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Input Protection between overload and Max

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Input Protection between overload and Max

Postby tomfield » Wed Aug 10, 2011 6:26 pm

On my 4227, usable input is +-20v and Overload protection is said to be +-100v. Is there some kind of clamping action or reduced input impedance for inputs above +20v or below -20v. Suppose, for example, that I am measuring a 500v DC level with a 100x probe (with ~99ohm series resistor into the 1meg impedance of the 4227 scope in DC mode). 500v divided by 100 is 5 volts which should be well within the +-20v input spec. But suppose that I, but accident, select AC on the 4227 for that channel. Now we have 500v through a ~99 ohm resistor into the internal 4227 AC capacitor. If there is clamping action between 20v and the overload protection of 100v, then all should be well after transient settling, but if there is no clamping action the 100v protection level could be exceeded on a transient basis or the internal AC coupling capacitor rating could be exceeded.

I would hope that there is clamping action between +-20v and +-100v
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Re: Input Protection between overload and Max

Postby PeterF » Fri Aug 12, 2011 9:32 am

Hi Tom,
For safety, when measuring high voltages we would always recommend our active differential probes. If you must use a passive x100 probe make sure that you leave the scope set at DC input (the default setting) if there is DC present greater than the voltage rating of the scope. By starting with DC and auto ranging, you can measure the DC present and, if safe, select AC which will allow you to increase the input sensitivity to view the AC component. You are unlikely to accidentally select AC as the DC/AC selection pull-down is rarely used and is a "Two-click" process.
Regards,
PeterF.
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Re: Input Protection between overload and Max

Postby tomfield » Fri Aug 12, 2011 12:31 pm

Can you tell us about how overload protection works? Is there any clamping on the inputs above 20v working range? It seems unlikely that the input amplifier or ac coupling switching would present 1megohm impedance all the way to the 100 volt overload protection point.
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Re: Input Protection between overload and Max

Postby Martyn » Tue Aug 30, 2011 9:28 am

We are unable to give out design details for our devices on a public forum.

If you follow the advice from Peter you should not experience any problems in use. However if you require more detailed information then please contact us directly at support@picotech.com
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