Author Topic: Putting multimeter for current measure in series with circuit  (Read 5951 times)

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Offline mariush

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Re: Putting multimeter for current measure in series with circuit
« Reply #25 on: July 22, 2016, 05:40:24 am »
An observation that I think nobody mentioned :

Even with a 0.1 ohm resistor, at higher currents you will have some voltage drop. So if you power a product with 3.3v and it uses 1A and you put a 0.1 ohm in series with the circuit, you'll drop 0.1 volts on the resistor and your product will see only 3.2v

May seem like very little, but I've tried this trick on digital cameras with 3v input and was wondering why my camera reset itself while charging the flash, and it took me a while to realize for a few ms the flash charging drew upwards of 3A from the power source, dropping the voltage to less than around 2.7v and causing the camera to turn itself off.
 

Offline ez24

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Re: Putting multimeter for current measure in series with circuit
« Reply #26 on: July 22, 2016, 07:23:26 am »
As another solution along with the great ones given. 

I use cheap Harbor Freight meters for currents.  Always start at the highest range.  Since the wires do not change, I do not make the mistake of using the wrong jack.  Also the meters are accurate.
Since they are cheap, you can use several in your circuit.

Funny thing is since I started using them (only for current) I have not blown a fuse.  The only bad thing is they use 9v batteries.
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Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #27 on: July 22, 2016, 07:48:12 am »
As another solution along with the great ones given. 

I use cheap Harbor Freight meters for currents.  Always start at the highest range.  Since the wires do not change, I do not make the mistake of using the wrong jack.  Also the meters are accurate.
Since they are cheap, you can use several in your circuit.

Funny thing is since I started using them (only for current) I have not blown a fuse.  The only bad thing is they use 9v batteries.

I assume you are speaking about the $6 model? Who thought they could do it?  Reviews though are decent.  How is the accuracy.  This is what they listed
Accuracy
(@0mA-200mA)
1.2%±2D; (@10A)
3%±2D

The $20 model even has a temp jack and capacitance.  Pretty cool and would pick one up just to have as an extra. 
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #28 on: July 22, 2016, 09:06:50 am »

 along with a Data Sheet for some Caddock CVRs. You can order a large handful of these in various different resistances and power handling ranges for less than the cost of the single Shunt you listed in that other thread. The popular component houses stock them and can have them delivered to you in a few days.

Wound up ordering the Caddocks, though I could not find any   800 series and went with the 900, 930's to be exact. One 0.1, and a 1.  At some point I'd like to do a circuit using a current sensing monitor so these might be sufficient for that idea as well.  Thanks!
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #29 on: July 22, 2016, 09:12:53 am »
An observation that I think nobody mentioned :

Even with a 0.1 ohm resistor, at higher currents you will have some voltage drop. So if you power a product with 3.3v and it uses 1A and you put a 0.1 ohm in series with the circuit, you'll drop 0.1 volts on the resistor and your product will see only 3.2v

May seem like very little, but I've tried this trick on digital cameras with 3v input and was wondering why my camera reset itself while charging the flash, and it took me a while to realize for a few ms the flash charging drew upwards of 3A from the power source, dropping the voltage to less than around 2.7v and causing the camera to turn itself off.

I don't know much but I'm thinking that if the meter itself isn't causing the circuit to fail adding a 0.1 resistor probably won't make much difference.  I could be wrong.  The 100mV drop though would be significant but I should be able to work that into the math for a more accurate reading. 
 

Offline mariush

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Re: Putting multimeter for current measure in series with circuit
« Reply #30 on: July 22, 2016, 01:04:29 pm »
Another tip

If you need to constantly measure current of various devices and the voltage drop on resistors or in multimeters can screw things up, you can "upgrade" to DC clamp meters or hall effect sensor ICs or  current  transformers.
With DC clampmeters you don't have to break the circuit, you just put one wire through the middle of the clamp, and you get the current measured. However, the accuracy of the measurement depends on where the wire is positioned inside the area inside the clamp and  at low currents DC clamp meters are not so precise. You get maybe 2-5% accuracy when you measure.
Oh.. with dc clampmeters a simple trick can be to just twist the wire around the clamp once or twice to double, triple, etc the current value measured making it easier to measure low currents (but you lose precision).

Here's a good and cheap clampmeter reviewed here on eevblog: http://www.eevblog.com/forum/testgear/uni-t-ut204-clamp-meter-review-and-tear-down/


Hall effect sensor chips are awesome, the internal resistance of such chips is very small (somewhere around 1.2 mOhm or 0.0012 ohm) so the voltage drop is almost inexistent, and the output error is also usually small, at around 1-2% and if you really want super exact measurements you can characterize such chips to know how the output deviates when the temperate is much higher than normal and so on.

An example of such chip would be Allegro ACS712, here's a datasheet :  http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashx?la=en
Depending on chip, you can measure +/- 5A , +/- 10A , +/- 30A ... basically you power the chip with 5v and it outputs 2.5v at 0A, and goes up or down with a certain number of mV for each A of current, so you can just measure the voltage on the output and convert that value to current.

Such chips work better than some multimeters - some multimeters can measure currents up to 10A but have limitations, like for example they say "measure up to 15 seconds, then wait 5 minutes" or something like that - that's because if the current is high, the internal current shunt heats up and then the value of the current shunt changes and measurements are no longer correct... and if the current shunt heats up too much, it desolders itself from the pcb.
Chips like ACS712 having a fixed 1.2 mOhm (approx) resistance are not affected by this, you really can't push enough current through them for this resistance to overheat the chip.
« Last Edit: July 22, 2016, 01:06:09 pm by mariush »
 

Offline Brumby

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Re: Putting multimeter for current measure in series with circuit
« Reply #31 on: July 22, 2016, 02:30:28 pm »
Chips like ACS712 having a fixed 1.2 mOhm (approx) resistance are not affected by this, you really can't push enough current through them for this resistance to overheat the chip.

In normal situations I would agree with you - but I'm sure Photonicinduction could challenge that.   ;)
 
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Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #32 on: July 23, 2016, 05:18:53 am »
Another tip
Hall effect sensor chips are awesome.........

An example of such chip would be Allegro ACS712
Such chips work better than some multimeters - some multimeters can measure currents up to 10A but have limitations, like for example they say "measure up to 15 seconds, then wait 5 minutes" or something like that - that's because if the current is high, the internal current shunt heats up and then the value of the current shunt changes and measurements are no longer correct... and if the current shunt heats up too much, it desolders itself from the pcb.
Chips like ACS712 having a fixed 1.2 mOhm (approx) resistance are not affected by this, you really can't push enough current through them for this resistance to overheat the chip.

Thanks for the information.  I did look at the ACS712 briefly and it maybe a consideration when I get around to a constant current setup.   I did like the fact that it integrates pretty easily with an Arduino.  Right now a little inaccuracy is fine for what I'm doing which is learning how various types of circuits work.  I think my meter is okay for a $60 model.  I think if I can work out a circuit mathematically and measure it within reasonable degrees things will be good.
 

Offline ez24

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Re: Putting multimeter for current measure in series with circuit
« Reply #33 on: July 23, 2016, 06:22:25 am »
Be sure to get a HF IR meter, if you do not have one
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Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #34 on: July 23, 2016, 06:54:45 am »
Be sure to get a HF IR meter, if you do not have one

I will.  They are opening one somewhat close in a few months so it'll be a good time to take my coupons in there.
 


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