I want to make a traffic light circuit but the problem is that I don't have a 0.1µF capacitor. I do have a 1µF capacitor, would it be OK to replace it?
The circuit diagram I am using is shown below and is taken from an online tutorial:
\$\begingroup\$
\$\endgroup\$
2
-
\$\begingroup\$ It should work fine, that is just a little filtering cap across the supply. \$\endgroup\$– R DrastCommented Apr 22, 2015 at 16:48
-
1\$\begingroup\$ Hans Camenzind's original bipolar 555 design draws some nasty spike currents from the power supply at switching- if you use a CMOS version it will use less current (battery will last longer) and decoupling is less of an issue, but yes 1uF ceramic (and probably electrolytic and probably nothing at all) is just fine for this non-critical circuit. \$\endgroup\$– Spehro 'speff' PefhanyCommented Apr 22, 2015 at 17:28
Add a comment
|
4 Answers
\$\begingroup\$
\$\endgroup\$
6
Seeing how it is across the 9V supply rail it is just decoupling for the power supply rail. Better designs (as most people here would be quick to point out) would have a decoupling cap local to each Vcc pin per IC, but for a simple design like this, one cap across the rail is probably fine. Using a 1uF cap is okay. Just make sure it is rated for >9V. Safest bet would be 16V caps, 10V is cutting it close.
Truthfully this design would probably work with no cap too, you might just experience weird glitches every once in awhile if the power supply blips from sudden current demand changes.
-
5\$\begingroup\$ This answer is correct for what it says, but leaves out the all-important mention of ESR. For example, if the OP's 1 uF cap is electrolytic, then using it to replace the 100 nF cap in the schematic is NOT OK. \$\endgroup\$ Commented Apr 22, 2015 at 17:21
-
2\$\begingroup\$ Your definition of 'works' and mine are clearly different. 'glitches every once in a while' is not what I would call working, I'd call it badly designed ;) \$\endgroup\$ Commented Apr 22, 2015 at 17:27
-
\$\begingroup\$ Oh nevermind, just typical pedantry. Unless OP has built this circuit in an EMI test cage and is blasting it with high frequency noise a 1uF electrolytic is fine. \$\endgroup\$– ACDCommented Apr 22, 2015 at 17:30
-
\$\begingroup\$ @TomCarpenter It's funny, I only put that in there because had I not someone would've said "Well what if you have no decoupling and your power supply load suddenly changes by 600A, it could cause a glitch on the reset pin hurr durr" \$\endgroup\$– ACDCommented Apr 22, 2015 at 17:32
-
\$\begingroup\$ No, 1 uF electrolytic does not have "negligible ESR" or impedance at high frequencies. A 1 uF ceramic, on the other hand, does have very low ESR and would be fine in this role to replace the 100 nF in the schematic. \$\endgroup\$ Commented Apr 22, 2015 at 17:35
\$\begingroup\$
\$\endgroup\$
5
Yes, you can replace the 100 nF (in engineering you keep 1-3 digits left of the point and adjust the units multiplier accordingly) with a 1 µF, but only if it is low ESR (equivalent series resistance).
The 100 nF you show is almost certainly a ceramic or some other type with very low ESR. Replacing it with a 1 µF electrolytic is not a good idea because this single cap is apparently also the bypass cap for the two ICs. This means it must be able to shunt the high frequency power currents generated by the ICs.
The right way to do this is to put a 100 nF to 1 µF or so ceramic cap immediately across the power pins of each IC. That shunts the high frequency power currents generated by the ICs immediately around the IC. This does a better job than a single cap for the whole board because of the lower series parasitic inductance in the shorter leads to the cap, and will radiate less due to the smaller loop size.
answered Apr 22, 2015 at 17:19
-
2\$\begingroup\$ In electrical engineering, your first paragraph is not always the case. 1000pF is almost universally more common than 1nF. Digikey is one of many good examples. \$\endgroup\$– ACDCommented Apr 22, 2015 at 17:36
-
1\$\begingroup\$ @ACD: There are still some bad habits out there from the ancient days, but that doesn't make it a good idea. Long ago there weren't even pico-Farads, just micro-micro-Farads. Even worse, micro-Farads were sometimes abbreviated "mF". Old ways die hard, but that doesn't make them right. Distibutors are sometimes afraid of perception and prefer to be on the trailing edge of trends. Proper use of milli, micro, nano, and pico is pretty common now. We should be pushing forwards here. \$\endgroup\$ Commented Apr 22, 2015 at 18:05
-
\$\begingroup\$ I use nF too, largely because I'd rather avoid using decimal point when using larger values. It's possible for a decimal point to be overlooked or to become illegible due to bad printing or photocopying etc. \$\endgroup\$ Commented Apr 22, 2015 at 18:36
-
1\$\begingroup\$ I have never in my life seen a schematic with markings in nF. \$\endgroup\$ Commented Apr 22, 2015 at 21:16
-
2\$\begingroup\$ @EJP: Then you haven't seen the many schematics I and lots of other people have created in the last 15 years or so. When I started as a EE working for HP in 1980, I would have written "1000 pF". It's been probably 15 years at least that I've been writing the same thing as "1 nF". Do you still write 15,000 uF too? You wouldn't say 15,000 uH, but for some reason people are resistant to using proper units multipliers for Farads. Here in 2015 that's just silly. \$\endgroup\$ Commented Apr 22, 2015 at 21:23
\$\begingroup\$
\$\endgroup\$
2
It's unclear exactly what the designer intended the 0.1uF capacitor for, but it would be very common to have a 0.1uF (100nF) capacitor across the power rails of each IC for decoupling.
There would usually be a 0.1uF capacitor for both the 555 and 4017 ICs, close to the power pins.
You could use 0.1uF and in this circuit it will likely work just fine (it might work fine without, why not try it?).
However you should probably buy a large pack of 0.1uF/100nF capacitors for use in the future because they will appear in most circuits.
-
5\$\begingroup\$ I cannot recommend strongly enough buying a 50-pack of 100nF ceramic caps early in your career. \$\endgroup\$ Commented Apr 22, 2015 at 16:53
-
3\$\begingroup\$ Actually nowadays you'd be better off getting a strip of 100 1 uF caps instead of 100 nF. With modern multi-layer ceramics, there is very little downside. The 1 uF will have lower impedance over all the frequencies you care about in a typical project, even above the self-resonant frequency of the 1 uF. Take a look at some impedance versus frequency graphs. \$\endgroup\$ Commented Apr 22, 2015 at 17:12
\$\begingroup\$
\$\endgroup\$
1
No need of that 0.1uf. That's for power stability. You may avoid that capacitor.
-
\$\begingroup\$ It's not for 'power stability', and there isn't enough information here for you to make that seeping assertion. \$\endgroup\$ Commented Apr 22, 2015 at 21:19