Hi people, this is my first post in this comunity, so be nice plz

well, the situation is:

i have to build an analog multimeter for a proyect. I've make the schematics of the main multimeter, is very simple, the display is a galvanometer so the circuit is basically a bunch of resistances for the scales, and the galvanometer itself...

i want to protect my multimeter, since it's so simple, is very easy to blow the galvanometer accidentally just by using a wrong scale or a bad connection and naturally i dont want this

so my idea is, to have a voltaje or current regulator toghether with the galvanometer so that once the maximun value of voltaje or current goes off the limits of the galvanometer, prevent it from get burned, and activate some led in order to warning, sounds simple...
in the galvanometer the maximun current is 55 mA, and the maximun voltaje is 83 mV, it has an impedance of about 1.5 ohms aprox.

i've seen voltage regulators with zener diodes, and it is possible to handle the situation with a zener, the problem is that there are no zener diodes of 83mV. a small fuse could work, but once you reach the max current, it'll blow off instead of the galvanometer of course, but no led will be activated, and the fuse need replacing in order to work, wich can be kinda annoying. a fuse will work as a partial solution and i will go that way only if i dont find anything else to do before...

so please, if is there any suggestion or advice you people can give me in order to solve my problem, post it, and thanks

PD: <<<i've included the schematic of the multimeter>>>

in this design, in the ohm scale the zero value is on the right part of the galvo, and the infinite is on the left (oposite to the ammeter and voltmeter), so when you measure 0 ohms the current in the galvo will be 55 mA. that's why the potentiometer is there, as the 9v battery degenerates with time, the user needs to set the 0 ohm using the potentiometer to regulate the current.

the other part are just current and voltaje dividers

cya Smile

Attached Images

galva.gif (51.3 KB, 23 views)

 

Most multimeters have a very high input resistance when measuring volts, so they don't load down the voltage being measured. Your multimeter has a very low input resistance.
Your meter movement takes a very high current and the battery will not last long.

Get a 1mA or more sensitive meter movement and buffer it to a very high input resistance with an opamp.

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Uncle $crooge

 

Yes, the original meter is FAR too insensitive, analogue meters are specified in 'ohms per volt', using a 50uA meter (which is common) gives you 20,000 ohms per volt. Even a 1mA meter is really far too insensitive for a multi-meter, although they were used pre-1960's or so.

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thanks for the answers

well i know the galvanometer is not to appropiate for a multimeter, becouse it is very insensitive... but this is for a proyect in the university, and here (i live in venezuela) those galvos are the only thing you can find (with a reasonable price), the other are just TOO MUCH expensive, so this will work for the proyect...

anything about the regulator??

thanks

 

You don't need a regulator, the usual method is simply diodes across the meter movement, they conduct at 0.7V and prevent too much current flowing through it.

You would need to check the suitability of such a scheme for your insensitive meter though.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk