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Old 10th May 2004, 01:25 PM   #1
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Default Measuring current with an LCD meter

How can I measure current (DC) with an LCD meter?

I would guess that I would mesure the voltage drop over a seres resistor, but I have no idea how to implement this.

Any links, circuits or advice appreciated.
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Old 10th May 2004, 01:27 PM   #2
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To measure current you need to 'break' into a circuit. That is, you disconnect some part of the circuit and enter in your meter. Current through -- Voltage across, remember that and you will know how to measure. The number on the meter will be the amount of current flowing from your + lead to your - lead.

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Old 10th May 2004, 01:28 PM   #3
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Ohm's law That's it.

U = I*R =>

200 mV FSD you need a resistor with value

2 A => 0.1 ohms

200 mA => 1 ohms

etc.
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Old 10th May 2004, 02:10 PM   #4
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Default Re: Measuring current with an LCD meter

Quote:
Originally posted by Matttcattt
How can I measure current (DC) with an LCD meter?

I would guess that I would mesure the voltage drop over a seres resistor, but I have no idea how to implement this.

Any links, circuits or advice appreciated.
Does this meter have a 'current' scale? If yes, you can 'break' into the circuit as described above, that is you put the meter into the line where you want to measure the current. If the meter only has 'voltage' mode, you need a shunt resistor in the line, and measure the drop across the resistor. You want to keep the resistor as small as possible to avoid changing the circuit by the measurement process.

Jan Didden
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Old 11th May 2004, 10:20 AM   #5
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I need to re-phrase my question. By LCD meter, I meant an LCD meter module, like the one shown in the picture. They are designed to mesure voltage. How can I connect one of these to mesure current?
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Old 11th May 2004, 10:28 AM   #6
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you still haven't mentioned what current are you trying to measure.


high voltage? AC? DC? how many amperes? how much allowable voltage drop? what is the measurement range of the LCD module?
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Old 11th May 2004, 10:38 AM   #7
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This seems to be a classical 3,5 digit dvm, which measures +/- 199,9 millivolts without any additional circuitry.
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Old 11th May 2004, 07:41 PM   #8
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Quote:
Originally posted by djQUAN
you still haven't mentioned what current are you trying to measure.


high voltage? AC? DC? how many amperes? how much allowable voltage drop? what is the measurement range of the LCD module?
This is for in car use.
12v DC
>100A
Dont know...
I dont have a particular module in mind.

Quote:
Originally posted by HBarske
This seems to be a classical 3,5 digit dvm, which measures +/- 199,9 millivolts without any additional circuitry.
Yes
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Old 11th May 2004, 08:13 PM   #9
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If you find out I'd like to know. I want to monitor charging and usage of current on a boat, and 100A fsd would be about right (for now). The problem is the shunt resistor for that current. RS Components do some shunts for 200mV dmms, but not 100A ones. A Hall-Effect transducer could be used, but this is not without problems.
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Old 11th May 2004, 11:47 PM   #10
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A Hall-Effect transducer could be used, but this is not without problems.
http://www.allegromicro.com/sf/0752/#sample_request


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The problem is the shunt resistor for that current. RS Components do some shunts for 200mV dmms, but not 100A ones.
i would use a piece of copper cable.
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