Re: Momentary Switch Circuit

ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote:

>Terry Pinnell wrote:
>> ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote (to the OP):
>>
>>
>>>I'm curious. Why didn't you try this?
>>>
>>>+Vcc ---o o---+---|<---- Vout
>>> \ | Zener
>>> o |
>>> | |
>>> [C1] [R1] 220K
>>> | |
>>>Gnd ------+-----+
>>>
>>>Vzener is slightly below Vcc - eg 4.7 volts if Vcc is
>>>5 V, 11 volts if Vcc is 12 volts etc.
>>>Ed
>>
>>
>> Ed: That looked such an attractively simple circuit that I tried it
>> myself yesterday. I may have missed something but aren't there a
>> couple of downsides to it?
>
>Yes, absolutely.
>
>I don't think the OP specified what sort of
>> pulse he wanted, but my starting assumptions were a clean, +ve, full
>> supply signal.
>
>He's replacing (or using in place of) a momentary switch.
>Both the toggle and the momentary are subjecty to switch
>bounce. Since whatever circuit he's driving will work
>on a momentary, then switch bounce from a toggle is not
>a factor.
>
>But your observation is correct. The circuit will not
>provide a clean, +ve, full signal supply. It is a definite
>downside to the circuit for general use - it is good only
>for use where switch bounce is irrelevant.
>
>
>Your circuit:
>>
>> 1) Delivers only a low amplitude spike, so needs amplifying to get a
>> 'full' pulse.
>
>That's wierd. It should deliver close to Vcc. Your
>Vcc is 14 volts and the zener is 12. It (the zener)
>should conduct until Vcap drops to 12, meaning that
>the pulse amplitude has to be at least 12. Maybe
>you are scoping at the input to the zener? The
>duration depends on the load, and to a lesser extent,
>on the 220K resistor, but primarily on the zener.
>It will be a spike, but it should be a spike whose
>amplitude is over the zener voltage. RC is ~ 9.5 mS
>(way too short) down to ~1/3 vcc - but it is collapsing
>only 2 volts or about 14% (instead of ~ 63%) when the zener
>shuts it off. You need a bigger cap! The circuit "looks at"
>just the top of the cap discharge curve, which is steep.

I think I need a much *smaller* cap! If I make it 10nF instead of 1uf,
then I see Vout as an initial *very* brief spike.

>We disagree on spike amplitude, but that doesn't change
>the fact that the output is another downside, as you
>indicated. You get what amounts to a spike instead of a
>nice robust square wave.

>Using say a simple NPN stage that results in a -ve going
>> pulse, which may then need inverting. (BTW, such amplification would
>> presumably be simplified if the headroom voltage is rather larger than
>> your examples?)
>
>Yes - you need that with a low impedance load, or a long
>duration pulse, or if the pulse must be square. And as long
>as you stuff enough drive current into the base, it drives
>to Vcc minus the transistor Vce. So you can have plenty of
>headroom between the zener and Vcc and still get square wave
>output.
>
>I got way over 40 seconds with a 120(?) ohm relay driven
>that way through a darlington with Hfe > 1000 in a delay
>off circuit I made. Had to add a resitor in parallel with
>the cap to get it down to the ~40 second target in that
>circuit. I don't remember all the values, but I can find
>them if they are of interest.

You've lost me there. Can you draw that circuit please?

>The headroom was to make the pulse voltage unambiguous.
>RC is ~ 63% discharged - the circuit is "looking"
>at the top ~6% of discharge (eg from 5.0 down to 4.7).
>I didn't want the the circuit that is being driven
>to "see" an ambiguous voltage that was not clearly
>either a high or a low. By using the top of the
>discharge curve, the voltage will be at a high
>until the zener cuts it off. It has a wicked slope,
>but all of it will be above the zener V. Also, I picked
>common zener voltages. I run into lots of 11 and 4.7v
>zeners, but rarely a 4.3.
>
>>
>> 2) Transmits switch noise.
>
>Absolutely. As mentioned earlier, I don't think
>that's a factor for the op, but it certainly is
>in many applications.
>
>>
>> I used a 14V supply and a nominal 12V 1W zener, with this circuit:
>> http://www.terrypin.dial.pipex.com/Images/Momentary-Ed1.gif
>>
>> Here are a few screenshots of the results I actually saw:
>
>Thanks for these - they're great! I have a question, inline
>below.
>>
>> Relatively 'clean' switch
>> http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-1.gif
>>
>> Noisy switch
>> http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-3.gif
>>
>> Noisy switch, detail
>> http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-5.gif
>>
>> Noisy switch, detail, after NPN stage
>> http://www.terrypin.dial.pipex.com/Images/Toggle-Ed-8.gif
>
>I'm puzzled by the Vout curves. It should "fall off the cliff"
>at 12V when the zener stops conducting, but the traces all show
>Vout curving down below 12 volts.
>
>I think the output (without the noise, which I can't draw)
>should look like this:
>
>+14 |\
>+12 | \
> | |
> | |
> | |
>0 ---- ----
>
>Can you determine why there is that curve down to 0
>volts? I wonder if your zener doesn't "zen" :-) ?
>
>>
>> BTW, I'm puzzled by the roughly triangular shape of the output. I
>> tried one simulation and got a squarish result:
>>
>> http://www.terrypin.dial.pipex.com/Images/Momentary-Ed-SIM1.gif
>> Maybe it was just the choice of zener?
>>
>>
>
>Regarding the simulation - my guess is that they treat
>the switch as noiseless, no bounce. But they don't
>show the Vout "falling off the cliff" pattern - unless
>the yellow is supposed to be Vout. The green looks
>more like a capacitor discharge curve than the yellow,
>but the green doesn't fall off the cliff. And if the
>yellow is supposed to be Vout, the trailing edge is
>correct and everything before it is wrong. So I'm
>clueless.
>
>Ed

I'm out for rest of day, but I'll get back on the case tonight. I
suspect that 'triangle' I saw for Vout might have been an artifact of
some sort, due to poorly chosen PC-based 'scope settings. All the
simulations I've tried show a square signal, amplitude Vcc-Vz, *apart*
from that initial spike which they now show, following the massive
reduction in the value of C.

--
Terry Pinnell
Hobbyist, West Sussex, UK
.

Relevant Pages

  • Re: Momentary Switch Circuit
    ... >>He's replacing a momentary switch. ... Since whatever circuit he's driving will work ... but primarily on the zener. ... >>It will be a spike, but it should be a spike whose ...
    (sci.electronics.design)
  • Re: Momentary Switch Circuit
    ... >>>He's replacing a momentary switch. ... Since whatever circuit he's driving will work ... but primarily on the zener. ... >circuit I illustrated above, with Vout taken from zener's anode, yes? ...
    (sci.electronics.design)
  • Re: Momentary Switch Circuit
    ... I am hoping to find a circuit that will provide the momentary closure when the toggle switch is first closed and then not again unless the toggle switch is opened and then closed again. ... to +Vcc, and charges quickly. ...
    (sci.electronics.design)
  • Re: Momentary Switch Circuit
    ... 11 volts if Vcc is 12 volts etc. ... Since whatever circuit he's driving will work on a momentary, then switch bounce from a toggle is not a factor. ... Vcc is 14 volts and the zener is 12. ...
    (sci.electronics.design)
  • Re: The RCS: Voice Transcript
    ... Have you gone through the circuit analysis for the circuit that ... sustained power to the heater is what did it. ... That thermo switch ... It is now known, however, that the tank contained two protective ...
    (sci.space.history)