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  #16  
Old 02-15-2014, 02:00 AM
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Tubejunke Tubejunke is offline
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Quote:
Originally Posted by Alastair E View Post
.A Voltmeter was attached across the resistor to monitor current draw.
The current with the 33K is limited to a little over 4mA (4.4mA ish)
An volt meter in parallel to measure amps. Is that possible?
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  #17  
Old 02-15-2014, 10:29 AM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by Tubejunke View Post
An volt meter in parallel to measure amps. Is that possible?
Sure. Ohm's Law

I=E/R
R=E/I
E=IR

Where
I= Intensity (current in amps)
R=Resistance (in ohms)
E=Electromotive force (voltage)

So if you have a 33K (33,000 ohm) resistor and
current thru it is 4.4 ma. (.0044 amp)

What is the voltage across the resistor?
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  #18  
Old 04-17-2014, 06:49 PM
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Quote:
Originally Posted by Alastair E View Post
've been doing some experiments lately on increasing emission of mediocre/poor CRT's.

A Voltmeter was attached across the resistor to monitor current draw.
The current with the 33K is limited to a little over 4mA (4.4mA ish)
You can't measure current (Amperes) with a volt meter. And it is essential to remember that current measurements must always be taken with the meter in series with the circuit to be measured , not "across" it or in parallel. Probably just a type-o, but I saw the statement and thought that it would be worth noting and possibly saving your meter.
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Old 04-17-2014, 09:09 PM
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Quote:
Originally Posted by Tubejunke View Post
You can't measure current (Amperes) with a volt meter...
Sure you can if you are measuring the voltage drop across a resistor caused by the current flowing through it just like old_coot88 said.
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  #20  
Old 04-17-2014, 09:53 PM
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Electronic M Electronic M is offline
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Quote:
Originally Posted by Tubejunke View Post
You can't measure current (Amperes) with a volt meter. And it is essential to remember that current measurements must always be taken with the meter in series with the circuit to be measured , not "across" it or in parallel. Probably just a type-o, but I saw the statement and thought that it would be worth noting and possibly saving your meter.
Wrong. You CAN measure current indirectly with volt meter. Old coot's post does a good job of explaining that if you measure the voltage drop across a resistor of known value and use Ohm's law (I=E/R) you can compute the current flowing through that resistor quite accurately.

In fact most modern Digital Multi-Meters (DMMs) use that principle. In my freshman year of college I built a digital multimeter kit and it used that exact principle. It had a short length of thick wire shorting the + and - current measurement terminals and we had to adjust the length of wire that we soldered between those terminals so that it was exactly 1 Ohm. At that value 2A through it meant 2V across it, 1A -> a 1V drop and so on. The circuit simply measured the voltage drop across that wire that shorted the current measurement terminals and scaled the value according to the range it was set to.
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Last edited by Electronic M; 04-17-2014 at 10:11 PM.
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