Quizzes & Puzzles5 mins ago
Digital multimeter, readings and auto ranging
I'm getting readings between 1.30v dc and 0.60 v dc from two easily identified contacts on a particular device.
Some of my colleagues around the world say they get readings around 9v dc
I measure the same on my digital and my old analogue multimeter, set to 20v range. Only a couple of others get these measurements where more are saying 9v plus or minus 0.5v.
At least one of the 9v guys is using an 'auto ranging' meter, is it possible that this is moving the decimal point?
Help. how can i triple check myself?
Some of my colleagues around the world say they get readings around 9v dc
I measure the same on my digital and my old analogue multimeter, set to 20v range. Only a couple of others get these measurements where more are saying 9v plus or minus 0.5v.
At least one of the 9v guys is using an 'auto ranging' meter, is it possible that this is moving the decimal point?
Help. how can i triple check myself?
Answers
1 to 9 of 9
It depends on the impedance of the circuit you are measuring and the impedance of the meter itself.
A digital meter has such a high impedance that it can easily get a reading of voltage induced into a high impedance circuit. As such the value can be anything.
Putting a resistor across the point being measured will lower the impedance and give more...
A digital meter has such a high impedance that it can easily get a reading of voltage induced into a high impedance circuit. As such the value can be anything.
Putting a resistor across the point being measured will lower the impedance and give more...
07:54 Tue 07th Aug 2012
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It depends on the impedance of the circuit you are measuring and the impedance of the meter itself.
A digital meter has such a high impedance that it can easily get a reading of voltage induced into a high impedance circuit. As such the value can be anything.
Putting a resistor across the point being measured will lower the impedance and give more sensisble readings.
It also depends what kind of circutry is connected. 0.5 volts is often found in circuits that involve diodes or transistors since this around about the forward drop of a PN junction.
It could also explain why two readings are popping up. For example an external connector may have a zener diode across it to short out voltage spikes. It might have a nine volt rating which means the junction breaks down when it reaches this potential. In reverse it will show somewhere around half a volt which is the forward drop of the diode.
These reading may appear on a high impedance circuit from the induced voltages.
A digital meter has such a high impedance that it can easily get a reading of voltage induced into a high impedance circuit. As such the value can be anything.
Putting a resistor across the point being measured will lower the impedance and give more sensisble readings.
It also depends what kind of circutry is connected. 0.5 volts is often found in circuits that involve diodes or transistors since this around about the forward drop of a PN junction.
It could also explain why two readings are popping up. For example an external connector may have a zener diode across it to short out voltage spikes. It might have a nine volt rating which means the junction breaks down when it reaches this potential. In reverse it will show somewhere around half a volt which is the forward drop of the diode.
These reading may appear on a high impedance circuit from the induced voltages.
Hello BillyBB
beso's answer quite valid and sound advice, but there might be something a little more complex going on.
If you connect a DC coupled oscilloscope to the test points on the device, you might find that whilst you have a general level of 9 volts DC there is a large AC component (a rapidly varying voltage). This would confuse your digital meter into displaying a varying reading. If it's high frequency, then switching the meter to AC won't help. Your connecting an analogue meter is probably the best way to get a general idea as to what's happening, as it will integrate the AC waveform and give you an average although very approximate reading but the oscilloscope is the only way to be certain as to what's going on.
You can easily check for an AC component on a DC voltage by connecting a non-polarised capacitor of a few hundred nano farad in series with one of your analogue meter test leads and set it to the AC range. If you get a reading, then it must be AC that's causing it as the capacitor will block all DC from getting to the meter. This test will only work with your analogue meter as the digital one will have a very high input impedance which will be a disadvantage in this case.
Regards
Peter
beso's answer quite valid and sound advice, but there might be something a little more complex going on.
If you connect a DC coupled oscilloscope to the test points on the device, you might find that whilst you have a general level of 9 volts DC there is a large AC component (a rapidly varying voltage). This would confuse your digital meter into displaying a varying reading. If it's high frequency, then switching the meter to AC won't help. Your connecting an analogue meter is probably the best way to get a general idea as to what's happening, as it will integrate the AC waveform and give you an average although very approximate reading but the oscilloscope is the only way to be certain as to what's going on.
You can easily check for an AC component on a DC voltage by connecting a non-polarised capacitor of a few hundred nano farad in series with one of your analogue meter test leads and set it to the AC range. If you get a reading, then it must be AC that's causing it as the capacitor will block all DC from getting to the meter. This test will only work with your analogue meter as the digital one will have a very high input impedance which will be a disadvantage in this case.
Regards
Peter
1 to 9 of 9
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