[sdiy] First DIY synth (555 osc.'s)

[Happy Harry]

A "Crowbar" circuit is designed to remove power from a circuit
in an overload condition. Usually an SCR is triggered directly
across the power supply, removing power before (hopefully) damage
occurs.

Probably originates with using a crowbar to discharge supply
capacitors (a BAD idea, use a bleeder resistor or "eat molten
metal motherfvcker".....).

The same term is used differently when two devices in a totem
pole output (i know... next you ask what's a totem pole... ;^)
conduct at the same time... causing a short arcoss the power supply.

In CMOS, this condition is an SCR latch condition so the name fits
exactly...

In the 555... the pull up transistor on the output and the pull-down
transistor on the output overlap for a brief time. This allows current to 
flow from plus supply directly to ground... just internal to the chip (you 
DO NOT want) instead of in the load.

In an inverter, this condition is called a "shoot through"

In most cases, crowbar or shoot-through are considered a "bad thing"

H^) harry


>From: MountainMan <mtman at cloud9.net>
>To: Scott Gravenhorst <music.maker at gte.net>
>CC: synth-diy at node12b53.a2000.nl
>Subject: Re: [sdiy] First DIY synth (555 osc.'s)
>Date: Tue, 8 May 2001 12:54:32 -0400 (EDT)
>
>
>"crowbar effect" ??
>
>Elby
>
>
>On Tue, 8 May 2001, Scott Gravenhorst wrote:
>
> > Take a look at the specs for an ICM7555.  It's a drop-in
> > equivalent for the 555, but CMOS.  The data sheet says that
> > there is no crowbar effect during switching as there is
>

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