EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Felicitus on February 13, 2014, 12:29:30 am
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Hi,
I have studied quite many schematics recently, but none of them could answer my question so far: How do multimeters do auto-ranging without relais and without blowing up their analog switch?
I have attached an example voltage divider as example. So if I apply 1kV at MEAS_IN, the divider would divide down to 1:20, which results in an input voltage of 50V at the 4051, which is certainly too much (and multimeters won't supply +.50V to Vcc/Vee).
I then thought: "Hey, why not add clamping diodes to each divider?". In theory, this was a good idea, however, I didn't realize that they clamp all the time, so if the clamping diodes at the 1:20 node clamps, I get the clamped output voltage divided down, not my input voltage.
I also have looked at several schematics for open source multimeters, including the OSHW Multimeter Project by stoney49 (which didn't show the input voltage divider at all) and the heise/c't Voltmeter for their lab (who use relais to switch inputs), so I couldn't answer this for now.
Any pointers in the right direction are appreciated.
Felicitus
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simple they use analog switches without protection. some of them use external jfets to perform the switching. others (like the agilent 34401) use asics with floating mosfets to do this work.
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Can you give me a pointer to such a part? Every analog multiplexer I have checked so far has a certain (low) value for their absolute maximum ratings for the input voltages, and often it's something like Vcc+2V/Vee-2V-
I also might have lack in understanding analog switches. If I have a look at the CD4066 and place a clamping circuit at the outputs, would the CD4066 be damaged if e.g. 100V is applied to the input pin in the OFF state (hence no current is flowing)? Also, if the device is in the ON state, would the clamping circuit at the output be fast enough to turn on, and protect the CD4066?
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This has been answered by "accident" in this thread (https://www.eevblog.com/forum/projects/caddock-1776-c48-odd-resistor-dividor/msg386619/#new). Big thanks to Jay_Diddy_B for giving me the pointer in the right direction!