Multimeter Question

[matthewwren]

I am having touble finding out what is wrong with a circuit I have designed. So I have gone out and bought a multimeter to check what voltages are going in and coming out of the various IC pins. I just wanted to ask if there are any gotchas i should know about regarding the use of multimeters on circuits?

 

[bananasiong]

Many fuse in the digital multimeter in my college were blown because many of them measure the current as the voltage.

 

[BeeBop]

I am having touble finding out what is wrong with a circuit I have designed. So I have gone out and bought a multimeter to check what voltages are going in and coming out of the various IC pins. I just wanted to ask if there are any gotchas i should know about regarding the use of multimeters on circuits?

Do your circuit calculations first so you know what to expect in your measurements.

You can avoid the problems bananasiong is talking about by always measuring voltage, rather than trying to measure currents in your cct. You must break the circuit to measure current, however voltage can be measured across components. Measure the voltage across a resistor, etc., then use Ohm's law to find the current.

You should also know that most multi-meters cannot, reliably, measure AC.

 

[matthewwren]

My multimeter comes with an rs232 adapter and software, so I am guessing this would cover everything an oscilloscope does too, or are oscilloscopes more reliable and have extra functions?

 

[dknguyen]

My multimeter comes with an rs232 adapter and software, so I am guessing this would cover everything an oscilloscope does too, or are oscilloscopes more reliable and have extra functions?

I wouldn't say necessarily more reliable, but definately much more capable (more graphical, more functions (and not useless gimmicky functions either) and much much faster and higher bandwidth. They cost $2K+ for a reason (though you can get hobby student PC-based models for like $200).

 

[JimB]

You should also know that most multi-meters cannot, reliably, measure AC.

A very sweeping statement!
Would you care to elaborate?

JimB

 

[aljamri]

My multimeter comes with an rs232 adapter and software, so I am guessing this would cover everything an oscilloscope does too, or are oscilloscopes more reliable and have extra functions?

What type of multimeter is that, name or photo will tell more.

Points to be aware are :

- AC and DC measurements,
- Current on one side, Voltage, Resistance, Frequency, Continuity Test on the other side,
- Special Functions like: Diode test, Caps Measurement and Temp
- Multimeter Setup.
- use of proper terminal leads ( Pin, Alligator ... )

and the best point of start is to read the Manual.

 

[BeeBop]

A very sweeping statement!
Would you care to elaborate?

JimB

Perhaps it is very sweeping, however the DMMs I've used will give an average value for an AC voltage up to 100Hz only. I use a scope for AC measurement.

Why, do you have a meter which will handle AC well?

 

[matthewwren]

It's just a simple multimeter that cost £20+ ...

https://www.uni-trend.com/UT60A.html

 

[Leftyretro]

Perhaps it is very sweeping, however the DMMs I've used will give an average value for an AC voltage up to 100Hz only. I use a scope for AC measurement.

Why, do you have a meter which will handle AC well?

You get what you pay for. Many DVM models can be purchased that do true RMS AC measurement. My several Flukes include that feature.

Lefty

 

[BeeBop]

My several Flukes include that feature.

OK, I've been forced to update my knowledge here; surprising what the voice of a teacher can do (I was told years ago, not to use a DMM for AC!)

I looked in the manual for my old 8012a (Fluke) and according to that, AC is good up to 200V at 50 kHz with a + or - 5% error. It is true RMS too.

Thanks!

 

[quixotron]

I am having touble finding out what is wrong with a circuit I have designed. So I have gone out and bought a multimeter to check what voltages are going in and coming out of the various IC pins. I just wanted to ask if there are any gotchas i should know about regarding the use of multimeters on circuits?

You're not measuring microwave frequencies are you? The voltage and current characteristics differ greatly across line lengths and loads. You won't get a consistent measurement.

 

[Dean Huster]

My multimeter comes with an rs232 adapter and software, so I am guessing this would cover everything an oscilloscope does too, or are oscilloscopes more reliable and have extra functions?

All that interface will do is provide automated data logging. A DMM is still a DMM and connecting it to an oscilloscope doesn't turn it into a scope. The data points it logs could be used by the computer to plot the measurement vs. time, but that's not the same thing.

A Simpson 260 was good to around 150KHz on ACV. Some of the lower-priced Fluke multimeters started falling off at around 1KHz. Better and/or newer DMMs will have higher AC frequency capability, but still not as good as a true AC voltmeter, e.g., the Hewlett-Packard 400E (10MHz, average-reading) or 3400A (10MHz and true rms). Of course, some oscilloscope models will measure up to 1GHz real-time while most run around 100MHz or more. However, the scope does not give you the accuracy and/or precision of a DMM. Satisfy your needs wisely!

Dean

 

[matthewwren]

Thanks for your help ppl ... the multimeter diagnosed my problem just fine and my circuit is working as intended now.