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I'm looking for some circuit design tips to design a LED board with a built-in strobe feature, something like this:

  • 16x high power leds (250 mA max, Vf ~ 3.2V)
  • 24V power supply
  • One 24V input for low power constant operation (let's say 50 mA, total 800 mA)
  • One 24V input for strobe operation (at the full 250 mA, total 4 A)
  • Charge rate limitation for the strobe capacitors (let's go with 1 A total input current)

My initial I-dont-really-know-what-I'm-doing kind of thought is just to have two constant current led drivers, each set at the proper current, and just have the inputs drive those. One would have to be driven through a timer to limit the strobe time, maybe a 555 would do the trick?

Stuff I figure I need to consider:

  • The capacitor stuff for providing the strobe power is not something I really know much about.
  • The case where the board is in constant operation and charging the caps at the same time. (I guess charge rate for the caps should be 200 mA max)
  • What happens if both inputs are on? The strobe input probably has to suppress the other input.
  • And what happens if the strobe gets triggered before the caps are charged?
  • Do I need to discharge the caps when power is lost?
  • I've also considered just throwing a small PIC on there.

Lots of fun stuff to consider!

Another possibility might be if I could vary the value of the led driver's sense resistor. Not sure if that's possible or not.

So, as you can see I have no clue about this stuff. I suppose I could just have someone that works with this stuff design it for me, but where's the fun in that! Any suggestions where to start?

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  • \$\begingroup\$ Easy answer is use an uC and 1 wire driver IC such as WS2811 which enables individual addressing. Different inputs on the uC trigger different modes and the modes are all configurable in your code so can be tweaked or reprogrammed in future as well. \$\endgroup\$
    – D-on
    Sep 15, 2016 at 18:15
  • \$\begingroup\$ @D-on Yeah that does make sense. I have seen several drivers that take a PWM input for dimming. \$\endgroup\$
    – Chris
    Sep 15, 2016 at 18:53
  • \$\begingroup\$ Rioraxe has given you a pretty good answer, but I think he has missed a few key points that might make or break your project. E.g. what is the nature of this "24 V Power Supply"? Is it batteries? An AC-DC supply? You describe two " 24 V inputs". Are these two different power supplies? Perhaps, a preconceived notion that you will need two different power inputs for the two different modes of operation? What is the current limit of the 24 V supply(s) and how does this compare to the "4 A" total you need to get the desired brightness from the LEDs? If you need the capacitors, more issues arise. \$\endgroup\$
    – FiddyOhm
    Sep 17, 2016 at 3:21
  • \$\begingroup\$ @FiddyOhm This is for industrial automation, the machines have standard switched power supplies, so I can't rely on that to limit the current. With 24V inputs I just mean 24V signals/triggers from a controller, don't really know what the correct terminology is. One machine could have several of these illuminators, which is why I want to limit the current. 10 x 4A x 24V would not go down well with my electrician ;) What issues arise with the caps? Safely discharging? \$\endgroup\$
    – Chris
    Sep 17, 2016 at 16:34
  • \$\begingroup\$ We have to be careful with our terminology here because there are closely related operational parameters that may be confusing to one who is unfamiliar with the topic. There is a distinction between the current "limit" of the power supply itself, and the current "limiting" needed to drive the individual LEDs properly. I will use the term "current capability" to denote the amount of current (in Amps) your 24 V power supply can safely deliver continuously to a connected load. What is the "current capability" of one of your 24 V "switched power supplies"? Is it 4A as you imply, or more, or less? \$\endgroup\$
    – FiddyOhm
    Sep 18, 2016 at 14:24

2 Answers 2

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A traditional strobe light works by dumping the energy stored in a capacitor to the light element at high voltage for short duration (a few hundred us).

A LED cannot work this way. Typically, you can over-drive a LED to no more than 2 times the maximum rated current for short duration at low duty cycle (for example, 500 us at 1% duty cycle).

Here are the suggestions. Organize the 16 LEDs into 4 strings. Each strings would take up to 3.2V x 4 = 13V at maximum rated current of 0.25A. This can be driven comfortably with very cheap and common LED driver from 24V.

You can try to drive all 4 strings in parallel with one driver. In this case, you may want to hand match the forward voltages of the 4 strings. I think using 4 drivers for the 4 strings could be a better choice.

Pick a driver that has a current setting input. Set the current to 0.05A for dim mode. For the strobe mode, pulse the driver's enable input for the duration desired and at the same time force the current setting to full power. Full power could be 0.25A or you can consider over-driving of up to 0.5A for a short duration strobe.


In response to the comments: a current limit plus a large input capacitor can allow the LED circuit to operate at higher power in short duration than the power supply can provide.

There is a common current limit circuit based on the LM317. One disadvantage of the LM317 circuit is the waste of around 3 to 4V.

Another simple alternative is as followed. The waste can be as low as 0.7V near current limit. Size R2 to the Vbe of Q1 for the current limit. The precision is based on the Vbe so you do need some head room between the current needed and the current available for setting the limit in between.

schematic

simulate this circuit – Schematic created using CircuitLab

Above circuit edited to add R3 if needed. R3 is for reducing Vgs which is very likely needed for 24V operation.

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  • \$\begingroup\$ Yeah I suppose strobe isn't really the right term in this case. I was planning on using one driver per string to avoid cascade failure if one string goes. This + µC is probably the way to go. I still need some capacitors to limit the max current the entire thing draws though. Any good search terms I can hit google up with to learn about that? Not quite sure how to limit the current to just the capacitor and not the entire thing. \$\endgroup\$
    – Chris
    Sep 15, 2016 at 19:28
  • \$\begingroup\$ I = C dV/dt. Let's say the excess current draw I is 1A. The tolerable voltage drop dV is 2V. The duration of excess current dt is 1ms. C = I dt/dV = 1A x 1ms / 2V = 0.5mF. \$\endgroup\$
    – rioraxe
    Sep 15, 2016 at 21:22
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    \$\begingroup\$ A key variable here is the tolerable voltage drop, for a typical buck LED driver, it can probably easily tolerate 16V when driving a 13V LED string, so 24V - 16V = 8V. So now the question becomes whether the system and power supply can tolerate such voltage drop. If the 24V power supply can gracefully enter and exit current limit, then the answer could be yes. So you can implement a simple current limit circuit supplying just the big input cap and the LED circuit. \$\endgroup\$
    – rioraxe
    Sep 15, 2016 at 21:22
  • \$\begingroup\$ What kind of current limiting circuit would you recommend? I had a quick google and using a LM317 seems like a simple way of doing it that doesn't require many components. \$\endgroup\$
    – Chris
    Sep 16, 2016 at 6:38
  • \$\begingroup\$ Thanks for the edit, looks good. I think all this is coming together in my head pretty well now. \$\endgroup\$
    – Chris
    Sep 17, 2016 at 17:44
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You should use a 48V power supply and and a buck driver for a single string of 16 LEDs. If you have to use 24V then you'll need a boost driver.

If the forward voltage is really 3.2V then a 54V power supply. I have done many 16 LED strings and none have gone over 43V. I typically use Cree XPE or Luxeon Rebel Color ES and Luxeon White(e.g LXML-PWC2)

A good simple buck driver for 16 LEDs is the TI TSP92511.

Below is another and probably a very good option for your project. All you have to design is the strobe signal.

Use a Meanwell HLG-40H-48B ($30 at sager.com) constant current 48V LED power supply.

Then you just need to toggle the dimmer between 0 and about 3V for your strobe.

Below is 16 LED strip (using 16 of 64) powered by a Meanwell HLG-40H-48B

enter image description here

enter image description here

The board will accommodate 32 Cree XPE and 32 Lumiled Rebel Color (4 strings of 16 LEDs).
The 16 Cree XPE are on and the bottom 16 Luxeon are off.

enter image description here

The holes are for mounting a heatsink.

enter image description here

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