What is duty cycle? Also, this is not for human eyes, just for photography.

 

What is duty cycle? Also, this is not for human eyes, just for photography.

Duty cycle is the ratio of on time vs. off time.

A 10% duty cycle means 1mS of each 10mS, 1mS on, 9mS off. That is for a 100Hz pulse rate.

For different frequencies, the times would change, for example a 2kHz pulse rate (for 500 μS) means for every 500μS on, 4.5mS would need to be off. Or, the LED can only be on for 1 in 10 cycles at a given frequency.

 

Duty cycle is the ratio of on time vs. off time.

A 10% duty cycle means 1mS of each 10mS, 1mS on, 9mS off. That is for a 100Hz pulse rate.

For different frequencies, the times would change, for example a 2kHz pulse rate (for 500 μS) means for every 500μS on, 4.5mS would need to be off. Or, the LED can only be on for 1 in 10 cycles at a given frequency.

I'm still confused... If I'm only using a single pulse, what does any of that matter?

 

you said you want to light 4 of those star type LED's, ur supply is rated at 12V but you did not tell me how much current ur supply can hold. Or are you using batteries.

For how long do you need them to light ?

 

I'm not trying to be an ass, but honestly, why don't people read other posts?

I am using a 12V battery supply at the moment and I only need to light them for a maximum of 500 microseconds, usually even less.

I can easily get a higher voltage supply, but at the moment that is what I have.

 

I must have missed that...aaaand you dunno who I am and where I am ?

Anyways.
So tell me what components do you have ?

Do you like to drive the LED's via a current source or any other method will do ?

Do you want the monostable to be retriggerable or just timed ?

 

I'm still confused... If I'm only using a single pulse, what does any of that matter?

You said it was for a flash. I didn't know if you were using a still camera or were synchronizing with a video camera for high speed videos. We get requests for both often here.

If it is for slow photography/under 10 frames per second, you are at a fractional percent duty cycle, and won't run into any problems. It was a precautionary question is all to avoid having it melt down later because we forgot to ask a question.

 

I must have missed that...aaaand you dunno who I am and where I am ?

Anyways.
So tell me what components do you have ?

Do you like to drive the LED's via a current source or any other method will do ?

Do you want the monostable to be retriggerable or just timed ?

Right now, I have a 555 monostable circuit, firing at 5 μs, triggered by a signal from my camera.

I'm not sure what you mean by "via a current source". If you mean by batteries or mains, I need this to be from a battery source.

Uhh.. I'm not sure what me knowing who or where you are has anything to do with what we've been talking about. Maybe I missed something.

You said it was for a flash. I didn't know if you were using a still camera or were synchronizing with a video camera for high speed videos. We get requests for both often here.

If it is for slow photography/under 10 frames per second, you are at a fractional percent duty cycle, and won't run into any problems. It was a precautionary question is all to avoid having it melt down later because we forgot to ask a question.

Sorry, when I think of a flash I imagine and camera flash, not a constant source of light that would be needed for video.

 

Right now, I have a 555 monostable circuit, firing at 5 μs, triggered by a signal from my camera.

Does it work? Buy a bag of 2A fuses and see if you get enough light out of it, that's one way of going about it for a quick and dirty test.

 

Does it work? Buy a bag of 2A fuses and see if you get enough light out of it, that's one way of going about it for a quick and dirty test.

It all worked with a 200mA 10mm LED and a .5 watt high power LED. I don't have the 1A LEDs yet, they are still in the mail on the way here.

 

Aah ...I see the problem.

U are trying to make a Flash using those star LED's and adding more to make a bright flash.

What the members trying to tell you is that a LED driven with a μs pulse won't give you the flash that you need even if you add more LED's or drive them with 2 or even 10Amps of current.

A flash is ignited with high voltage and a LED won't Flash that quick.

But you can trigger the LED and give 500ms and check if the light output suits ur need.

This has to be a trial and error run kinda thing, u know.

 

Aah ...I see the problem.

U are trying to make a Flash using those star LED's and adding more to make a bright flash.

What the members trying to tell you is that a LED driven with a μs pulse won't give you the flash that you need even if you add more LED's or drive them with 2 or even 10Amps of current.

A flash is ignited with high voltage and a LED won't Flash that quick.

But you can trigger the LED and give 500ms and check if the light output suits ur need.

This has to be a trial and error run kinda thing, u know.

I realize that I need more time to get the full output from LEDs, and at the same time, I know that this can be done. The circuit I had made with just one tiny .5 watt LED put off a substantial amount of light for being only 200mA at 5 μs. Unless all LEDs will put off that same amount of light in that time period, then I can do this with more powerful LEDs.

 

So ..what do u need to know now ?
Put in simple terms.

 

So ..what do u need to know now ?
Put in simple terms.

I need to know how to power these LEDs at 2A each... I was pretty sure that was what the original post was all about.

When I did the math I got ridiculous numbers. Such as needing a 70 watt resistor. I didn't know if there was a better way to do it.

 

The 70W resistor would be in conjunction with using 2A continual current.

If you are taking one photo per second, leaving the LED on for 1mS of each second, then the resistor only needs to be physically large enough to not explode when the current pulse gores through it.

Pulsed power will probably be easier for you to tune by trial and error rather than calculations which require calculus.

Use the resistor value that was suggested for the 2A scenario, but add a 3A fuse, and make the resistor 2W instead. It will have plenty of cooling down time between pulses. The reason 70W resistors are so bulky and expensive is cooling fins so they don't overheat with the load. Since your load is fractional percentage duty cycle, there isn't that much power to dissipate.

 

The 70W resistor would be in conjunction with using 2A continual current.

If you are taking one photo per second, leaving the LED on for 1mS of each second, then the resistor only needs to be physically large enough to not explode when the current pulse gores through it.

Pulsed power will probably be easier for you to tune by trial and error rather than calculations which require calculus.

Use the resistor value that was suggested for the 2A scenario, but add a 3A fuse, and make the resistor 2W instead. It will have plenty of cooling down time between pulses. The reason 70W resistors are so bulky and expensive is cooling fins so they don't overheat with the load. Since your load is fractional percentage duty cycle, there isn't that much power to dissipate.

If I have 4 of these 2A LEDs, how should I arrange them? If I had a 24V source could I just hook them all up in series, so I only need 2A rather than 4 or 8? They voltage drop ranges from 3.9-5.4V at 25 C.

 

If I have 4 of these 2A LEDs, how should I arrange them? If I had a 24V source could I just hook them all up in series, so I only need 2A rather than 4 or 8? They voltage drop ranges from 3.9-5.4V at 25 C.

That would be one way to start, just be sure to have some sort of resistor to limit current. Once Vf of an LED is met, it essentially "looks like" a short circuit, and so the current must be limited once it is lit. Though for the very tiny duty cycle, you have more room to play on the amount of resistance side of things.

I'd put a 100Ω, 2W resistor in series with each LED and a 6V lantern battery(Always handy to have around, especially if you get the alkaline w/screw terminals kind!) to measure the 1.2A Vf of Each Separate LED and write it down/color code them/whatnot.

See if they are all similar (±0.05V), if they are, a series string would be fine. If they aren't, they'll be different brightnesses if put in series, and you'll probably notice that during the test as well.

 

See if they are all similar (±0.05V), if they are, a series string would be fine. If they aren't, they'll be different brightnesses if put in series, and you'll probably notice that during the test as well.

I don't think a different forward voltage changes the brightness, it is the amount of current that determines the brightness. In series, they all have the same current. Then if they are physically well made they will all have the same brightness.

 

You cannot put 2A into that LED.

You will get around few flashes before the LED gives up.

Putting more current just kills the LED. They are not made of wires u know.

To get the full brilliance all you need is to pulse it long enough with the specified current which is ≈ 1000mA = 1A at rated Voltage.

For them to efficiently flash at 1A, you need a current source to power the LED. Drive the LED with a fast MOSFET driven by a low gate resistance with a 500ms first to see u get enuf light. then shorten the pulse.

Using Current source u don't need any series resistor. You need just one source of 1 Amps. and a voltage of around 4 li-ons.
Which will give you the desired voltage for four LED's in series.

4 X 4.2V = 16.8 Volt.
You can power the current source with this and the LED's.
Just put a 12V Zener and a resistor to give power to the 555.
12V is enuf to drive a MOSFET to full ON

 

For 2µS he can use 2A. Even 2ms he could use 2A. The problem is when it is stuck on, then the LEDs would blow.

Are you asking for a starting schematic? Like one of the other guys said, you can tweak it later.