Multiplexing - Brightness

[Sceadwian]

I've always been curious how to calculate the maximum pulse for an LED though Nigel. I mean how much peak current can you really toss through a common white LED if you can pulse it fast enough? I mean if you can calculate the maximum energy the junction can withstand you could charge a small capacitor up to a very high voltage for some insane pulses. Something I've always been curious about, I just don't know what the real limits are. I got a bag full of LED's would be fun to see what the highest power pulse I could get out of one is for strobe photography.

 

[Pommie]

I've always been curious how to calculate the maximum pulse for an LED though Nigel. I mean how much peak current can you really toss through a common white LED if you can pulse it fast enough? I mean if you can calculate the maximum energy the junction can withstand you could charge a small capacitor up to a very high voltage for some insane pulses. Something I've always been curious about, I just don't know what the real limits are. I got a bag full of LED's would be fun to see what the highest power pulse I could get out of one is for strobe photography.

Elektor did a project along these lines **broken link removed**. I can't remember the details but I seem to remember it discharged a capacitor through the LEDs to get a large current for a very short period.

Mike.

 

[Sceadwian]

I got nothing to do today, a bag full of LED's to burn and a flash charger that will go up to 300 volts. Maybe I'll play around some =) That ring flash looks very nice, I'll book mark it. It's happening again though =O I get so any bookmarks from this website you should see what I have...

 

[3v0]

I was about to look at the article but it requires 10 Elektor Credits.

Do you get credits when you register with their site or do you have to purchase them? Sounds silly because I could register and find out, but I would rather not.

Silly me

Register now for ELEKTOR.com – the world-leading network for electronics and receive 10 free Elektor Credits. Subscribers to the magazine get 30 Credits! Elektor Credits allow you to download a pdf version of any magazine article. After successful registration, you also have write/reply/create topics rights on our forum as well as access to the news archive.

 

[Pommie]

Just found the magazine and the unit uses a HV unit from a disposable camera. The unit contains a 160uF capacitor that's charged to 300V. The cap is discharge into 32 white LEDs with a 470Ω 5W series resistor. From the text it states that 10 1/100th of a second flashes can be fired before you have to worry about recharging the capacitor.

Mike.

 

[Sceadwian]

I was thinking more along the lines of a couple tens of micro seconds for a strobo scope like app.

 

[DanskEl]

It's not JUST average current Nigel, you pulse too much current through a P/N junction the junction will vaporize outright, the heat can't dissipate fast enough, even if the average is within it's tolerances. But I didn't know remotes used 1 amp pulses. I've always been afraid to do anything over 200ma's for any LED, but I believe ya =) If the transistors can handle it it's the easiest sollution.

Latching shift registers is the other, but that requires more IC's. They're pretty simple Dan, you shift bits into the registers one at a time via a clock line, and then when you trigger a certain I/O line those bits are all pushed out onto the IC So you can update a theoretically infinite number of I/O lines by chaining them together, it's limited only by the speed you can clock the serial data in. I dug around on Digikey and found this one.
https://www.electro-tech-online.com/custompdfs/2009/03/397-1.pdf
But that's just the first one I found.
For 70 elements (You're doing 10 7 segments right?) you'd need 9 of them.
Which would put you out all of 5 bucks total.

Re-reading some of the posts I got back to this one. This idea sounds more or less like placing a seperate 4511 for each 7-segment display and removing the resistors. We could then use the latch in the 4511 to make sure they're turned on all the time, thus elminating the dim LED's.
If none of the other methods work we'll probably end up doing that.

 

[dougy83]

Sir you add 7 NPN transistors to the output of the 4511 (via Base resisters) & give Emitters as outputs to the SSDs & Collectors will goto VDD via separate current limiters.

No base resistors required here.