Electronics > Beginners

4017 IC Driving LEDs Using Only One Resistor

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HandyAndy:
Thanks, but how would that cause issues with the simulator?  If I Block Copy it breaks the simulation the next time i try to play it nothing works, but if I Copy to Clipboard and paste it then both circuits still work fine when in start the simulation.

Zero999:

--- Quote from: Audioguru on September 14, 2018, 01:04:43 am ---Using a single current-limiting resistor is bad because it causes all the LEDs that are turned off to have a reverse voltage and many LEDs have a maximum allowed reverse voltage of only 5V.

--- End quote ---
It depends on the LED. Old red LEDs should be fine with more than 5V of reverse bias, even 20V or so won't do any harm, especially if the current is limited. In this case you'll get the supply voltage, minus one LED forward voltage drop, which will be about 7V, for a 9V supply and shouldn't be a problem with red LEDs. On the other hand more modern blue LEDs are much more sensitive and can be destroyed by reverse bias. In this case it would give 5V to 6V of reverse bias, which will be on the upper limit of what a blue LED can stand.

mikerj:

--- Quote from: ArthurDent on September 14, 2018, 12:52:41 am ---In the single resistor version when Q0 goes high the anode of the 1st LED goes close to Vcc and the cathode goes to around Vcc - about 2 volts, depending on the LED, and this voltage is fed through all the other LEDs (which act as forward biased regular diodes) to the 4017 with most output pins at near zero volts, which isn't good. Use the first version.

--- End quote ---

The LEDs on the non-active outputs will be reverse biased, not forwards.

ArthurDent:
mikerj - "The LEDs on the non-active outputs will be reverse biased, not forwards."

You are absolutely correct. I shouldn't be posting late at night. Thanks for catching that-I have corrected my original post.

Audioguru:
How can your simulation program work when your schematic shows no supply voltage?

Years ago I made some chasers using 74HC4017 ICs, Hewlett Packard 3.2V blue and bright green LEDs and powered from 6V. A problem was caused since only one current-limiting resistor was used for 10 LEDs. Some of the LEDs leaked current when they were reverse biased (only 2.8V of reverse bias) causing some of the other LEDs to dimly glow when they should be turned off.

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