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Current limiting resistor?
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Current limiting resistor?

by hberg32 on Thu Feb 25, 2021 11:23 am

Are the IR LEDs able to survive without current limiting resistors because there are 4 of them? I'm putting together a smaller version with just 1 IR LED but the LED overheated and cooked almost immediately. Having only 1 LED, I omitted the PNP transistor and just have the output from the ATTiny85 go to the base pin of a 2N3904 via a 120 ohm resistor (a setup I swiped from the the diagram here: https://learn.adafruit.com/tv-b-gone-ki ... -1094910-1)

Henry

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Re: Current limiting resistor?

by adafruit_support_bill on Thu Feb 25, 2021 12:37 pm

The 4 LEDs each have their own drive transistor, and the battery pack can source well over 1A in short bursts.
The LEDs are pulling more than 100mA each when on - but don't overheat because they are only driven in short pulses.

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Re: Current limiting resistor?

by hberg32 on Thu Feb 25, 2021 1:12 pm

Would you expect the IR LEDs with this hardware design to melt if they were being kept on for too long?

I'm wondering now if I have a timing problem. If I put in a rando green LED from the box o' bits in place of the IR LED everything is fine and I can see it flicker (so the MCU is at least doing something). Swapping the green LED for IR leads to melted LED in seconds. So perhaps this means I have a problem with the clock setup which is causing the whole program to run slowly and leave the LEDs on for too long?

Do you by any chance have the command line params you use to flash the .hex file onto the ATTiny when you make the kits? After reading up on how to set the fuses I came up with the following but maybe I'm wrong: -Uefuse:w:0xff:m -Uhfuse:w:0xdf:m -Ulfuse:w:0xe2:m

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Re: Current limiting resistor?

by hberg32 on Thu Feb 25, 2021 1:21 pm

Oh, wait, nevermind, the fuse settings are in the makefile. Nice.

burn-fuse:
$(AVRDUDE) $(AVRDUDE_FLAGS) -B 250 -u -U lfuse:w:0xfd:m -U hfuse:w:0xdf:m

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Re: Current limiting resistor?

by adafruit_support_bill on Thu Feb 25, 2021 1:51 pm

The specs allow for a continuous forward current of 20mA - with peaks of up to 1A (<= 1% duty cycle) https://cdn-shop.adafruit.com/datasheet ... asheet.pdf

Since the forward voltage considerably lower than that of a green LED, the current will be considerably higher. With a 3v source voltage, you are going to see quite a bit more than 100mA.

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Re: Current limiting resistor?

by hberg32 on Thu Feb 25, 2021 3:33 pm

Ah, the penny dropped when you mentioned 3v, thanks. I'm powering the circuit on the breadboard from a USB programmer which must be feeding it 5 volts, not 3. And as it clearly states all over the project documentation 5v = bad things.

Thanks.

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Re: Current limiting resistor?

by hberg32 on Sat Feb 27, 2021 9:36 pm

I like to learn from my dumbidity so could you tell me if I'm on the right track with using a 3.6 ohm resistor in this case? I just read the page on KVL. So if the forward voltage drop of the IR LED is 1.4 and I want to allow the current to peak to 1 amp and my supply voltage is 5v, then the resistor needs to account for 3.6v. 3.6v * 1 amp = 3.6 ohm. Right?

Thanks,
Henry

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Re: Current limiting resistor?

by adafruit_support_bill on Sun Feb 28, 2021 7:20 am

Have another look at the table on page 3. Forward voltage is not a constant. It changes with the current. The Vf for those LEDs is 1.4v @ 100mA. At 1A, the Vf is 2.6v.

Also read the fine print regarding pulse width and duty cycle at those currents: "Pulse Width≦100μs ,Duty≦1%"

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