Decoupling capacitor choice for 4017 decade counter

Hi all,

I've been having some problems with a chain of divide-by-ten counters firin g off prematurely and messing up the division. I'm guessing this is a decou pling issue (after all, what else could it be, since I'm using a hi-quality filtered power supply that should be well up to the job). I seem to recall from somewhere that these chips need larger than usual cap s for this purpose. Someone even said 47uF or 100uF for the purpose but tha t does sound VERY large and we're into electrolytic territory.

Can anyone offer any advice on the matter?

TIA.

Reply to
orion.osiris
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Reply to
David Eather

Check the maker's datasheet, 4017 is a pretty common part-number and devices from different sources may be subtly different. That said, I also would expect 100nf to be plenty.

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Reply to
Jasen Betts

This may or may not be relevant to your case, but I used to get strange triggering issues with 4013 D-type flip-flops Back In The Day. Only seemed to happen on the proto-board, not the finished PCB... but of course the whole point of the proto-board was development for the PCB... grrr!

Anyway, I guessed that there was some sort of coupling / capacitance issue with the proto-board. (The plastic kind with rows and rows of socket holes for chips and component leads.) The cure was to put 10K resistors in series with each chip's clock pin, apparently (together with chip or proto-board C) slowing down the edges just a tad.

Turned out other guys in the lab had similar problems that this worked for, and it became known as the "10K cure".

Hope this helps!

Bob Masta DAQARTA v7.10 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

ing off prematurely and messing up the division. I'm guessing this is a dec oupling issue (after all, what else could it be, since I'm using a hi-quali ty filtered power supply that should be well up to the job).

aps for this purpose. Someone even said 47uF or 100uF for the purpose but t hat does sound VERY large and we're into electrolytic territory.

Do you have a 'scope? You can look at the power supply rails. Is the circuit on a nice PCB, a rat's nest on copper clad, or plugged into white protoboard? As other's have said I think 0.1uF should be plenty. Maybe you should look at the layout and ground 'issues'???

George H.

Reply to
George Herold

I'm inclined to agree. It's always a problem with breadboard, because you c an't get remotely close to zero lead length on the important bits and there 's no ground plane. Plus the newest piece of breadboard I'm using came from China and the connections just feel like poor quality upon component inser tion/removal. I'm rebuilding the thing properly on etched copper clad using the tips others have suggested and hope that will clear up the problem. I' ll know in a day or so. Anyone else had problems with Chinese breadboard?

Reply to
orion.osiris

Are you sure it's decoupling? How long is your clock line, is it terminated, are you using a CA4017 or a 74(mumble)C4017, etc.?

If you're using a fast part (i.e. 74ACT4017) and your clock line either has a slow transition or a fast transition and enough wire between the source and the counter for it to ring, then your problem may have nothing to do with decoupling at all.

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Reply to
Tim Wescott

can't get remotely close to zero lead length on the important bits and the re's no ground plane. Plus the newest piece of breadboard I'm using came fr om China and the connections just feel like poor quality upon component ins ertion/removal. I'm rebuilding the thing properly on etched copper clad usi ng the tips others have suggested and hope that will clear up the problem. I'll know in a day or so. Anyone else had problems with Chinese breadboard?

I rarely use the white protoboard these days. (The last time was a chaos circuit with a max frequency of ~2kHz.) Mostly I do the pcb directly if it's a circuit I've done before, or a rat's nest on copper clad if I'm not sure how something will work.

George H.

Reply to
George Herold

And make sure the input is de-bounced

Reply to
David Eather

I'm not using any mechanical switches here; the first 4017 is pulsed by the output from a 555 timer which I believe I'm entitled to assume will be nice and clean. It had better be, anyway, since I don't have a DSO to examine the pulse train in detail!

Reply to
orion.osiris

555 + 4017 ia a popular combiation, I've used it myself and the internet is full of pages dedicated to it. it should just work.

the "10k fix" mentioned earlier is worth trying if you're using a solderless breadboard.

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Reply to
Jasen Betts

output from a 555 timer which I believe I'm entitled to assume will be nice and clean. It had better be, anyway, since I don't have a DSO to examine the pulse train in detail!

Back when 555s were the latest rage, I tried some and found that they caused enormous switching spikes. By the time all the extra decoupling caps were added, not to mention all the parts that the 555 needs to begin with, these chips didn't seem worth the trouble. True, they offered pretty good timing performance, but it seemed like the vast majority of my needs were easily met by simpler (if less precise) circuits using a CMOS gate or two and an RC. Or I needed high precision (Xtal), or VCO, or something else the 555 was just mediocre at (especially considering its big supporting parts footprint).

So I never got into the habit of using them. But I notice that people continue to use them like popcorn, so I figure maybe they've solved the spike problem? Or maybe not, judging by your experience.

Just for grins, you might want to try one of those CMOS Cookbook-type gate-and-RC timers to see if that solves your problem.

Best regards,

Bob Masta DAQARTA v7.10 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

Reply to
David Eather

off prematurely and messing up the division. I'm guessing this is a decoupling issue...

for this purpose. Someone even said 47uF or 100uF

4000 series CMOS can (if you're using 10-18V power) put enormous spikes on the power buss, and the '4017 is a fairly large chip (30 gates, five flip/flops). I'd think a 22 uF to 100 uF Al electrolytic capacitor within two inches of the chip should suffice. In tantalum, you might be able to go lower. One capacitor for every four chips is enough.

The dominant current requirement of 4000 series CMOS occurs in short times during logic transitions; it's ALL spikes, unlike TTL or ECL. At lower voltages, you can use smaller capacitance (because of smaller spikes, and slower responses).

The clock input on '4017 is already a Schmitt trigger type, you needn't add a Schmitt gate to obtain that particular advantage.

Reply to
whit3rd

OK, thanks for all the tips, guys. All duly noted. Nearly finished transferring the components to CCB with a sprinkling of your suitable, recommended caps around the place. See how we go from here! :)

Reply to
orion.osiris

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