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I can think of some reasons why modern LED streetlights would likely be pulsed;

  • Efficient voltage conversion starting from line voltage would likely include an AC or switching step higher than 60 Hz.
  • Highest efficiency operation of LEDs often occurs at a current greater than can be sustained continuously due to heating issues.
  • Conversion back to steady DC (edit: at line frequency 50/60Hz) would require additional components that could fail, and would have no benefit that would offset the reduced efficiency operation.

There is a short section in Wikipedia about pulsed LED operation but it just introduces the concept without addressing how widespread pulsed operation is in the field.

As long as the frequency were sufficiently high that there were no chance of flicker perception, it seems to me that LED streetlights would be pulsed - or at least the blue LEDs used to excite the phosphor. The phosphor could have a long enough half-life to make most of the spectrum of the resulting emitted light steady even if the LEDs were pulsed.

Because some white light LEDs rely much more on LED's primary blue light than others, I'm going to ask my question primarily about the LEDs themselves, rather than the emitted light.

Are LEDs in modern streetlights usually pulsed? If so, roughly what frequency? 100 Hz, 1 kHz, 10 kHz? While there could be substantial variation in some regions, I'd expect in regions where cities are implementing widespread conversion from gas (mercury, sodium) to LED, there must be some commonalities or general trends/convergence in design.

uhoh
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    Borderline "too broad" unless you give an example of a streetlight because there will obviously be all kinds of solutions, but still an interesting question. – pipe Jun 14 '17 at 07:37
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    @pipe I understand the concern, but in this particular case at this time, there is a good chance that standardization or standard practices have set in. How about we give this one a day or two to see if a clear answer emerges? If not I'll edit it. I'm after a good answer here so I'll keep an eye on what happens. In the mean time, if you can suggest or even make a helpful edit, that would be great! Anything short of insta-closing would be appreciated. – uhoh Jun 14 '17 at 07:39
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    Roughly between 50Hz and 50kHz. – PlasmaHH Jun 14 '17 at 07:45
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    @uhoh I'm not going to close, I upvoted it, but I have no control of other users. :) – pipe Jun 14 '17 at 07:47
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    I seem to recal a lot of people from the engineering comunity arguing **against** LED streetlighting, as it has a number of dissadvantages when compared to cold sodium-vapour (for example, I believe professional astronomors like SV lights more because they can just use a filter to block out the two spectral lines and get rid of a lot of the issues with light pollution. SV lights are also very efficient in terms of percieved light intensity, perhaps even better than LEDs although with the modern increase in efficiency that could no longer be true) – Joren Vaes Jun 14 '17 at 08:12
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    Efficacy goes DOWN with increasing current. The Vf vs I and Lm vs I curves are not straight but slightly curvy. PWM is used because it's a fair bit easier to implement a constant voltage source than a variable constant current one. So you set the output voltage to produce the maximum current and pulse that to achieve brightness control. – Barleyman Jun 14 '17 at 08:52
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    @Barleyman *starting from zero*, it first goes up, then at higher current it goes down again, *ergo* there is a maximum in the middle. Let's investigate the whole elephant, or if you prefer [brontosaurus](https://www.youtube.com/watch?v=cAYDiPizDIs) – uhoh Jun 14 '17 at 08:55
  • For any meaningful current range the efficacy usually keeps falling when current grows. I've actually spent a few occasions on drawing lines on blown up Lm vs If curves to estimate efficiency at various currents. So for the LED I'm looking at right now from something like ~2mA to 130mA the efficacy keeps dropping. If is not defined below ~2mA so very possibly LED could work differently. Usually you wouldn't go that low in any case as it's not easy to design a (current controlled) SMPS that's happy with two orders of magnitude difference in output current. – Barleyman Jun 14 '17 at 09:05
  • @Barleyman picture being worth a thousand words and all that, if there's any way you can add a plot of such data with an imgur link or add an answer here rather than just describing it verbally, that would really be great! – uhoh Jun 14 '17 at 09:13
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    @JorenVaes - You're correct - and that's why you can buy LED streetlights which are bright orange - they're made to have the same (or at least very similar) emission spectrum to the SV ones. – brhans Jun 14 '17 at 11:11
  • LED streetlights are driven using power supplies like [this](http://www.usa.lighting.philips.com/products/oem-components/xitanium-led-drivers) or [this](https://www.osram.com/osram_com/products/electronics/electronic-control-gears-for-led-modules-and-dimmers/outdoor-ecg-for-led-modules/constant-current-non-dimmable/optotronic/index.jsp). Although they're switch-mode supplies, their output doesn't pulse like a PWM. – brhans Jun 14 '17 at 11:15
  • @brhans that sounds like it's the makings of a definitive answer to my question. Can you consider posting as such? Thanks! – uhoh Jun 14 '17 at 11:28
  • @JorenVaes I'm supposed to like them -- the better colour-rendering of white LEDs is supposed to make spotting cyclists easier. In practice I'm not hard to spot in the dark, and the streetlights being the same colour as headlights removes a useful piece of information at some junctions – Chris H Jun 14 '17 at 12:23
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    @joren that might have been true in the LPS (low pressure sodium) days, but nowadays everything is HPS. LPS is really bad for visibility and has a CRI of practically zero (literally -44). – Nick T Jun 14 '17 at 13:27
  • @NickT LPS is the orange light right? I believe they are still common in some places, at least here in belguim we practicly only use those. Does CRI even matter at night? Seems to me all you care about at night is seeing things, not decerning their color (and since you'll mainly be relying on your rods at night, and not your cones, your color vision is going to be horrible anyways) – Joren Vaes Jun 14 '17 at 13:43
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    @JorenVaes LPS and HPS are both orange, LPS is basically monochromatic, HPS has a widened spectrum due to some mercury. CRI is still important in [mesopic](https://en.wikipedia.org/wiki/Mesopic_vision) situations because it allows people to recognize objects better and faster, which, when driving is a gigantic safety issue. A whiter but *dimmer* light is [better for safety](http://www.lrc.rpi.edu/researchAreas/pdf/GrotonFinalReport.pdf). If you're at such low illumination levels where you have zero color vision ([scotopic](https://en.wikipedia.org/wiki/Scotopic_vision)), you're *not* driving. – Nick T Jun 14 '17 at 14:21
  • @NickT Thanks for the clarification, I didn't know that. I was just going of off what I've been told by others, and given my (limited) understanding of the different factors at play there was some sense to it. Now I know better! – Joren Vaes Jun 14 '17 at 14:23

4 Answers4

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LEDs used for street lighting usually employ a DC/DC converter of some kind, with tight current control at its output. So, providing a steady current isn't reducing efficiency nor does it add unnecessary components which could fail, nor does it reduce the lifetime of the LEDs.

It's the simplest and most efficient way to drive a high-power LED array. Steady current, provided from a "pulsed" source.

Janka
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  • Are you saying that LEDs are driven most efficiently with DC, and not with pulsed current? Are the LEDs themselves converting electrical power to light with the highest power/power efficiency when the current is DC? Pulsed operation does not improve power/power efficiency for high power white light LEDs used in street lights? – uhoh Jun 14 '17 at 07:56
  • I think he means DC/DC to not drive them directly from the AC line, instead turning it into DC and then pulsing that. – Joren Vaes Jun 14 '17 at 08:18
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    The circuit of most DC/DC converters has a coil on output, which —despite the pulsed nature of its working— automatically produces a more-or-less steady current. That's the very core function of such a DC/DC converter. – Janka Jun 14 '17 at 08:20
  • Pulsing LEDs makes only sense when you want a high power output for a very short time with a LED not really designed to provide that power steadily. E.g. in an IR remote control. Over the glimpse of the thermal time constant (seconds), power needs to be balanced out to account for the heat dissipation. – Janka Jun 14 '17 at 08:27
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    I'll go do some reading and try to find some data. Starting from very low voltage (where there is heat generated and no light) and ramping up, the efficiency starts low, since it starts at zero. There is only non-radiative recombination. At you increase the voltage and the current increases, the ratio of radiative to non-radiative recombination improves, as do other aspects. I had thought that the plateau in efficiency occurred at a point beyond where heat could be removed for continuous operation, and so for the most watts of light per watt of electricity, it was better to add LEDs and pulse. – uhoh Jun 14 '17 at 08:32
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    Efficiency at low voltages is low because you cannot have light emmission if you don't jump over the band gap directly. But as soon you do, there is no sense in increasing the voltage any more — the overshoot energy is simply turned into heat. – Janka Jun 14 '17 at 08:40
  • Real III-V MWQ LEDs are more than a little complicated inside, so arguments based on ideal diodes are helpful but incomplete. Let's find some data... – uhoh Jun 14 '17 at 09:08
  • Can you add some link or supporting information? "LEDs used for street lighting usually employ a DC/DC converter of some kind" could very well be true, but I need something more than just saying so in order to accept. Thanks! – uhoh Jun 14 '17 at 12:07
  • Does http://hrcak.srce.hr/file/125028 help you? Please note how it first says PWM dimming is a great solution if you don't want the color temperature change with luminance but then discards it completely in favor of an extensive discussion of DC/DC converters without any additional PWM on output? Because PWM is a feature of dimmable LED lighting, and a feature of lighting were color temperature is important. Both isn't the case for street lighting. – Janka Jun 14 '17 at 12:24
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    @uhoh, the current at which the efficiency stops increasing is quite low. Efficiency is then almost flat before it falls off at least partially due to slef-heat (including heating from contact resistances). I went to a conference on this and other III-nitride matters a few years back: It was true then and it's more true now, that once you've got useful amounts of light coming out, driving them harder makes these LEDs *less* efficient. – Chris H Jun 14 '17 at 12:27
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    @ChrisH one person's *then* is not necessarily the same as another person's *then*. My *then* is farther back than most people's *then*. I did some further reading in the last few hours and I can see that the more recent stuff looks just as you mention, a really broad plateau followed by a very gradual drop at much higher currents. There was a lot of work done over the decades; better crystal growth, then sapphire substrates, substrate lift-off too, then better MBE technology, plus better heat flow and solid-state modeling, etc. They don't give out Nobel Prizes for nothing you know. – uhoh Jun 14 '17 at 12:33
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    @uhoh It's more the definition of "few years". This was after the really big breakthroughs but before the Nobel. – Chris H Jun 14 '17 at 12:37
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You're making a wrong assumption that the efficacy goes up with higher power level. The opposite is true, to any meaningful power levels the efficacy decreases whenever you increase the current.

PWM is used because it's very easy to implement. If you set your current to the maximum you want to use, you can have linear brightness control by just adjusting the duty cycle. Adjusting current has nonlinear response requiring calibration table if absolute accuracy is important (it often is not). enter image description here

As you can see from this 1W white led Lumen vs current curve, doubling the current does not double the light output. Whether this is important depends on your application. If you're dealing with a >1kW advertising backlight, the electricity bill easily exceeds the up front cost of the display module. There are also thermal considerations, with better efficacy you have less waste heat on your system.

enter image description here

To make things worse is that efficacy drops even more with higher junction temperature. This graph shows ambient temperature but essentially junction temperature works in similar fashion. They're just being difficult about it. Now PWM will average the heat output but still, worse efficacy requires higher average current which means higher junction temperature..

One downside of a PWM is that the load is nasty from SMPS point of view, you're effectively imposing constant radical transients to the poor thing. At the very least you need a large output capacitor to buffer the voltage dips and peaks at edges.

A problem with constant current driving is that it's more complicated, especially if you want adjustable output current. There are further complications with local dimming applications as Vf varies with output power level so your current regulator has to dissipate the difference.

Edit added bit about junction temperature.

Barleyman
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    Wow! To double check, is the plot really showing DC current, not PWM average current? And an LED that can handle 400 mA DC continuously is still at least locally linear all the way down to around 5 mA DC (give or take)? https://i.stack.imgur.com/dSQbw.png This is all really measured, and not just a plot/extrapolation of some parameterization? – uhoh Jun 14 '17 at 10:15
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    @uhoh Yes, that is DC. With PWM the active pulse would behave much the same way. – Barleyman Jun 14 '17 at 10:35
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    There's also a second order effect, light output goes down with junction temperature. So PWM with higher current will run with worse efficacy to start with. It will also run hotter for the same light output that makes things worse. So it's definitely better to use constant current from system efficiency point of view. – Barleyman Jun 14 '17 at 10:44
  • That's pretty amazing, it seems GaN devices have improved a bit over the years, it's almost Nobel Prize-worthy! OK would you go one step farther and suggest that this might fairly well rules out any intentional and/or substantial pulsing of the LED current; that there would be no reason from the LED side to pulse it? Further, as long as the frequency from the power system is filtered somehow, that the LED current would really most likely be DC in a typical outdoor/streetlight application? I'd like to accept the answer as long as it addresses the question sufficiently. – uhoh Jun 14 '17 at 10:46
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    @uhoh The only reasons to use PWM is that if you need very high instaneous light output (optical sensors for example) or you want to control brightness. For lighting there's no reason for PWM if you don't want a dimmer. – Barleyman Jun 14 '17 at 11:09
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    But the efficiency that matters is the efficiency of the power supply + LED and power supplies ussually have increased efficiency with higher power. So is the "cascade" of both efficiencies that matters in the end, and that could be a design decision as well. – Andrés Jun 14 '17 at 11:10
  • OK I'll hold off on accepting anything until I get something definitive about street lighting, but your answer and discussion are really helpful. Thanks! – uhoh Jun 14 '17 at 11:17
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    @Andrés PWM on low duty cycles is a pretty nasty load for PWM thought. You would like to have a large capacitor to supply majority of the transient load. Not that 29mA is a great load if your max output would be 2.9A. – Barleyman Jun 14 '17 at 11:45
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    @uhoh that efficiency graph he presented is in the data sheet of every LED emitter I've ever seen. Have you read any LED data sheets? They are "enlightening" (ha ha) to say the least. [Here, pick any random one](http://www.mouser.com/Optoelectronics/LED-Lighting/LED-Emitters/_/N-8usfd/) (you can filter on specifics) and click "Datasheet". – Harper - Reinstate Monica Jun 14 '17 at 15:42
  • @Harper I used to work with people who were making QW LEDs, but that was a while ago. I do look at data sheets, but not for high power LEDs, at least not yet. Once I click your link, that will all change. Thanks! – uhoh Jun 14 '17 at 16:01
  • While this is an interesting discussion about driving LEDs, and does address some of the assumptions stated in the Question, it does not actually address the primary questions stated in the Question: "Are LEDs in modern streetlights usually pulsed? If so, roughly what frequency?" – Makyen Jun 14 '17 at 17:30
  • @Makyen sometimes working towards an answer takes a little time. In this case my question hypothesiszed that they would be pulsed based on some thoughts of optimum current. This answer is examining that premise and finding some issues with it. When an easy/immediate answer is not forthcoming, sometimes one has to work a little harder. If you know the answer, please post it! – uhoh Jun 14 '17 at 17:54
  • There's multiple helpful answers here, but this answer zeroed in on what it was that was making me think they might be pulsed and then zeroed it out. Thanks for your help! – uhoh Jul 15 '17 at 13:14
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To summarize: They wouldn't do it because it's not efficient, wouldn't keep the lights in safe spec, and is not viable as a way to control a large group of lights (owing to the distance and lack of versatility).

In street lighting, the name of the game is efficiency.

LEDs are inherently a difficult customer, since at their high-efficiency ranges, they are too non-linear** and most be driven by a constant-current power supply.

Operative word: "constant".

Since they already must drive it with a constant-current supply, if they also wanted to do PWM, that would add unnecessary complexity. And there's a much better way to dim LEDs using the constant-current supply already present. Here, look at this datasheet on Page 11. Forward voltage vs forward current. Note this graph is very distorted, for normalized, look at my endnotes.

enter image description here

If you're driving the LED at 3000ma and want to dim it, cut current to 1000ma, and voila. Of course it doesn't quite drop by 2/3, look at "flux vs current", same page.

enter image description here

At 1/3 the current, luminous flux drops from 235% to 95% of spec. It's much more efficient at the lower current. Voltage drops too, which nibbles a bit of the efficiency difference, but not by a whole lot.

Would someone deliberately use more emitters to improve efficiency? Absolutely. Many commercial and industrial customers are looking at total life-cycle cost, and emitters are a small part of that. If $100 more emitters saves $300 in electricity over the life of the fixture, it may be a smart move. I had a guy who specced three LEDs at redline max 1400ma. It gave the needed light. However heat was the key issue. I respecced using the datasheet "normal" current of 350ma and seven emitters. Got the same light at half the heat.

Now that I've positively shown lower power is more efficient for LEDs, you can see where PWMing them is not efficient. Running 3000ma at 33% PWM is worse than running 1000ma continuous.

Why would anyone PWM then?

In a perfect world, all dimming would be via something like the 0-10 volt signal widely used commercially, and each LED module would use the "adjust the output of the constant-current supply for perfect dimming" method. However.. that does not work everywhere. Fact is... PWM is an efficient way to propagate a dimming signal.

Consider the lowly "LED strip". A narrow strip of PCB, every 50mm (2") it has a CUT line, three LEDs and a resistor. Or for an RGB strip, three RGB LEDs and three resistors. And with RGB, of course, they want to dim each channel individually. How do we get three dimming signals down to hundreds of little segments? Cost makes it impossible to put adjustable-output constant-current power supplies on every 50mm segment. The only workable dimming method is PWM.

It gets better. PWM is both the power and the signal. If the PWM controller can only drive 3 amps, and you want to run seven 6A strips, you can use an amplifier: it receives the controller's output as a signal and uses it to gate its high-current outputs, tapping out PWM in lock-step. The versatility is hard to beat.

And this works for any of a huge variety of LED lighting (whose purpose is notably, not efficiency.) Nobody really cares about the lumens per watt here:

enter image description here src

Why not street lights, then?

It's not entirely unreasonable to dim LED street lights. They could ease on at dusk, burn in excess of legal requirements to 11pm, then roll back in the haunting hours when hardly anyone is out. But they wouldn't use PWM. The signal won't propagate well over an installation the size of a town.

An LED street light takes high voltage (240-277V or even 480V which they tap off the nearest power line without metering, that means that PWMing the power line is right out)***. Internally, a street light has a sensible number of large emitters - ideal for series connection to a high voltage constant-current supply. This would be best dimmed by current adjustment. They would either use radio - or if they were hardwiring an expensive signal wire, they would use it for a lot more stuff than dimming. They might work with the power company to power-line-encode a data signal similar to how power companies can remotely shut off smart meters. Adding $20 a unit for the transceiver is not a "deal breaker" on a $1000 street light.

** Incandescents are linear once lit, so sending 120V to them will reliably produce 60W. Discharge lighting (fluorescent, neon, low/high pressure sodium, mercury vapor and metal halide) is totally non-linear: once struck, they are a dead short and must be current-limited by a ballast/driver. In the case of LEDs, their voltage-current curve is quite steep, You remember the Voltage vs Current chart from This data sheet page 11. Look again: The scale is distorted, and volts don't start at zero. If corrected, the graph would look like this:

enter image description here

That's what you call non-linear. Remember, this line moves a little depending on temperature, age, binning, etc. and when the line is that steep, a little is a lot. Send 3.05V and who knows what'll happen! The manufacturer only guarantees what'll happen if you send 2500ma. Every other chart in the datasheet is based on current, for that reason.

*** The power company and the city agree how much power a normal street light draws, and the power company simply multiplies by the number of lights, and bills them.

Harper - Reinstate Monica
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  • This is an interesting perspective, thanks! You might consider adding a tl;dr at the top. It is a yes/no question and I think you have a definite conclusion on that, why not add a "yes" or a "no" somewhere in the beginning of the answer as well? – uhoh Jun 14 '17 at 18:06
  • 2" is 50.8mm to pick some nits, You memorize some common imperial values fairly quickly.. Chinese companies (a metric country) always reply to me in mils and inches when I specify everything in metric as a random observation. WRT PWM being complex to implement, not really. A humble MOSFET in series with the LEDs will get the job done. Put it on the negative side and you don't have to deal with high voltages either. WRT $20 extra expense, you underestimate how far beancounting does. Extra $2 expense on a product costing something like $5k gets frowned upon. WRT remote control, GSM would do it. – Barleyman Jun 15 '17 at 09:00
  • I'd implement PWM control by transmitting desired current value OTA and using locally some 8/16-bit cheap microcontroller to produce PWM if I wanted to use PWM control to start with. See my answer with similar content. – Barleyman Jun 15 '17 at 09:01
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In general, there are two methods of dimming LEDs, PWM dimming and Amplitude dimming. What you refer to as DC dimming is amplitude dimming. In professional lighting applications, PWM is no longer used for dimming , mainly due to health concerns over the generated flicker. With street lighting, another issue is the stroboscopic effect. You will find today that virtually all professional LED drivers including street lights use amplitude dimming. You can read more about flicker and dimming here.

Update: In response to some of the comments, I would like to extend my answer. By professional lighting applications, I am referring to constant current dimmable >20W LED drivers such as these, not cheap and nasty halogen or bulb replacements or computer backlight applications.

There are two causes of flicker, one is caused by the mains ripple propagating to the output. Cheap single-stage LED drivers such as those used in bulb replacements suffer from this phenomenon.

The second type of flicker is caused by PWM dimming. This may be perceptable or imperceptable. The IEEE PAR1789 is a recommendation of how high the PWM frequency needs to be for it to be considered imperceptable. That said, you will find in industry that high-quality LED drivers for professional applications almost exclusively use amplitude dimming (DC dimming).

mr_js
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  • PWM is most definitely used in in professional display backlight applications. Constant current is an exception. Flicker is usually a non-issue once you have high enough frequency. 90 to 360Hz is typical range. – Barleyman Jun 14 '17 at 09:46
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    @mr js good article about flicker .I hate cheap nasty flickering leds . – Autistic Jun 14 '17 at 10:18
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    @mr_js: The article you link to is almost entirely about flicker due to the mains supply (which is at a rather low frequency, usually 50-60 Hz, typically giving rise to flicker at 100-120 Hz). Professional lighting applications do use PWM for dimming, but will typically use a much higher frequency (tens of kHz). – psmears Jun 14 '17 at 10:58
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    PWM is broadly used. It just isn't used much anymore *at mains frequencies* (100-120Hz effective) partly because switching supplies are cheaper than copper windings these days. Unfortunately, General Motors *didn't get the memo*, and GM car taillights are simply the brake lights PWM'd to a "dimmer" light level, and in a visible range. In fact they are as bright as brake lights *when they're on*, and as your eyes sweep-scan the road, they leave tracks across your corneas. Maddening! – Harper - Reinstate Monica Jun 14 '17 at 15:47