555 driving two decade counters

Thread Starter

quadhed

Joined Jan 13, 2016
29
Does anyone know if a 555 chip can power two 4017 decade counters in parallel? I tried this and, for some reason after a brief time the 555 was fried.
 

jbeng

Joined Sep 10, 2006
83
Ordinarily, a 555 could likely do this with ease.
Please attach a schematic showing exactly how your circuit is connected. That would go a long way in helping us to help you figure out what is going wrong with your circuit.
 

Audioguru again

Joined Oct 21, 2019
4,536
An NE555 or LM555 is normally used to clock a CD4017, not to power it. They can both use the same power supply.
A Cmos 555 (LMC555, TLC555 or ICM7555) can also clock a CD4017 and use the same power supply.
 

AnalogKid

Joined Aug 1, 2013
9,965
I initially didn't include a schematic because I didn't see the need.
Really - ? The word "Circuits" is right there in the title.

There are several errors in the schematic.

R6 is in series with the U3 GND connection. The chip will not function reliably.

There is no current limiting for the lower left group of LEDs.

There are several places where you have two LEDs in direct parallel. This does not work well, because no two devices are perfectly matched. You always will have one device hogging current away from the other to some degree. One way around this is to connect each pair of LEDs in series. This might work, but we do not have any information about the LEDs you are using. Please post the datasheet.

Because the circuit is powered by 12 V, there is around 10 V of reverse bias across the off LEDs. The reverse breakdown voltage varies depending on the part, but it usually is less than this. This can damage these parts because the current through them has no current limiting resistor in series. White and blue LEDs are particularly delicate in this regard.

The 4017 does not care about the input clock pulse width or duty cycle, so having D1 in there to make a narrow output pulse is not needed.

Your circuit requires that the CD4017 outputs supply over 30 mA to the LEDs. As stated in the datasheet, this is way above its rated output current.

Add decoupling capacitors across each chip's power pins.

And - the two 4017 circuits appear to be identical. If this is the intent, you would be better off to have only one 4017, and add driver transistors to its outputs to handle the LED current. A common part for this is a ULN2004. It has 7 darlington transistor driver stages capable of 500 mA.

ak
 
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Audioguru again

Joined Oct 21, 2019
4,536
I agree that the parallel LEDs should be in series. A series diode should be added to the LEDs to protect them from damage by reverse voltage.
Notice that you have two outputs on each CD4017 shorted together to make smoke. Use diodes to separate the outputs.
Your schematic had tiny parts and text very spread out. I cropped the parts closer together and enlarged the schematic. I fixed R6.
 

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Thread Starter

quadhed

Joined Jan 13, 2016
29
Really - ? The word "Circuits" is right there in the title.

There are several errors in the schematic.

R6 is in series with the U3 GND connection. The chip will not function reliably.

There is no current limiting for the lower left group of LEDs.

There are several places where you have two LEDs in direct parallel. This does not work well, because no two devices are perfectly matched. You always will have one device hogging current away from the other to some degree. One way around this is to connect each pair of LEDs in series. This might work, but we do not have any information about the LEDs you are using. Please post the datasheet.

Because the circuit is powered by 12 V, there is around 10 V of reverse bias across the off LEDs. The reverse breakdown voltage varies depending on the part, but it usually is less than this. This can damage these parts because the current through them has no current limiting resistor in series. White and blue LEDs are particularly delicate in this regard.

The 4017 does not care about the input clock pulse width or duty cycle, so having D1 in there to make a narrow output pulse is not needed.

Your circuit requires that the CD4017 outputs supply over 30 mA to the LEDs. As stated in the datasheet, this is way above its rated output current.

Add decoupling capacitors across each chip's power pins.

And - the two 4017 circuits appear to be identical. If this is the intent, you would be better off to have only one 4017, and add driver transistors to its outputs to handle the LED current. A common part for this is a ULN2004. It has 7 darlington transistor driver stages capable of 500 mA.

ak
Hello. Thank you for your response. I meant for the 4017's white 3V 20mA leds to split the 30 mA between the two in parallel. I've read that the 4017 can source only 25mA of current but cannot find that in the datasheet. This 25 mA is still enough for my led's purposes. The 555 is adjusted so the flash rate somewhat matches the model of the Galactica. All leds share the 270-ohm resistor for each flash. I don't understand how the leds should be protected from reverse breakdown if they are susceptible to that. The power supply is regulated AC adapter so I don't understand the purpose of decoupling capacitors.
 

dl324

Joined Mar 30, 2015
14,323
I've read that the 4017 can source only 25mA of current but cannot find that in the datasheet.
Don't see how anyone could say that.

1640984244411.png
1640984282242.png
1640984311820.png

If you allow the output voltage to drop below the test conditions, you're not operating the counter as intended and maximum current isn't characterized.
 

dl324

Joined Mar 30, 2015
14,323
Where does it say 25mA? I'm a newbie to datasheets in every way.
My point is that it doesn't. In the CD4xxx family, the only devices that come close to that are CD4049, CD4050, and maybe 7 segment decoders.

AT 12V, you'd get something between the 10V and 15V parameters. Maybe a few mA, but nothing close to 25mA. Letting the output voltage drop lower than what would be recognized as a logic HIGH would get you more current. But the devices aren't characterized for that mode of operation.
 

sghioto

Joined Dec 31, 2017
3,061
I've read that the 4017 can source only 25mA of current but cannot find that in the datasheet.
1640989249993.png
According to the data sheet the max current allowed per device at 12 volts would be 41ma and per output transistor is 8 ma.
A 270 ohm series resistor would certainly be overloading the outputs of the 4017.
 

Audioguru again

Joined Oct 21, 2019
4,536
The datasheets from Texas Instruments show that almost all CD4xx ICs produce a typical shorted output current of 22mA when the output high is shorted to ground, but then the heating in the chip is too high.

The datasheet shows an absolute maximum allowed output transistor power (heating) of 100mW.
A graph shows power supply voltages of 5V. 10V and 15V output sink and source currents with output voltages of typical and minimum.

So with your 12V supply, an output high that is shorted to ground typically produces 23mA and heats with 12V x 23mA= 276mW to produce lots of smoke. 100mW occurs when the output voltage is 6.5V (5.5V across the output transistor) and the output current is 18mA. With a white 3V LED as the load then the output transistor has 9V across it and 22mA of current for a heating of 9V x 22mA=198mA which will kill the IC.
With the two white LEDs in series then the typical current is 17mA and the transistor heats with 6V x 17mA= 102mW which is a little too high. You need the two 3V white LEDs in series and in series with at least a 27 ohms resistor.
EDIT: You can connect hundreds of 3V LEDs in parallel and the total current is still 22mA.

A minimum spec CD40xx IC produces half that current.
 

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