Q on useage of Fluke 8845A
- From: Robert Baer <robertbaer@xxxxxxxxxxxxx>
- Date: Thu, 19 Apr 2007 20:19:49 GMT
Say one has an adjustable current limiter driven by a 15V supply, the load is a zener in series with the 100 ohms current monitor (Fluke input Z in 100uA mode), and the zener voltage is also monitored by the Fluke.
Now add in a program to switch relays where in one position the Fluke is connected to measure the voltage but the diode current goes thru a dummy 100 ohm resistor; the other postion connects the Fluke current monitoring (as originally mentioned) and disconnects the voltage side (and connects a 10Meg dummy resistor).
The program switches the Fluke to the desired voltage or current scales and switches the relays.
Now comes the problem.
If the relays are switched first, then the current terminals (or voltage terminals) get voltage before the Fluke is switched to the appropiate mode. Ditto if the range (voltage / current) of the Fluke is switched first.
I think it is safe to assume the external relays will switch slower, so the programming "solution" of switching both "at the same time" will still result in the fault conditions mentioned.
Nobody (not even Fluke) talks about voltage across the voltage terminals when the meter is in current mode, also, nobody talks about voltage across the current terminals when the meter is in voltage mode.
So, what is the answer?
What are the safe limits (both cases)?
.
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