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### Topic: 74HC595 Current Draw? (Read 15391 times)previous topic - next topic

#### scswift

#30
##### Mar 03, 2011, 07:02 pm

Quote
Maybe the heat dissipates faster as the led heats up.  Or maybe it's linear.  Who knows?

Newton that's who.

Newton's law of cooling states the rate of loss of heat is proportional to the excess temperature.

I see.  So the hotter something is relative to the air around it, the faster it cools.  But as it cools, the difference between the ambient temperature an the object which is being cooled changes, so the rate of cooling slows down.

#### mechengr

#31
##### Mar 03, 2011, 07:14 pm

If you look at the datasheet an LED, it will tell you the max current you can put through them continuously, but they will also often specify a much larger amount of current that you can put through it if you use a certain duty cycle.  I think one I looked at said something like you could put 120mA through, but only if the led was on for 1/1000th of the time or something.

Yeah, I saw that in one of the datasheets, but I am not sure how I can use that to my advantage here.  I mean, I could arbitrarily set a current to get a higher brightness, but I run the risk of blowing LED's.  I really don't want to re-build a 12x12x12 cube.. almost 1800 LEDs!

Another thing to consider about heat dissipation is that you can't just say I'll have my LED on for 8% of the time, because that doesn't tell you how long the led is actually on for.  If you have a 50% duty cycle, is the led on for 1ms and then off for 1ms, or is it on for 16ms and then off for 16ms?  With the former it might be okay to put 60mA into the LED because it won't heat up fast enough to blow.  But with the latter it might get too hot.  Both are 50% duty cycles though.

Well, let me run the calculation real quick.  If I am shooting for a refresh rate of approximately 240 Hz, then the entire cube will be refreshed every 4.16 ms.  That means, each layer (and each LED) will be on for 0.347 ms and off for 3.82 ms (8% duty cycle).

I think I might go hook up a PIC later and just simulate the layer multiplexing with a few LED's.  See how the brightness changes with refresh rate.

I think the problem is people buy LEDs from crap suppliers that don't supply data sheets.
Here's  a spec for a part from superbrightleds.com
Has everything you need to know.

http://www.superbrightleds.com/cgi-bin/store/index.cgi?action=DispPage&Page2Disp=%2Fspecs%2Fw18015_specs.htm

>> Continuous Forward Current IF 30 mA  <<
>> Peak Forward Current (1/10th duty cycle, 0.1ms pulse width) IFM 100 mA <<
>> Reverse Voltage VR 5 V <<
>> Forward Voltage VF 4.2 3.7 V IF=20mA <<
>> Luminous Intensity IV  18000 mcd IF=20mA  <<

If one goes out & buys a bag of 1000 LEDs from e-bay from China, what do you get?  There-in lies the problem. So you are left to do some testing on your own and see what the parts will tolerate.
Write a loop that puts out pulses as specified, put in a variable resistor to control current flow, ideally put in a shunt resistor so you can a scope across it for accurate current measurement, and see what is achievable.

And that's the situation I am in right now.. no data sheets available other than what the supplier told me (following).  I am trying to get a data sheet out of him, but I figure it's unlikely.
Emission Color: Blue
View Angle: +/- 120 degrees
Forward Voltage 3.0V - 3.3V (Max 3.
Peak Emission Wavelength: 465nm
Luminous Intensity: 5000 mcd
Reverse Current: 20 mA
Other: Low Power Consumption

The reverse current bit confuses me... unless he means forward current?  He's got a return policy though, so if they don't work, I can return them.

Quote
Maybe the heat dissipates faster as the led heats up.  Or maybe it's linear.  Who knows?

Newton that's who.

Newton's law of cooling states the rate of loss of heat is proportional to the excess temperature.

This is only directly true for conduction heat transfer, not for convection.  Convection heat transfer is largely the primary means for heat transfer in this case and the equations get very messy.  Your convection heat transfer coefficient varies with temperature and is non-linear.

As a general rule of thumb though, the hotter something is, the higher the heat transfer rate .  I don't think anyone will argue with that.

Quote
for a multiplexed 5x5 matrix, each LED will only be on for 4% of the time

That is only true if you only turn one LED on at a time, LED cubes don't do this. When you are multiplexing you usually turn on a whole row or for a cube a layer. This makes the on time much longer.

Correct, I was just using it as an example.  If a 5x5 matrix LED has an on-time of 4%, how come they can still look pretty bright?  I was just addressing the concerns of LED brightness with a layer on-time of 8%.

#### MarkT

#32
##### Mar 03, 2011, 07:24 pm
The reason you can put high currents for short periods of time is to do with the thermal circuit - the path heat follows to escape the LED chip and reach the ambient air.  The chip itself has a certain heat capacity (amount of heat it takes to raise its temperature per unit mass).  So if you run it at a high current it will simply heat up until it gets too hot.

However on a small duty cycle you stop the current and the chip gets a chance to pass its heat to the substrate its mounted on (the rate of which depends on geometry and the thermal conductivity of the chip and substrate).  Thus it cools down enough in time for the next pulse of high current.

The whole setup acts a bit like a low-pass filter - on average the LED produces the net heat dissipation but internally the chip is heating and cooling on every cycle.  What matters is the maximum temperature of this cycle.  Both the duty cycle and its frequency affect the maximum temp.

So long as you keep the current on for a short enough time you can put a lot more current than the steady state.  However eventually other mechanisms limit the current (bonding wires melt - this depends on the mean squared current).  And thermal cycling causes mechanical stresses too, which can affect device reliability and lifetime.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### mechengr

#33
##### Mar 03, 2011, 07:29 pm

The reason you can put high currents for short periods of time is to do with the thermal circuit - the path heat follows to escape the LED chip and reach the ambient air.  The chip itself has a certain heat capacity (amount of heat it takes to raise its temperature per unit mass).  So if you run it at a high current it will simply heat up until it gets too hot.

However on a small duty cycle you stop the current and the chip gets a chance to pass its heat to the substrate its mounted on (the rate of which depends on geometry and the thermal conductivity of the chip and substrate).  Thus it cools down enough in time for the next pulse of high current.

The whole setup acts a bit like a low-pass filter - on average the LED produces the net heat dissipation but internally the chip is heating and cooling on every cycle.  What matters is the maximum temperature of this cycle.  Both the duty cycle and its frequency affect the maximum temp.

So long as you keep the current on for a short enough time you can put a lot more current than the steady state.  However eventually other mechanisms limit the current (bonding wires melt - this depends on the mean squared current).  And thermal cycling causes mechanical stresses too, which can affect device reliability and lifetime.

While I am sure there is a way to model this maximum thermal loading, I assume the best bet is simply to play with it and see what works or do you have any recommendations?

#### MarkT

#34
##### Mar 03, 2011, 08:35 pm
Read the datasheets for the LED in question - or if not available datasheets for LEDs of similar construction.  In general at high frequencies you can get away with a lot more.  If the LEDs are cheap you can afford to experiment!
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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