Forgot to include the links to latest screenshots
BTW, note that in these tests I'm typically toggling the switch on and off again after only a sub-second pause. In practice it doesn't matter how long it remains in the On position (providing you remember to switch it off before requiring a subsequent action).
-- Terry Pinnell Hobbyist, West Sussex, UK
You have a fundamental misunderstanding on how zener diodes function. I recommend studying the (reverse) voltage vs. (reverse) current graph closely in a zener diode data sheet. When a zener diode is reverse conducting, it will have a voltage drop across it of appoximately the rated zener voltage. In your circuit, immediately after the switch is moved to the right, Vout should be close to Vcc-Vzener, not close to Vcc.
Why do you want a brief spike? The circuit is intended to give something roughly 10 mS long.
+12 ---+-----+---[RLY]---+-----+ | | | | | +---[D2]----+ | | | OFF o o ON | | /c o---+---[Zd]---+-------| Q1 | | \\e [C1]470uf [R1]220K | | | | Gnd -------+----------+--------+In essence, the darlington's gain multiplies the coil resistance and places it in parallel with R1, which affects RC.
I'm not surprized. The larger the C the less you will see the spike. But the spike is most definitely not the thing wanted in this application. Intended is a positive voltage near Vcc that stays up for a relatively long time - milliseconds. The "voltage up" period is the time for the voltage to drop from Vcc to the zener voltage.
Ed
The second image with the 5 uf cap and noisy switch begins to show a more desireable waveform at the beginning of the capacitor discharge. Stuff a big cap in there and see what you get. I still don't understand the voltage levels. It's like your zener is shorted.
Ed
Thank you. Ed
Our ongoing conversation and different observations bugged me, so I breadboarded the thing yesterday, and my results agree* with yours. I can't post the waveforms like you can. :-( What do you use? The postings are great! :-)
*= essentially. I'm using different values to get the whole picture on the display - I can't see 5 seconds.Some observations: I missed the time scale on your 5 second waveforms and the voltage scale on the right. My scope doesn't do that! :-) Took me a while to catch on to how to read them.
Your comment about Vout = Vcc-Vz is correct. Henry also pointed that out. I saw your first waveform showing a 14 volt Vcc with a 12 volt zener yielding an "8" volt output, which makes no sense either way - but now that I understand how to read the waveform, that "8" volts is actually 2 volts, which does make sense. So toss the zener from the circuit.
During the switch noisy time, I get some beautiful transitions on the bounces like you have in the first transition in Ed-11, but I view it at .1 ms per div. How wide is your first transition? Mine varies from .1 to .2 mS.
Ed
Good - that discussion was really perplexing me!
I use a PC-based 'scope. It's an ADC-200/50 from Pico Technology
I take a screen capture (various ways; I use Snagit), and paste it into PaintShop Pro 7. Occasionally I might edit it there, adding some explanatory text etc.
Was there meant to be a link here to your display?
Yes, I'd begun to suspect that you weren't reading the red scale .
I'd go further. To mis-quote an old Python sketch: That circuit is deceased, no longer with us, kicked the bucket... .
Not exactly sure what you mean. As shown by the X-axis timescale, in Ed-11 I was using 20 ms per division, and 100 ms in Ed-10. The former was with a 5 uF cap, and the latter with 1 uF. In both cases I was starting with toggle open, and then quickly closing and re-opening it (about 300 ms), and then not repeating for several seconds.
BTW, that particular switch is one I reserve when I want to be sure of seeing the effects of *lots* of noise.
But it's all rather academic now!
-- Terry Pinnell Hobbyist, West Sussex, UK
A couple of other points have since emerged:
1) My simulation circuit was screwed up! To correctly simulate Ed's SPDT toggle switch, *two* VCS elements are obviously necessary, not the single VCS I used.2) From feedback I've had from other CM users, the zener models are definitely seriously flawed. Quoting an extract: "Most of their zener models are created by connecting a diode in series with a voltage source. While this approach makes a very coarse voltage clamp it doesn't accurately reflect a true zener in circuit and in fact has a number of problems. One particular problem ... is that the diode in series with the voltage source has a reverse leakage current. This leakage current can create error voltages in simulated circuits."
Here's my revised simulation:
I reckon that 14V spike at the output is the spurious result of the unrealistic modeling described above.
(I'll post this in the cad group too.)
-- Terry Pinnell Hobbyist, West Sussex, UK
No. The only way I can capture scope waveforms by taking a picture of them. :-(
Nope. The circuit works sans zener for devices that accept momentary swicthes as input and have a built in debounce.
Right. What I'm calling the first transition starts at 0 mS, goes up to 2 V then down to 1V, where the greatest amoubt of noise starts. I'm wondering what is going on during that period. I get about the same waveform, and the period for the first transition varies from ~ .1 mS to ~.2 mS - I'm using a Tek 485, & get the same thing on a Tek 475.
The former
Yup. But it got me to wonder if anyone else uses toggle switches in place of momentary. I'd never considered or heard of doing that until the OP made his post. I did make a relay "one-shot" once - OP needed a relay to energize for one pulse at shut down. The supply energized a relay when power was on. The open point charged a cap. When power was turned off, the relay dropped out and transfered the charged cap to a second relay:
+Vcc ---+---[RY1]---+-----+--- Gnd | | | o NO [C1] | \\ | | RY1-1 o---------+ | | o NC | | | +-----[RY2]-------+I never did learn why he wanted that circuit.
Ed
That was major motive for buying the PC-based scope. A digicam is OK at a pinch, but not in the same league.
OK, agreed.
OK, I see what you mean. But, if they do indeed follow that sort of pattern no idea why. Switch bounce is something of a black art . Several toggles I tried were exceptionally low on noise. That particular one exceptionally high. Presumably, if pole and both contacts are connected to +Vcc and 0V respectively, swings must always be across full supply, with no intermediate values possible. Maybe if I'd set my timescale down to say 5 us I'd see even shorter pulses?
Did you also see my later follow-up yesterday? news: snipped-for-privacy@4ax.com
-- Terry, West Sussex, UK
Yes. If both VCS1 and VCS2 are simultaneously energized, D1 input "sees" the input to VCS1 normally open point - not a good thing. The 14v spike might be that condition?
Ed
Good point, I'll have to think about that. I vaguely thought I'd be OK by ensuring both VCSs (identical) had a non-zero threshold. I arbitrarily used a VCS I'd previously set at 10 mV. Their full specs are: VT: Threshold voltage 10.000m VH: Hysteresis voltage 0.000 RON: On resistance 1.000 ROFF: Off resistance 1.000t
But I didn't think it through, and I think you may be right. Maybe some hysteresis would fix it? Or maybe I need to design a simulation with a definite changeover time.
Until your reply, I was happy to accept the following from one of my CM-user friends, whom I'd also asked about that spike: "The spike is caused by the Cjo capacitance of the zener which is 92 pF. This would have a time constant with the 10 k output resistor of
0.92 us. The horizontal resolution of the waveforms shown is too long to see the wave shape of the spike, but if you expand the spike you should see a RC discharge shape with a time constant of around 1 us.The rest of the waveform looks good and is what would be expected from the circuit given, except for the ringing after the main RC pulse. There is nothing in the circuit that would account for the ringing so it is probably just a simulation artifact."
FWIW, this is another simulation I've been playing with since seeing your post:
-- Terry Pinnell Hobbyist, West Sussex, UK
The trouble here is that we don't know how the sim program models the zener. Your friend may be right - I have no clue what the program does. On the real waveforms you posted, I don't see the spike. On my breadboard, with a 6v zener, 1uF, 15K load and 17V Vcc, I see a triangular spike something like this:
7.5 |\\ 7.0 | \\ 6.5 | \\ 6.0 | \\_______________ That's a far cry from the 14 volt spike. The actual circuit, however, would be something like this:/ +---o o---+ | | Vcc---||---+---||----+-----|| your post:
On the sim, why not do something like this to guarantee an offset between R2 and R3 energized time: + | -----VCS1 / | | Gnd | + | | VCS3 / | | | R2 | | +---+ Gnd
Your sim shows the voltage drop at C during the voltage rise at D, and I'm wondering if it will delay things to account for VCS3 transfer time.
Ed
Thanks, I'll experiment further. I am close to a better SPDT simulation, but I've digressed into trying to generalise it for future use. Reckon I'll pass on simulating varying contact switch capacitance though! Presumably, for the SPDT used here, we'd include *two* such, Cs1 and Cs2. Although their values must be infinitesimal (ignorable?) until contact is extremely close on either side?
FWIW, here's the CM model of 1N4742 12V Zener
IS: Saturation current 9.67e-15 RS: Ohmic resistance [0,] 4.260 N: Emission coefficient 1.000 * TT: Transit-time [0,] 50.10n CJO: Zero-bias junction capacitance [0,] 94.20p VM: Grading coefficient 330.0m EG: Activation energy 1.110 * XTI: Saturation-current temperature exponent 3.000 * KF: Flicker-noise coefficient 0.000 * AF: Flicker-noise exponent 1.000 * FC: Foward-bias depletion coefficient 500.0m * BV: Reverse breakdown voltage 11.91 IBV: Current at breakdown voltage 21.00m TNOM: Paramameter measurement temperature 27.00 *
-- Terry Pinnell Hobbyist, West Sussex, UK
I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'Momentary Switch Circuit', on Mon, 19 Sep 2005:
Would you also please consider snipping you very long articles in this thread now.
-- Regards, John Woodgate, OOO - Own Opinions Only. If everything has been designed, a god designed evolution by natural selection. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
I'll certainly try, as I'm a snipping fan. But it's sometimes quite difficult when the quotes are in-line sections.
FWIW, I did initially try composing my last reply with some heavy snipping, but got into such a mess I regressed to the easier option .
Any thoughts on the substance of the post?
-- Terry Pinnell Hobbyist, West Sussex, UK
Assuming we're still discussing the same circuit configuration, then I get this result now that I've corrected my SPDT design:
The output is about 40mS, with an amplitude of about 11 V (Vcc-Vz).
If I throw in some capacitance, and increase the timing resolution, then I get this:
-- Terry Pinnell Hobbyist, West Sussex, UK
I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'Momentary Switch Circuit', on Mon, 19 Sep 2005:
What is unexpected? The 100 pF will transfer the 17 V to the output until it charges up. Time constant is 1.5 us, so after 5 us most (96.4%) of the spike will be gone.
-- Regards, John Woodgate, OOO - Own Opinions Only. If everything has been designed, a god designed evolution by natural selection. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Thanks. Nothing unexpected with that one as far as I'm concerned! But it differs from what Ed is apparently seeing.
My earlier reply was asking whether you had any comments about the points in my previous post. For example, how significant would contact capacitance be in simulating a SPDT toggle switch?
-- Terry Pinnell Hobbyist, West Sussex, UK
I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'Momentary Switch Circuit', on Mon, 19 Sep 2005:
It isn't something you can generalise about. A switch with 3 pF capacitance may be disastrous in one circuit, while another with 30 pF may be quite OK in another circuit. It's just the same with contact resistance, but that also involves 'wetting current'. Some mains toggle switches are open-circuit in all positions when tried in low-voltage, low-current circuits.
-- Regards, John Woodgate, OOO - Own Opinions Only. If everything has been designed, a god designed evolution by natural selection. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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