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Topic: 74HC595 and LEDs (Read 1 time) previous topic - next topic

kustom

Really simple question, I was reading through this thread, and I'm still not sure what is the answer to this:
is it safe to control 8 leds using this chip, seeing the max current 74HC595 can provide is 70mA and 1 LED uses about 20mA?

Grumpy_Mike

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is it safe to control 8 leds using this chip, seeing the max current 74HC595 can provide is 70mA and 1 LED uses about 20mA?

No.
8 * 20 is 160. This is > 70.

takao21106

This is the rating for the continuous current.
If you use 6x multiplex, it's only 16.6% modulation.

A trick is to use 4 LEDs on the H drive and 4 on the L drive.

At 3 volts the currents are lower than 20mA, so it can work 50% or 100%.
I also have PIC related web domain.

takao21106

there are 10 to 20 factors that limit current, otherwise the IC (Arduino) would really use 3 Amps, using 10 pins shorted to ground. It would pop up instantly. There are limiting factors but different from using LEDs, shorting too many I/O will destroy most smaller controllers.

The lowest gap, 0.5 volts, has really large increase in current. What becomes the limit then are the MOS channels, to the current distributes. They work like resistors and that's OK for them. 120 ohms, but i do not know it can change due to voltage. not a ohm resistor.
I also have PIC related web domain.

Grumpy_Mike

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This is the rating for the continuous current.
If you use 6x multiplex, it's only 16.6% modulation.


But much simpler is to increase the resistor so that the 75mA is spread over the 8 LEDs, so that is just about 9mA per LED.

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At 3 volts the currents are lower than 20mA, so it can work 50% or 100%.

This is just nonsense, you can't run these chips at 3V from a 5V arduino without level shifting. Please remember this poster is a beginner, describing half baked answers is not helpful. The brightness of an LED depends on current not voltage.

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there are 10 to 20 factors that limit current, otherwise the IC (Arduino) would really use 3 Amps

That is simply not true.

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The lowest gap, 0.5 volts, has really large increase in current. What becomes the limit then are the MOS channels, to the current distributes. They work like resistors and that's OK for them. 120 ohms, but i do not know it can change due to voltage. not a ohm resistor.

This is so poorly expressed that it makes no sense at all.

takao21106

Yes it's poor wording from a poor Windows programmer soul.

I have good news: use 4x 2.2k Ohm resistors is OK for modern clear LEDs.

So the OP has no need to bother about my precautions. Think Grumpy_Mike many other people will read this thread. So I only add relevant additional info.

The Answer to the OP realy is: Yes it's safe for my purposes. If you know what can go wrong, it can be safe. If you don't have the LED datasheet, or a beginner, IT IS NOT SAFE.

You see Grumpy_Mike it's difficult to give a valid answer, unless you think backwards, only to satisfy people who are new to this technology. Is this a teacher or school technique?
I also have PIC related web domain.

takao21106

I mean Grumpy_Mike, I use 2.2k resistors for each LED, and 4x switching.

So my circuit is totally on the safe side at 5 volts! Guarantee.  :smiley-sweat:

I'm happy that I can know it's total correct circuit.
I also have PIC related web domain.

kustom

To sum it up, it's safe to use 8 leds with larger resistors, like 500ohm or 1k?

Grumpy_Mike

Yes keep the current down to 9mA and it will be OK. The actual resistor value will depend on the voltage drop of the LED.

kustom

Thanks. Only thing I don't understand is why they are recommending this chip together with 220Ohm resistor here then: http://www.arduino.cc/en/Tutorial/ShiftOut

Makes no sense.

Grumpy_Mike

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Makes no sense.

A lot of those tutorials contain errors but try as we might all requests to change them go unheeded.

You might also notice the 1uF capacitor on the latch line. This must not be fitted, it will damage your arduino if you do.
Put a capacitor between the +5V pin and ground if you get flickering problems.

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