> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of neilraf@btinternet.com
> Sent: Friday, October 27, 2006 5:24 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] - scope input circuit
> 
> 
> 
> 
> > fun too.  Hanging the input protection on the output node of the 
> > divider will raise havoc with the frequency response too.  Lots of 
> > fun...
> 
> Which protection?
> 

What do you suppose would happen if you tried to measure a voltage far
outside the working range of your circuit without some way of limiting
both the voltage and current to the FET follower he shows on another
page.  You let out the magic smoke and stuff stops working.

Generally, an oscilloscope worth its salt can accept about 200V or maybe
a little more more at the probe tip with a x10 probe without damage.
Higer than that and you need to consider x100 probes (and take the
bandwidth hit) or differential measurements.  For really high voltages
you start building custom attenuators, specific to the waveshape.  A
good example is a test rig for HV spark arrestor testing and
high-tension insulation testing.

The simplest form of input protection would be a pair of zener diodes
following the attenuator and before the FET follower that limit the
voltage to something a little below the power rails.  But a simple zener
like that doesn't work well when the power supply is off or has failed
and so a real protection scheme is a bit more involved.  Plus it can't
interfere with the intended oscilloscope bandwidth.  Some of the old
tube powered TEKs used a combination  of fast glass diodes, honking big
zeners and gas discharge tubes.  Neat to see the gas discharge tube
light up.

Rob

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