes Quarks, D25As, and an SC52 (which actually was the least efficient SL despite spec claims).

There's an easy way to visually detect even HF PWM in everyday life, but you may not want to know - it's ruined some good lights for me.


----------

