Re: Initializing a Flip-Flop on Power-up

On Fri, 17 Feb 2006 10:08:46 -0600, the renowned Dan Charette
<dan@xxxxxxxxxxxxxxxxxxxx> wrote:

Hi All!

I've got a small toggle circuit that I'm using to drive a shutdown
circuit on an audio power amplifier. The circuit is made from a
CD4093 Schmitt trigger NAND configured as a debouncer for my mute
switch input. The output of the 4093 is fed into a CD4013 fo provide
toggling cabability. I've got S and R tied directly to ground. But,
I'd like to explore the possiblity of providing a predictable power-up
scenario. Currently, the circuit upon power up will default to one
state or the other. What are some methods that you fellas have used
to achieve a known state consistantly with a flip-flop like this when
the power supply is from a single source. First gut instinct that I'm
thinking is to make some sorta RC timing circuit on the power supply
pin so that I can provide the set or reset pins with something known
before the power supply pin comes up. Any thoughts? Also, I don't
want to add too much additional circuitry... if I can solve this
purely with passive components or a few transistors, that'd be great.

Thanks!

Use something like this to generate a reset pulse:
http://ww1.microchip.com/downloads/en/devicedoc/11187f.pdf

Only one part required. BTW, don't let the "microcontroller" in the
description throw you, they are suitable for POR of any 3-5V CMOS
logic circuit.

While you can use an RC (typically about 0.1s time constant with a
diode to help out the protection diode to Vdd, a series resistor to
limit the current in case of a Vdd-to-ground short, and perhaps a
drain resistor from Vdd to Vss to make sure that Vdd falls reasonably
quickly), it's far from foolproof, more complex and not much cheaper.
Another low-cost solution is to use a zener, a BJT and a few
resistors, with or without the RC, but that consumes more power, and
is even more complex.

.

Relevant Pages

  • Re: astable multivibrator issues - 2N3904s work, but not TIP31As
    ... the circuit would not oscillate unless I briefly disconnected then re- ... The main issue folks have is that they forget to bypass the power supply, ... As a side note, the easy way to build a square wave oscillator from a cmos 555 is to connect the output pin to a resistor, then connect the other side of the resistor to ground through a capacitor. ... For your application, you might use the discharge to directly drive one LED, and have the other LED driven by a power PMOS or PNP transistor. ...
    (sci.electronics.basics)
  • Re: 12 LED resistance circuit help
    ... be more humble and merely post "Circuit designer"? ... >Perhaps "dissipate more power" would have been more appropriate than ... "Since there are two LEDs in series, one may hog more current than the ... >> dissipation spec of an LED with Vf max in series with an LED with Vf ...
    (sci.electronics.basics)
  • Re: The Rest of the Story
    ... The short path is the series path of two resistors composed of the source resistor Rs and the input to the 50 ohm transmission line measured by Vg. ... Power flows through both paths to Rs at all times, but because of the time differential in arrival timing, at some point Rs will receive power only from Vs, and at another point, receive power only from Vref. ... There is no further reflection from Vs or Rs. ... I like to resolve the circuit into one path, ...
    (rec.radio.amateur.antenna)
  • Re: Adapting An Electrophoresis HVDC Power Supply To Microampere Level
    ... the power supply is to be used. ... an expert in circuit design, but he could not give much time to it ... MOSFET, resistor, capacitor, etc. ... going to need a voltage sequencer from 500V to 10KV. ...
    (sci.electronics.design)
  • This guy is a crackpot, but what is the most obvious reason why?
    ... Analog Audio Power Amplifier Design ... This author's experience with analog audio circuit design extends over ... a period of 50 years -- the era of the vacuum tube, ...
    (rec.audio.tubes)