Really understanding Capacitors

[Dale]

I am building a shift circuit that utilizes 555 timers. One triggering the other by way of a mylar cap. 555 triggers can be touchy, so I wanted to transfer a good " pulse". After all my reading and studying I realized that I really have no understanding of how to pick a capacitor properly for this coupling pulse. I just purchased a new digital scope(which I love!) and decided to put in different caps and values and look at the results. I decided that the quickest time for a negative pulse and the maximum magnitude were what made the most sense. I was amazed to discover that on average, The pulse magnitude was 50 % greater by flipping the direction of my ceramic and mylar "NON POLARIZED" caps! That is significant. Also, the discharge - charge characteristics are different as well as the times. I have never see this addresed anywhere. Did I just reinvent the wheel, or is this something not on the radar screen? I would really like to hear from those a lot smarter than myself and with a lot more experience. As an afterthought, I never really saw any detailed info on using caps as pulse tranfers in digital circuits. They are there, but how they are chosen is a mystery. Thanks for the Forum - I love every issue of N&V's. Dale

[Robert Reed]

Dale
I have never heard of this phenomenon's or experienced it. The only thing that comes to mind is Ceramics- in the y and X7R series do have a voltage coefficient, the COG series do not.
Mylar - if it is a wound style might appear different to the pulse one way or the other
These are the only idiosyncrasies that come to mind and really seem like minimal properties for what you are doing. Any self inductance should not present a problem as the 555 is a relatively slow device.What does your trigger pulse look like?

[Dale]

Hi Robert, I am reading the cap at the second 555's trigger input which is tied to Vcc- 5 volts via a 10 K resistor. Pulses are between 5 and 10 milli seconds to the maximum negative value in a nice straight line. Recovery back to Vcc becomes very anolog and the time varies. I understand the non precision of this circuit ( however there are ten timers involved trying to shift a transmission as quickly as possible mechanically) But it is more of not building something that works, but really understanding the electronics with confidence. EX: a standard 503 ceramic cap from Mouser both directions are 10 milli seconds for the drop, however the magnitudes are 2.6 volts vs 1.1 volt.??

[dyarker]

Value range of capacitors (.0xuF, nF, pF)?

Solderless breadboard construction?

Cheers,

[Dale]

The cap values in uF are: 0.1 - 0.05 - 0.021 ( the best: 5 mS and 3 volts) - 0.033. In both ceramic and mylar the behavior is the same. Construction is breadboard. I have pulled the bottom plates and looked for broken jumper pieces shorts ect. Everything is O.K.

[chribec2]

Hi Dale --
.
I would like to throw some light on the subject if I can. As I see it, there are three areas up for discussion -
.
1. What's involved in triggering a 555 on pin #2.
2. Why the charge/discharge differences.
3. About coupling capacitors
.
1. It would appear that you are setting up a string of 555's as a serial-to-parallel shift register, thus each 555 is
configured as a monostable 'one shot' (O/S) multi-vibrator. As such, each O/S, as it times out, will trigger the
next O/S down stream etc., etc. To trigger a O/S, pin #2 must be brought low -- that is -- below 1/3 Vcc for
a period of more than one micro-second for reliable triggering. Once the O/S is triggered, pin #3 goes high for
the time determined by the R/C timing network. Pin #2 must return to it's pre-triggered condition before
pin #3 output times out, or else pin #3 will remain high until pin #2 loses it's negative going pulse ! ! This is a
"gotcha" that will take some time to figure out. (Don't ask me how I know this ! ! !). The input on pin #2 is
v e r r y voltage tolerant -- slow transitioning pulses are no problem. Differentiated pulses work just fine.
.
2. Charge/discharge differences in the timing network occur because the capacitor charges through the timing
resistor but discharges directly to ground (with only minor on-chip resistance involved.) Check the schematics
in the references.
.
3. When using the output of one O/S to trigger the input of a following O/S, it is best to go through a small
capacitor thus making the capacitor a 'coupling capacitor'. Now, remember, a capacitor blocks D.C. but passes
A.C. -- meaning -- the horizontal flat portions of a square wave (both the positive and ground portions of the
waveform represent the D.C. elements) are blocked (not passed), but, the rising and falling edges (these
represent the A.C. elements) go screaming on through -- although on the other side of the capacitor, these
rising (positive going) and falling (negative going) edges create positive and negative spikes, respectively. The
positive spike will be ignored by the O/S, but, the negative spike -- if it gets below 1/3 Vcc -- becomes --
WA - LAH -- the trigger pulse ! !
.
References:
1. "The TTL Cookbook" by Don Lancaster. Besides the TTL stuff, Don gets into the 555 quite deeply including
applications.
2. From the Internet -- "555 Timer Tutorial" by Tony vanRoon. Another indepth study of the 555 with apps.
.
Personally, I have successfully mixed 555's (running 12v) in the middle of my TTL set ups by biasing the 555's
inputs thusly -
The 555's pin #2 goes to the junction of two series connected resistors - a 6k2 ohm to 12v and a 4k7 ohm to
ground. A little over one millamp flows through this network - well above the 500 nanoamp minimum. This
gives me about 5.1v at pin #2, a nice cushion above the 4v (and below) needed for triggering. Now, one end of
a 680 pF cap connects to said junction with the other end of the cap connecting to the TTL gate output. This
gives me a negative going spike with a duration of about 10 microseconds at the 4v level. This spike does
reach ground. The output at pin #3 goes to a 470 ohm resistor/5.1v 1w zener. The resistor/zener junction goes
to the next stage. I tried the 555 at 5v, but, I was getting some instability at this level, so, with 12v, everything
was rock solid.
.
I hope this meets your need a little bit. Good luck on your project - " . . . trying to shift a transmission . . .)?
Sounds interesting. Can we hear more about this?
.
Phil Potter.
.