Re: 4017 Counter skips under load

jcargile2001@xxxxxxxxx wrote:

On Jul 2, 3:24 pm, "David L. Jones" <altz...@xxxxxxxxx> wrote:

On Jul 3, 7:11 am, jcargile2...@xxxxxxxxx wrote:


I have a fairly simple circuit that consists of a 4017 decade counter
and nine relay/LED combinations. The relays are very small and the
circuit operates just fine with no load, or when I connect an LED to
the relay output. But when I try to operate the circuit under load
(it is being used to fire nichrome ignitors), then the counter simply
skips the loaded relay and moves right to the next output. So if I
send the counter a series of 5 clock pulses, and put a load on the
relay attached to output 3, then the count goes 1,2,4,5,6. The final
count ends up being one off, so it's almost like the output in
question doesn't exist. What I don't understand is why this is
happening, since the relay is what bears the load, not the 4017.
Shouldn't the output requirement on the counter be the same regardless
of what the relay is switching?

Yes, it should be, but only if you have used proper circuit physical
circuit layout techniques to ensure that there is no ground or power
bounce.
Think "star grounding" and power decoupling.
Very common trap for young players.

Dave.


I installed the final power setup (on the tests I just had the board
jumpered onto the power supply) and now it will fire the ignitors.
The problem is that the counter is now behaving irratically. It will
skip several counts when receiving a clock pulse and will jump around
with the slightest change in ground (even connecting a single lead
from the multimeter to any point on ground advances the count). I've
tried several different caps (.01-10 uF) connected directly to the
power leads of the 4017 with no luck. All of the info I can find on
decoupling keeps talking about selecting the capacitor based on
frequency. But I'm only sending a single pulse of about 25 ns every
10-20 seconds. Does this mean I can use a larger cap? I'm seriously
freaking out at this point cause I only have 24 hours to get this
working. Thanks for the help!


How on earth do you get a 25nsec pulse out of a CD4017? That's like clocking a moped at 100mph.

Anyhow, since things seem to become desperate over there I can only suggest to build it up again on an experimental board that has a ground plane. And no, you do not have to select decoupling caps based on frequency. A 0.1uF plus a nice 10uF electrolytic should do, more if your power supply is wimpy.

Can you post schematic plus photo?

--
Regards, Joerg

http://www.analogconsultants.com
.

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