The title explains it. I have an old voltmeter that is full of resister and i want to know if it is a high empedance voltmeter. How can i test it?
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how to check the impedance of a voltmeter.
- Thread starter longhaireddwb
- Start date
All voltmeters have a high measuring impedance so they do not affect the circuit under test by loading it. Look up the specs for your meter. It is usually found right on the meter face itself. It should say something like 10,000 ohms/volt or 20,000 ohms/volt. That means that the input resistance of the meter is that number times the full scale reading selected. If you have a 20,000 ohms/volt meter and you select the 100 volt scale the resistance is 20,000 X 100 or 2 million ohms.
CK beat me to it. Incidentally, as you read that really fine print on the meter face down near the pivot, you might see something like "FS 50ua". That's the amount of current (50 microamperes) needed to drive the meter movement itself to exactly full-scale.
And mathematically it will also tell you the meter's sensitivity in terms of ohms per volt. Using Ohm's law in CKs example, I=E/R, so
current = 100 volts / 20,000 ohms = 0.00005 amperes = 50 microamperes.
And mathematically it will also tell you the meter's sensitivity in terms of ohms per volt. Using Ohm's law in CKs example, I=E/R, so
current = 100 volts / 20,000 ohms = 0.00005 amperes = 50 microamperes.
On the front of it , it says 5000 volts AC and 20000 volts DC.
This is a Heathkit Model MM1
I looked it up on line and am not coming up with much on it because it is so old.
Can anyone else come up with a number/rating of impedance on this thing. I use it to hook to the speaker wires of a CB and set the RX. I normally set it on AC when doing this. I've never tried it on DC. Maybe I should.
Any comments?
This is a Heathkit Model MM1
I looked it up on line and am not coming up with much on it because it is so old.
Can anyone else come up with a number/rating of impedance on this thing. I use it to hook to the speaker wires of a CB and set the RX. I normally set it on AC when doing this. I've never tried it on DC. Maybe I should.
Any comments?
Look at the bottom of the meter scale in the middle. See where it says Ohms per volt. The impedance is 5000 ohms per volt on the scale selected on ac and 20,000 ohms per volt when on dc. I have no idea what you are talking about hooking it to your speaker leads to set the RX. I have neen using all sorts of multimeters for nearly 40 years and that one beats me. I have used them to measure audio output levels if that is what you mean but they generally require a 600 ohm circuit to be accurate and the results are in db not voltage.
I built one of those MM-1s about 50 years ago. Mine said "5000 OHMS/VOLT AC 20000 OHMS/VOLT DC". Note the slash (/) in there; it means "per".
Using that and the range/function switches will give you an idea of the "impedance", although that would probably not be very useful for precision work. I doubt whether "setting the RX", whatever that is, qualifies as "precision".
Impedance is an AC term which involves resistance along with inductive and capacitive reactance. If we knew what you're trying to do with the meter we might be able to help more.
Edit: http://www.heathkit-museum.com/test/hvmmm-1.shtml
Here's an old picture which you've seen if you've been Googling. Note that Heath correctly calls the 5000 ohms/volt AC and 20,000 ohms/volt DC "sensitivity" rather than "impedance". Sensitivity is a measure of how much voltage must be applied to give precisely a full-scale reading. It's so much less sensitive on AC because the AC signal, of whatever frequency (!) has to be rectified and filtered before it can be applied to the meter movement. Lots more circuitry involved; lots more places for something to go rwong.
Using that and the range/function switches will give you an idea of the "impedance", although that would probably not be very useful for precision work. I doubt whether "setting the RX", whatever that is, qualifies as "precision".
Impedance is an AC term which involves resistance along with inductive and capacitive reactance. If we knew what you're trying to do with the meter we might be able to help more.
Edit: http://www.heathkit-museum.com/test/hvmmm-1.shtml
Here's an old picture which you've seen if you've been Googling. Note that Heath correctly calls the 5000 ohms/volt AC and 20,000 ohms/volt DC "sensitivity" rather than "impedance". Sensitivity is a measure of how much voltage must be applied to give precisely a full-scale reading. It's so much less sensitive on AC because the AC signal, of whatever frequency (!) has to be rectified and filtered before it can be applied to the meter movement. Lots more circuitry involved; lots more places for something to go rwong.
I see. Well it is a very old meter. I guess i should try to find something more up to date for this.
As far as what im doing, if you inject a signal into a cb on frequncy and have a meter hooked to the speaker them while ajusting the rx pots you tune for higher reading on meter. And if you have the speaker also working then you tune by ear more signal and less nouse. This is the way ive been doing it for a while. I jnow its not as good as a sinad meter or a vtvm or scope but it works well after some practice.
I just didnt want to load the radio down too much when doing this so i wanted high impedance.
Does this make sence or am i pissing in the wind.
As far as what im doing, if you inject a signal into a cb on frequncy and have a meter hooked to the speaker them while ajusting the rx pots you tune for higher reading on meter. And if you have the speaker also working then you tune by ear more signal and less nouse. This is the way ive been doing it for a while. I jnow its not as good as a sinad meter or a vtvm or scope but it works well after some practice.
I just didnt want to load the radio down too much when doing this so i wanted high impedance.
Does this make sence or am i pissing in the wind.
Does this make sence or am i pissing in the wind.
Yes to both.
Now I understand what you are trying to do. It is far from a good idea mind you as some pots are not supposed to ne tuned for max in the receiver to prevent overload but whatever. The meter should be on AC to be able to measure audio. Don't worry aboit loading the radio. Your meter measures in the millions of ohms or at the least several hundred thousand ohms. A speaker is 8 or maybe even just 4 ohms. Going this route the impedance or resistance of the meter is the least of your problems.