LED current in a Matrix

R

RonH

New Member
I have an 8X8 LED matrix, since it is multiplexed the output to the LED's is technically a PWM signal and therefore dimmer than 100% on.

I'm running each LED row through a 330 ohm Resistor @5v (15ma), I'm going to assume a max of 20ma constant current since I have no data sheet for this matrix.

My question is, since the LED is off 87.5% of the time can I safely increase the current beyond 20ma to brighten the LED without blowing it? Is there a formula to figure out a safe range for a LED driven by PWM?

Ron
 
You could safely increase the peak current some above 20mA for a PWM drive, but the increase in brightness may not be proportional. The brightness of an LED tends to saturate above its maximum current, so the brightness tends to flatten above that point. You can try it and see. For large duty-cycles, the maximum current would be 20mA divided by the duty-cycle. For small duty-cycles I would not increase the current above about 50% of its maximum, or 30mA.
 
In that situation, be mindful that if your mux circuitry has the possibility of stalling, you could push far too much current thru the LED's, damaging or destroying them, if it were to happen.
 

most 20mA LEDs are rated for 50mA pulsed, some for 100mA. in our muxed LED displays we usually drive them with 40-60mA, mind you i am doing LED matrices of 500 to 1000 LEDs

dan
 
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The datasdheet for most LEDs lists its max allowed peak current and its max allowed duration for each pulse. It also shows a graph showing the loss of efficiency at high currents.
 
Thanks for all the info, in my testing I will stick to 20ma and after I prove it works I'll play with a lower resistance since I will be stopping my program from time to time I don't want to blow the LEDs.

I would read the Datasheet if I had one but like I mentioned I don't have it and can't find it on the net. I bought these 16 modules at a surplus store.

Ron
 
You must guess at the spec's of surplus junk.
Maybe you will win and not destroy the LEDs.

A lower resistance (in series with the LEDs to limit the current) will increase the current which will destroy the LEDs.
 
audioguru said:
A lower resistance (in series with the LEDs to limit the current) will increase the current which will destroy the LEDs.
Click to expand...

I know this, after I have finished my programming I won't be stopping the program anymore and I can alter the current without worrying about blowing the LED's.

I did a little searching and I've noticed that Fairchild sells some modules that look similar, maybe they are the ones that make these. I looked at other data sheets they have and they state continuous current 30ma and peak 200ma 1/10 10kHtz.

Ron
 
I think what Crutschow said should be re-iterated.
RMS current with an LED does NOT mean RMS brightness. LED's are incredibly non-linear, they're diodes after all =)
It might be safe to pulse an LED beyond it's max rated current as long as the junction temperature is safe, however the efficiency of the array will suffer drastically, especially at the 'high' end of it's specs.
 
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actually the efficiency depends a lot on the diode... the old ones were very inefficient but intensity was pretty linear with current, the newer stuff peters off with higher current densities.

the flip side of the coin is that the eye is not a linear sensor... it responds to the peak and then has a bit of a memory - the end result is that the perceived intensity tends to be brighter than the actual average so the "efficiency" is higher than you would expect. it is more for higher current densities so it is less effective than it used to be.

dan
 
RonH said:
II've noticed that Fairchild sells some modules that look similar, maybe they are the ones that make these.
Click to expand...
Do you gamble? Same thing.
Maybe the cheap Chinese LEDs will survive only 20ma or they won't. The manufacturer has your money and will not chase you into his far away weird country.
 
audioguru said:
Do you gamble? Same thing.
Maybe the cheap Chinese LEDs will survive only 20ma or they won't. The manufacturer has your money and will not chase you into his far away weird country.
Click to expand...

I understand that there is a limit to the brightness, you can't just increase the current and expect the brightness to increase as well but... if I can get a little more out for just a few ma more then I'll try it.

Do I gamble? sure why not, this is only a hobby project not something I'm going to mass produce. I have a module that has 1 bad element, I will use this as my guinea pig, I can't harm it more since it's already useless.

I'm also attempting to increase my network speed (I2C) so that I can dwell longer and increase my scan rate.

Ron
 
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