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Fast LED Strobe for Macro
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chris_ma



Joined: 22 Mar 2019
Posts: 212

PostPosted: Wed Jan 29, 2020 12:58 pm    Post subject: Reply with quote

mawyatt wrote:
Pulsing the LEDs keeps the average power and temperature down, an LED current of 10A at 10V for 1ms is only 0.1 watt, whereas continuous is 100 watts.


is there a typo in there or am I missing something?

to me power (W) should be independent of time - maybe you meant energy in wattseconds (Ws) ?

1ms pulse:
10A * 10V * 0.001s = 0.1Ws

1 second pulse:
10A * 10V * 1s = 100Ws

continuous:
10A * 10V = 100W

not trying to be clever here, but genuinely interested because I only have very basic engineering skills that I'd like to improve.
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JKT



Joined: 28 Oct 2011
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PostPosted: Wed Jan 29, 2020 1:35 pm    Post subject: Reply with quote

mawyatt wrote:

Pulsing the LEDs keeps the average power and temperature down, an LED current of 10A at 10V for 1ms is only 0.1 watt, whereas continuous is 100 watts.

Indeed it does. Yet the shorter pulse will concentrate the generated heat even more so the maximum pulse energy likely drops as the pulse gets shorter. Unfortunately finding those limits likely requires burning quite a few LEDs. Very Happy

I had no idea that the controller was THAT flexible. Unfortunately I can't understand and thus appreciate the finer points.

mawyatt wrote:

Seems to me the LED is the limiting factor and finding acceptable LEDs at a reasonable cost is the task ahead. I suspect the LEDs will cost more than the controller Rolling Eyes


Well, yesterday there were two problematic parts. Now there's just one. Very Happy
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mawyatt



Joined: 22 Aug 2013
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PostPosted: Wed Jan 29, 2020 2:21 pm    Post subject: Reply with quote

chris_ma wrote:
mawyatt wrote:
Pulsing the LEDs keeps the average power and temperature down, an LED current of 10A at 10V for 1ms is only 0.1 watt, whereas continuous is 100 watts.


is there a typo in there or am I missing something?

to me power (W) should be independent of time - maybe you meant energy in wattseconds (Ws) ?

1ms pulse:
10A * 10V * 0.001s = 0.1Ws

1 second pulse:
10A * 10V * 1s = 100Ws

continuous:
10A * 10V = 100W

not trying to be clever here, but genuinely interested because I only have very basic engineering skills that I'd like to improve.


This is an area that gets confusing, power is in terms of Watts and energy is in terms of Joules or Watt-Seconds. So energy per unit time is power or Watts, then 1 Joule per second equals 1 Watt. Watts are what heats things up.

Average power was not a good usage on my part, it needs a time reference, sorry for the confusion. This assumes some reference use because the average power of anything that transitions from zero to some "peak" power and back is zero without a time frame specified (assumed integrated from 0 to infinity) unless it's repetitive (then integrates over the repetition period) but kinda defaults to 1 second where 1 Watt and 1 Joule are the same. This was the context I was using, 10A at 10V for 1ms referenced to 1 second (which I didn't say).

Actually this should be related to the thermal time constant of the system in question though. In electronics the instantaneous and average power are concerns. Most semiconductors specific or imply an instantaneous and average power dissipation, the instantaneous is usually related to peak current at maximum voltage and average is related to maximum device temperature. This ends up interplaying with the thermal environment since this influences the actual chip temperature, as does the thermal impedances as "seen" from the chip to the environment. Transistors have a parameter called "secondary breakdown" or Safe Operating Area where the device can't support a high voltage and high current simultaneously because the very thin junction temperature can almost instantly rise well above the overall chip temperature due to the thermal time constant & impedance from the thin junction to the chip substrate. So a very high thermal transient can destroy the chip or device, I've seen this before and the transistor had a tiny hole actually melted in the collector to base junction due to junction localized heating, and silicon melts at 1414C Shocked

The point of all this is that with a short pulse-width the LED and NMOS power devices shouldn't get too hot, and as long as you stay within the peak current, voltage and Safe Operating Area limitations everything should be fine. As you've shown, once the pulse width approaches 1 second or continuous time, then things change and the LED and NMOS driver will heat up considerably and require heat sinking.

Anyway, thermal design becomes another complex engineering subject, ain't nothin' simple anymore Rolling Eyes

Best,
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Last edited by mawyatt on Wed Jan 29, 2020 10:07 pm; edited 1 time in total
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mawyatt



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PostPosted: Wed Jan 29, 2020 2:39 pm    Post subject: Reply with quote

JKT wrote:
mawyatt wrote:

Pulsing the LEDs keeps the average power and temperature down, an LED current of 10A at 10V for 1ms is only 0.1 watt, whereas continuous is 100 watts.

Indeed it does. Yet the shorter pulse will concentrate the generated heat even more so the maximum pulse energy likely drops as the pulse gets shorter. Unfortunately finding those limits likely requires burning quite a few LEDs. Very Happy

I had no idea that the controller was THAT flexible. Unfortunately I can't understand and thus appreciate the finer points.

mawyatt wrote:

Seems to me the LED is the limiting factor and finding acceptable LEDs at a reasonable cost is the task ahead. I suspect the LEDs will cost more than the controller Rolling Eyes


Well, yesterday there were two problematic parts. Now there's just one. Very Happy


You shouldn't burn out any LEDs with the controller since it's current controlled and almost completely independent of supply voltage (within limits of course). NMOS devices tend to shed current with temperature so some protection here as well. So as long as you stay within the peak current LED limits and have some decent heat sinking if you use long pulses, or continuous, or highly repetitive use, you should be fine.

Agree the problem now is the LEDs, and getting a cost effective high output device. Seems these are going to be 18~36V rather than 9~18V devices, which will require some component changes (the caps and op-amp are 35V rated, the NMOS are 30V, but higher voltage devices are available in the same form factor). The op-amp is the issue and I'll probably need to use another higher voltage rated device, or use a regulator to protect it from higher voltages since ~>40V will be required for the 36V LEDs.

Best,
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JKT



Joined: 28 Oct 2011
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PostPosted: Thu Jan 30, 2020 12:44 am    Post subject: Reply with quote

mawyatt wrote:

You shouldn't burn out any LEDs with the controller since it's current controlled and almost completely independent of supply voltage (within limits of course). NMOS devices tend to shed current with temperature so some protection here as well. So as long as you stay within the peak current LED limits

I was thinking about forgetting that last condition. Cool

A stupid question: We all know that cheap flashes tend to vary their output and sometimes not fire at all. Is the problem in the tube or in the electronics? If it is in the latter... Laughing
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mawyatt



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PostPosted: Thu Jan 30, 2020 7:04 am    Post subject: Reply with quote

[quote="JKT"]
mawyatt wrote:


A stupid question: We all know that cheap flashes tend to vary their output and sometimes not fire at all. Is the problem in the tube or in the electronics? If it is in the latter... Laughing


Actually a good question, as I'll bet many of us have experienced this and ruined long stacking session results Confused

Without having the actual flash unit & setup it's hard to pinpoint but suspect this is caused by a few things. Here's a few that come to mind, I'm sure there are a few more.

1) Trigger pulse coming before flash has had time to fully recharge

2) Trigger not issued by controller or camera hot shoe for various reasons

3) With Wireless Trigger, interference or weak signal at flash receiver

4) Poor flash trigger design common with cheap flashes

5) Weak high voltage flash tube ignition ringing waveform common to cheap flashes

6) Weak batteries if Speed-light

7) Variations in recharge flash tube voltage thus effecting output power.

Note that the indicator LED and/or audio beep is just a lower level voltage indicator of the energy storage capacitors voltage which is still charging towards the peak voltage after the indicator annunciation. Poor designs on cheap flashes have the indicator level set lower so the user thinks it's recharging quicker, thus a better flash. To add insult to this the capacitor storage energy, which determines (along with PW) the optical output, is proportional to voltage squared, so a 20% lower voltage causes a 40% optical variation Shocked

This is why it's always a good idea to allow many seconds after the annunciation (especially with Speedlights) before issuing another trigger, to allow the capacitor voltage to reach final value. While trying to achieve quicker times between flashes for S&S work I've thought of designing a sense circuit to monitor this voltage and indicate when it's within maybe 1%, but for now I'll just monitor this voltage with an oscilloscope to "see" the charging waveform and timing. If you decide to do this be very careful, the capacitor voltage are usually around 400V and with many strobes the internal ground reference (capacitor negative) is NOT something you can rely on since it's part of a voltage doubler and actually moves with the AC input waveform, so not a common ground. Connecting this to earth ground will likely smoke your strobe, scope and other AC powered test equipment. You need to use an isolation transformer for your AC powered test equipment, or operate the strobes from batteries if they are compatible. Quality lab DVMs don't have this problem, they have floating negative (black) inputs, and of course handheld DVMs are battery powered so not as issue.

Anyway, I've had better luck with Strobes than Speedlights, but that's just my experience.

Best,
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Last edited by mawyatt on Thu Jan 30, 2020 7:38 am; edited 1 time in total
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JKT



Joined: 28 Oct 2011
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PostPosted: Thu Jan 30, 2020 7:28 am    Post subject: Reply with quote

I think 1) & 6) can be eliminated by mains power and sufficient time between shots. 2) Doesn't apply in a three flash setup with single controller ... or at least it is rather obvious if it happens.

Those cases it happens, it is almost always the very first shot, which makes me suspect some component was still sleeping. It is more common to loose just one flash and power variation is even more common ... or to some degree it is the rule.

3) is indeed an issue as increase in cases with only two flashes is usually cured by battery change for the transmitter. Wireless controller is not really necessary - it is just convenience to manage the power setting. It would likely work with wired trigger and the wireless as power selector.

That leaves 4) and 5), which are the most likely causes for power variation. A delayed flash could do that too, but then it shouldn't be even over the frame, should it?

Besides, separating the head from the controller, would give so much more freedom for diffusor design... Wink
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mawyatt



Joined: 22 Aug 2013
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PostPosted: Thu Jan 30, 2020 8:14 am    Post subject: Reply with quote

JKT wrote:
I think 1) & 6) can be eliminated by mains power and sufficient time between shots. 2) Doesn't apply in a three flash setup with single controller ... or at least it is rather obvious if it happens.

Those cases it happens, it is almost always the very first shot, which makes me suspect some component was still sleeping. It is more common to loose just one flash and power variation is even more common ... or to some degree it is the rule.

3) is indeed an issue as increase in cases with only two flashes is usually cured by battery change for the transmitter. Wireless controller is not really necessary - it is just convenience to manage the power setting. It would likely work with wired trigger and the wireless as power selector.

That leaves 4) and 5), which are the most likely causes for power variation. A delayed flash could do that too, but then it shouldn't be even over the frame, should it?

Besides, separating the head from the controller, would give so much more freedom for diffusor design... Wink


I edited and added 7 which also a consideration for variation in exposure.

I think a member Lou did separate the Speed-light head from the main body with good results.

The LED Controller should provide extremely uniform output in both time and exposure.

There are strobes that have been specifically designed for fast recycle and uniform & quick optical outputs. One such device is the Adorama Rapid 600 and Godox equivalent.

https://www.adorama.com/fplfrp600b.html?rfkref=productPage
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JKT



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PostPosted: Thu Jan 30, 2020 8:40 am    Post subject: Reply with quote

That could indeed have an effect, though the lamp comes on ~0.2 and the delay is 3.5 s. (1/64 setting) Come to think of it, the low setting is not helping at all. At 1/128 the variation is much worse.

Those studio flashes are just too big.

In any case, such development is way beyond me.
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mawyatt



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PostPosted: Thu Jan 30, 2020 9:20 am    Post subject: Reply with quote

Yes they are big and heavy!! I use a "C" stand with an 40" arm to position them up close to a light tent, works OK for me.

Just found a Cree CXB3070 that looks attractive, about $16 and produces over 10,000lm with 36V @ ~ 2A. The controller should handle this easily with higher voltage rated components.

Best,
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Lou Jost



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PostPosted: Thu Jan 30, 2020 11:46 am    Post subject: Reply with quote

Quote:
I think a member Lou did separate the Speed-light head from the main body with good results.


Yes, its easy to do and puts a vast amount of light in a small space. Doesn't help with flash variation though.
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mawyatt



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PostPosted: Thu Jan 30, 2020 1:17 pm    Post subject: Reply with quote

Lou Jost wrote:
Quote:
I think a member Lou did separate the Speed-light head from the main body with good results.


Yes, its easy to do and puts a vast amount of light in a small space. Doesn't help with flash variation though.


Lou,

Thought so!! Maybe my memory isn't that bad, sometimes my "selective" memory isn't that good tho Rolling Eyes

If these LEDs behave the way I expect (I've just studied some on the net), then this Current Mode Controller should produce a very uniform and precisely repeating timing optical pulse output without some issues common to Speed-lights and Strobes.

Now the quest is to find the optimum LED from a cost & output power standpoint, the mentioned Cree seems good. The controller design and PCB can be "tweaked" a little to optimize with the selected LED before committing to PCB fab.

Since the controller is removed from the LED element, this should have similar benefits as with removing the flash tube head from a speed light.

Best,
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mawyatt



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PostPosted: Thu Jan 30, 2020 8:03 pm    Post subject: Reply with quote

These Cree CXB3070 LEDs look very promising, with ~ 20 times the output of the CXB1304. We've modified the design to include the CXB3070 with a higher voltage supporting 36V at >2A using 63V rated capacitors, a pre-regulator to limit the higher voltage for some of the electronics and higher 60V rated NMOS output devices.

While thinking about how one might use this controller in a macro stacking application we realized that it might be useful to have a separate LED current control for continuous light similar to a modeling light on a strobe and another control for the peak LED current under pulsed conditions along with the pulse width control. This way you won't be fiddling with the controls when switching from continuous to pulsed and back again, just set the LED currents in each and switch back and forth without any adjustments.

The controller design now supports 4 higher voltage LEDs such as the Cree CXB3070 which provides ~10,000lm at ~2A 36V, and separate LED current control under continuous and pulsed conditions. The number of LEDs can be easily expanded beyond 4 by paralleling LEDs with a small ballast resistors (~1/2 ohm) for conditions where more LEDs are required.

We've just completed the initial design changes to incorporate the above, so getting closer to submitting the PCBs for fabrication.

Best,
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Lou Jost



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PostPosted: Thu Jan 30, 2020 8:07 pm    Post subject: Reply with quote

I was quite impressed by your memory, Mike!
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Saul



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PostPosted: Thu Jan 30, 2020 8:22 pm    Post subject: Reply with quote

mawyatt wrote:
These Cree CXB3070 LEDs look very promising, with ~ 20 times the output of the CXB1304. ....
The controller design now supports 4 higher voltage LEDs such as the Cree CXB3070 which provides ~10,000lm at ~2A 36V, and separate LED current control under continuous and pulsed conditions. The number of LEDs can be easily expanded beyond 4 by paralleling LEDs with a small ballast resistors (~1/2 ohm) for conditions where more LEDs are required.

Mike, great idea. But what about LED cooling ?
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