Common supply

[Arthur Harrison]

Crowbars are usually intended to prevent drastic 
consequences from gross, permanent supply faults, 
not transients.  Agreed that MOVs can be effective 
protection from spurious noise.

-Art

-----Original Message-----
From: Barry L Klein <Barry.L.Klein at wdc.com>
To: SYNTH-DIY at mailhost.bpa.nl <SYNTH-DIY at mailhost.bpa.nl>
Date: Wednesday, January 20, 1999 12:17 PM
Subject: RE: Re: Common supply


>Another approach you may want to consider is one I mentioned before -
>using TVSD's, which are transient protection diodes, much like big zeners.
> They just solder across the supply in question and will clamp
>overvoltages beyond the part's spec'd clamp voltage.  They are very fast
>and probably would do everything the crowbar circuit would, maybe faster. 
>The crowbar detects an overvoltage, fires an SCR to short out the supply
>to ground.  If you don't have protection fuses the supply may fry as a
>result, depending on its design.  The TVSD would clamp overvoltages to a
>fixed voltage level.  Momentary spikes get clamped and your circuit
>continues functioning - no fuse gets blown and all is well.  But if your
>supply dies and results in overvoltage, the TVSD will clamp and hopefully
>the fuse opens.  If you don't have a fuse the TVSD will probably get hot
>enough to fry your pcb or melt its solder connections, fall off, and then
>all your remaining parts would fry too.  We use them on drive designs here
>at WDC and it has resulted in a much reduced failure rate of the various
>IC's.  
>
>You can get TVSD's from Digikey and Mouser I think.  A 5V part will clamp
>slightly over 7 volts, a 12V part at 17 volts or so.  Much cheaper - no
>heatsink required for an SCR for example.
>
>These are not MOV's and don't degrade with time, BTW.
>
>Barry
>
>
>>
>The idea here is that if some fault causes the power supply unit to 
>put out too much voltage (say a resistor failure somwhere) then a SCR
>produces a short circuit across the output, hopefully triggering
>another current limiting ckt to shut down the PSU.. it doesn't have to
>be destructive.
>When the SCR triggers, it looks like a crowbar, resistance wise.
>
>
>
>