Hello
I've been working seven segment display using cut LED strip. I used ULN 2803 to drive those segments, however, when i keep it on for a longer period of time ( 2hrs), the ULN 2803 got burned. It seems that my driver can"t handle the load. Is there any other LED strip driver can i use, aside from MOSFET, to handle such high current load?
Thanks a lot.
Such high current load =
You can try ordinary transistors, not Darlingtons as in the ULN chips.
How do you limit the current through the LEDs?
Hi,
Can you link us to the LED strip, what voltage is it?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Thanks.. Tom.... 
Are the strips powered by 12V?
How many leds in each segment?
Tpic6b595 may be a better solution, depending on your answers.
on my board i have 6 74HC595 and 6 ULN2803. each ULN2803 drives 7 cut LED strips with 6 LEDS at each strip. At first the display works fine and going well for more than an hour but after almost two hours , the ULN 2803 begins to break down. All my LED strips are powered by 12 V 5A. When i replaced all burned ULN2803 it works perfectly fine again.
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I have a second thought of using MOSFET , but i am also considering the space which can be occupied, imagine i have to solder 70 pcs of MOSFET to drive all those strips.
Tpic6b595 is similar to 74hc595 but with 8 MOSFET built-in. Replace your 74hc595 with tpic6b595 and you will not need the uln2803.
HOWEVER...
At 6 leds per segment, the current for each segment should only be about 40mA. Even if all 7 segments are lit, this is only 280mA, which is well within the capability of the uln chip. It should not be getting burnt. There must be something else causing the problem. Post your schematic please.
what LED?
need the data sheet.
it might be easier to change the LED than to change the driver.
PaulRB:
Tpic6b595 is similar to 74hc595 but with 8 MOSFET bult-in. Replace your 74hc595 with tpic6b595 and you will not need the uln2803.HOWEVER...
At 6 leds per segment, the current for each segment should only be about 40mA. Even if all 7 segments are lit, this is only 270mA, which is well within the capability of the uln chip. It should not be getting burnt. There must be something else causing the problem. Post your schematic please.
he said he cut LED strips.
dave-in-nj:
he said he cut LED strips.
Yes. It is a 12V strip, the strip can be cut after every 3rd led because trios of leds are wired in series, with a resistor to limit the current to 20mA. So each segment of 6 leds should draw 40mA, and a digit should draw 280mA when displaying "8".
How did you interpret the OP's description of the circuit, Dave?
Hi,
Your circuit diagram does not show how you have gnds connected and also you should show pin numbers.
Can you post a picture of your project.
Can you please post a copy of your circuit a picture of a hand drawn circuit in jpg, png?
Trying to draw using paint is not easy.
Have you got a DMM to measure how much current is circulating in each of the outputs of the 2803 please.
Thanks.. Tom..
ok guys thanks for all your suggestions and comments... i will try to measure the parameters you asked. Now my project is running.. has a display of , from left to right, 1 1 1 and second row 1 1 1. start time: 3:00 pm. Im now observing again if its one ULN 2803 is heating up.
The reason i used the ULN 2803 is simply because of its compactness to drive 7 LED cut strips and it can handle 500 mA of load.
Thanks
How have you connected the COM pin (pin 10) of the ULN chips? It should be connected to 12V or left unconnected. Don't connect it to ground!
All the grounds are tied together. Means, arduino ground, 12V source grnd, 74595 grnd, uln 2803 ground, all tied together.
thanks
The com pin (10) of all ULN 2803 are all left unconnected.
PaulRB:
Yes. It is a 12V strip, the strip can be cut after every 3rd led because trios of leds are wired in series, with a resistor to limit the current to 20mA. So each segment of 6 leds should draw 40mA, and a digit should draw 280mA when displaying "8".How did you interpret the OP's description of the circuit, Dave?
based on the sketch the OP posted,
each LED uses 20mA, x 6 = 120ma per segment.
a 7 character digit would use 7 x 120 = 840 ma
but before I start offering that a typical ULN2803 package [ channel ] can handle about 500mA, I would want the OP to post exactly what strip he is using.
superbrightled rates a 5050 led strip with 60 LEDs for 808mA for 60, or 80.8mA per group of 6
80.8 x 7 is 554 ma and [ not close to the threshold of overheating the 2803 package if they are all on at the same time.
assuming a scoreboard showing all 0's is only 6 segments, x 80.8 = 498mA,
both cases would lead to failure over time, exactly as the OP has listed. [ only valid if this is a package not channel ]
if the strip properly attached to a heat sink ? what happens to that exact LED when it over heats ?
generically, just using larger drivers can fix some problems, however, I like to do the math using the data sheets.
[ did we confirm the ground between the 12v and the Darlington ? ]
[ max out of the 74HC595 is 70mA, ]
A confusing post, Dave! To me, never mind the OP.
ULN2803 is rated for 500mA per channel. That's fine if only one channel is sinking current at a time. But if more than one channel is active at once, the limiting factor becomes the max power dissipation of the chip. Have a look at @grumpy_mike's page on the subject here. Even without a heatsink, it should be able to handle 550~750mA in total.
If your estimate of 840mA is correct, it would explain why the chips are getting burnt. But I can't see how or why the current would be that high. In all the 12V strips I have seen, the leds are grouped into threes and each group is wired in series. So each group of 3 leds uses 20mA, not each led. Even if that 20mA where in fact 40mA, each digit would need 560mA, and the ULN chip should be OK.
So... @jimregister595, we really need you to measure the current flowing through a segment!
Thanks guys for the comments. I took one segment having 6 LEDS and measured around 90mA. I tried 2 segments ( parallel connected) and showed around 200 mA. So with this data, i pressumed that when all the segments are lit ( showing 8) probably the total current would be 200 mA x 3.5 = 700 mA roughly. If ULN 28003 can handle only a maximum of 500 mA, im pretty sure it would get burned. I just dont know if my analysis was correct.
Thanks for your help.
