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best way to measure current draw? |
felix le chat
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 Posted: Fri Sep 23, 2011 3:41 pm Post subject: best way to measure current draw? |
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Hello,
I need to know how much current some of my modules use. For this thread we will assume that this is always the +12V rail that draws more current.
I have a multimeter, but no idea how to do it safely.
For example, I could do the following:
* connect a ribbon power cable to the case power bus
* connect another ribbon power cable to the module to be measured
* connect a "crappy" DIY cable between the two ground pins of the ribbon power cables
* connect a "crappy" DIY cable between the two -12V pins of the ribbon power cables
* put the multimeter (DC current / Ampere mode) between the two +12V pins of the ribbon power cables and read the value
But is it safe? Especially, the multimeter connection will not be very solid so the module will power on and off many times, if you see what I mean.
Also, many risks for plugging modules backwards!
Any better solution?
Cheers
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sgnhh
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BrotherTheo
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| Posted: Fri Sep 23, 2011 4:13 pm Post subject: |
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I use a shunt resistor and measure the voltage across the resistor. then Ohm's Law gives you the current. Just make sure you use a small enough resistor so you don't drop too much voltage.
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Luka
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| Posted: Fri Sep 23, 2011 4:22 pm Post subject: |
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i have 2 distro cables one with a cut positive rail and one with a cut negative rail. you set your DMM to measure current then put it in series, ie power up the module with the DMM bridging the cut wires (make sure the DMM probes are in the correct holes). do positive, then switch distro cable and do negative
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felix le chat
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| Posted: Sat Sep 24, 2011 6:27 am Post subject: |
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| sgnhh wrote: | better solution: rtfm (if it exists).
most current draw is listed by the manufacturer. |
Well I only intended to measure the current when the manufacturer does not list the current draw.
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felix le chat
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| Posted: Sat Sep 24, 2011 6:29 am Post subject: |
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| BrotherTheo wrote: | | I use a shunt resistor and measure the voltage across the resistor. then Ohm's Law gives you the current. Just make sure you use a small enough resistor so you don't drop too much voltage. |
Thanks for this suggestion but...
What is the advantage of this solution compared to direct current measuring?
And how do you install the shunt resistor?
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felix le chat
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| Posted: Sat Sep 24, 2011 6:34 am Post subject: |
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| Luka wrote: | | i have 2 distro cables one with a cut positive rail and one with a cut negative rail. you set your DMM to measure current then put it in series, ie power up the module with the DMM bridging the cut wires (make sure the DMM probes are in the correct holes). do positive, then switch distro cable and do negative |
This is more or less what I intended to do at first, though I do not have cut power cables. Do you have a special method for "connecting" the cut power cables to the DMM probes? Or do you only put the metal parts in contact with your hands?
Maybe dominoes are an option
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felixer
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| Posted: Sun Sep 25, 2011 11:17 pm Post subject: |
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using a 'cut cable' can be dangerous: some chips blow if only powered from negative voltage. so if your connection between the cut positive->dmm->module is 'crappy' this might happen.
using a shunt resistor involves making a ribbon with a small resistor inline with the power cable: simply cut the positive cable and put a 1 ohm resistor between. you might as well do the same for the negative cable. be careful that those two can't touch each other! set them a few cm apart ... now measuring the voltage across the resistor will give you the current going thru it.
ohm's law: V=I*R or V/R=I. V=the voltage you measure, R=1 ohm, so I=the same value as V. set your dmm for mV as that will be the range we are talking about here ...
you could also put a resistor in line with the ground cable and get the total current (pos+neg).
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Big G
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| Posted: Mon Sep 26, 2011 4:47 am Post subject: |
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| felixer wrote: |
ohm's law: V=I*R or V/R=I. V=the voltage you measure, R=1 ohm, so I=the same value as V. set your dmm for mV as that will be the range we are talking about here ...
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By Ohm's Law if you put a 1 ohm resistor in a rail taking 1A you get a 1V drop--which will alter everything. A much lower value would be better, but it is easier to make two 4mm to faston (or whatever your distribution card and PSU use) cables and connect the meter in straight after the PSU. Multimeters already contain the precision low value series resistor of a much lower value than you can readily buy.
| Quote: |
you could also put a resistor in line with the ground cable and get the total current (pos+neg). |
No.
By Kirschoff's Law the current in the ground cable will be the difference of the positive and negative currents.
Putting a resistor in there also changes your 0V reference.
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felixer
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| Posted: Mon Sep 26, 2011 9:44 am Post subject: |
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| Big G wrote: | | By Ohm's Law if you put a 1 ohm resistor in a rail taking 1A you get a 1V drop--which will alter everything. |
true, but you won't be drawing 1A for a single module: more like 20-100 mA. i don't think the OP has any scientific aspirations here. i think he simply wants to know if he needs to up his psu.
and this cable isn't for running the module permanently. if it slightly affects the sound/performance: so what? it's just to quickly/easily/safely give an indication of the demands on the psu ...
| Big G wrote: | | easier to make two 4mm to faston (or whatever your distribution card and PSU use) cables and connect the meter in straight after the PSU. |
again: one mistake here and you might fry some chips. i don't think the OP will be able to fix that himself so that could become an expensive repair .... and your meter will be tied in that position. no way to quickly/easily check some other value (like neg current. or voltage drop ;-) elsewhere.
| Big G wrote: |
By Kirschoff's Law the current in the ground cable will be the difference of the positive and negative currents. |
meaning that if a module draws equal amounts of pos and neg current the current thru the ground cable will be 0 A? c'mon man, think logically ... anyway, total current isn't interesting if you want to calculate the demands on your psu ...
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Big G
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| Posted: Mon Sep 26, 2011 5:19 pm Post subject: |
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| felixer wrote: | | meaning that if a module draws equal amounts of pos and neg current the current thru the ground cable will be 0 A? c'mon man, think logically ... |
And what exactly do you think it will be?
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