# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: Stuartambient on July 21, 2016, 05:12:42 am

Title: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 05:12:42 am
I admit having no experience putting the multimeter in series with the circuit to measure current.  Now I'm following some text and a diagram but getting no results.  It's about as basic as it gets.

If I'm following the diagram correctly, the ground of the battery is going to the multimeter com, one positive is going to one leg of the lamp and the other leg of the lamp goes to the mA port on the meter.  Lamp not lighting though (confirmed it works) and no flow.

I'm wondering if it's something to do with my meter and how it might be in series with a circuit, maybe?

Title: Re: Putting multimeter for current measure in series with circuit
Post by: ProBang2 on July 21, 2016, 05:21:35 am

Strange...
If I want to see the picture of the circuit, then an error occurs:
"You are not allowed to access this section" WTF?

Back to the topic.
My first guess is: The fuse in the DMM is broken.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Paul Moir on July 21, 2016, 05:23:45 am
Yes, that's correct.  Likely the fuse in the multimeter is blown (it will not affect volts or resistance ranges, just current).  Put your multimeter in ohms, put the red probe back into the volts/ohms jack, and then put the probe tip into the current input jack.  If it reads open, the fuse is blown.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 05:26:40 am
So if the fuse is blown then the circuit won't work, i.e. the lamp won't light?
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Kilrah on July 21, 2016, 05:31:39 am
Given that current has to go through the meter, and the fuse is precisely there to open the circuit in case the current exceeds what it should not then yes, your circuit becomes open if the fuse is blown.

The first step when you do this is calculate the current that SHOULD be flowing and see if that's within your meter's range. If in doubt use the highest range/jack until you've confirmed current is less than the max of the mA range...
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 05:38:20 am
I guess it's possible I blew the fuse.
The lamp is rated at 20mA the meter says .315mA, so unless I did something wrong (more then possible) it shouldn't have blown.  Regardless I guess I'll open the meter up and take a look.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Kilrah on July 21, 2016, 05:42:13 am
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 05:47:44 am
T 1 ¾ Bi-Pin Base Incandescent Lamps
Specification    Value
Terminal Type   WIRE TERMINAL
Voltage (V)   6
MSCP (Mean Spherical Candle Power)   0.6

https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=210008 (https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=210008)
Title: Re: Putting multimeter for current measure in series with circuit
Post by: klunkerbus on July 21, 2016, 06:02:41 am
...The lamp is rated at 20mA...

Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 06:13:15 am
...The lamp is rated at 20mA...

Sorry, that is correct.

I'm puzzled regardless  as the DMM manual says 315mA overload protection but the label on the meter itself says 200mA MAX Fused.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: KL27x on July 21, 2016, 06:13:30 am
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.

In 10 years of doing this kinda stuff, I have used a current meter on a DMM maybe once or twice.

Put the shunt in place of where your DMM is, now. And measure the voltage across the shunt. This is exactly how your meter measures current. But it doesn't have as big (in wattage) of a shunt resistor inside that little case. With a large shunt resistor, you won't blow it out. And it's a lot more convenient than using your DMM as part of the circuit. You can move it around and measure other things, rather than needing multiple DMMs.

When using the 1R resistor, the amps = Volts.

When using the 0.1R resistor, you multiply Volts by 10 to get amps.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 06:18:14 am
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.

True, as I just got done with a thread here recently learning about shunt resistors.  Still I was thinking this was a small amount of current and rated to what this meter can handle.

Title: Re: Putting multimeter for current measure in series with circuit
Post by: Kilrah on July 21, 2016, 06:23:27 am
I'm puzzled regardless  as the DMM manual says 315mA overload protection but the label on the meter itself says 200mA MAX Fused.
That means 200mA is the max you can measure and should put through the meter, 315mA is where the fuse blows if you didn't comply.

An incandescent bulb has lower resistance when cold, so when you turn it on it can draw significantly more than what is rated during operation for a short while.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 07:54:34 am
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.

By the way, do those go together in series? I was kind of settled on getting a shunt, the type that mounts and is encased in metal or is metal(s).  Just haven't ordered one yet.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: tautech on July 21, 2016, 08:22:37 am
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.

By the way, do those go together in series? I was kind of settled on getting a shunt, the type that mounts and is encased in metal or is metal(s).  Just haven't ordered one yet.
You do have the leads in the DMM amps jacks I hope.
Black in common, Red in the highest range just to start with then a lower range if more resolution is needed and only if that lower range is not exposed to excessive current.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 08:35:33 am
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.

By the way, do those go together in series? I was kind of settled on getting a shunt, the type that mounts and is encased in metal or is metal(s).  Just haven't ordered one yet.
You do have the leads in the DMM amps jacks I hope.
Black in common, Red in the highest range just to start with then a lower range if more resolution is needed and only if that lower range is not exposed to excessive current.

Correct jacks and it was set to max range which is 200mA.  Should have thought it through better but I was hoping I'd clear the 315mA overload protection.  For a small bulb though it gives off a good amount of light  8)  Oh well lesson learned.

I'll try some of the precision power resistors as mentioned above.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Paul Moir on July 21, 2016, 08:39:19 am
Hi tautech,

From previous posts Stuartambient seems to have an Amprobe 5XP-A which only has a 200mA input jack, so no 10/20A range.

Stuartambient:  You would put one of the resistors in series with your circuit, then measure the voltage across the resistor.  1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp.  Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1.  They will get pretty hot at those currents though.

Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 09:17:05 am

Stuartambient:  You would put one of the resistors in series with your circuit, then measure the voltage across the resistor.  1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp.  Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1.  They will get pretty hot at those currents though.

Thanks Paul, trying to decide if I need precision .1% tol and how much of a difference would it make with a power resistor at 5% tol.  Big difference but maybe not to me? Could through mA's off by alot or if I can get a good reading off the resistor alone then work the difference into the math?
Title: Re: Putting multimeter for current measure in series with circuit
Post by: alsetalokin4017 on July 21, 2016, 12:25:43 pm

Stuartambient:  You would put one of the resistors in series with your circuit, then measure the voltage across the resistor.  1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp.  Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1.  They will get pretty hot at those currents though.

Thanks Paul, trying to decide if I need precision .1% tol and how much of a difference would it make with a power resistor at 5% tol.  Big difference but maybe not to me? Could through mA's off by alot or if I can get a good reading off the resistor alone then work the difference into the math?

Yes, you have almost certainly blown the fuse in your DMM.

Didn't we go through some calculations in that other thread, on how much the resistor or shunt tolerance value and the DMM's own accuracy affect the final accuracy of your measurement? I seem to recall posting some example calculations.

Here's a photo of two current-viewing resistor setups I use to measure current with the V or mV range on a DMM. One uses a 0.1 ohm and the other is 1.0 ohm.  I've also included the Data Sheet for the Ohmite precision non-inductive resistors I used, 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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Brumby on July 21, 2016, 12:43:51 pm

Stuartambient:  You would put one of the resistors in series with your circuit, then measure the voltage across the resistor.  1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp.  Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1.  They will get pretty hot at those currents though.

Thanks Paul, trying to decide if I need precision .1% tol and how much of a difference would it make with a power resistor at 5% tol.  Big difference but maybe not to me? Could through mA's off by alot or if I can get a good reading off the resistor alone then work the difference into the math?

The tolerance of the resistor will determine the accuracy of the voltage that appears across it.  5% is quite a big variation in measurement - but if you are happy that an actual 200mA current is showing as something between 190mA and 210mA, then you're good to go.

If you want to be a lot more accurate, you can measure the actual resistance of your shunt and do some V=IR math - but you may need to consider temperature effects on the shunt resistance, especially if it gets noticeably warm.

When you get down to it, you can use ANY resistor as a shunt - providing the burden voltage is acceptable - and use V=IR to calculate the current.  Using resistors in the decadic sequence of 0.01, 0.1, 1, 10, etc means you can read values directly from the meter, so long as you pay attention to where the decimal point goes.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 11:28:58 pm

Didn't we go through some calculations in that other thread, on how much the resistor or shunt tolerance value and the DMM's own accuracy affect the final accuracy of your measurement? I seem to recall posting some example calculations.

Yeah we did go through it and it wasn't my intention to rehash it. It was really the circuit that was confusing me.   I had been trying to decide between the resistors you reference here, a panel meter type shunt or building something using something like the TI INA series current sensing monitors.  I just thought for this little bulb measurement the dmm would be alright.  At least now I learned, without too much damage, what I need.

Outside of Digikey and Mouser are there any reliable Ebay vendors, U.S. based that sell reliable known precision resistors? I took a look yesterday will have to scour again today and see what I find.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 21, 2016, 11:38:01 pm
The tolerance of the resistor will determine the accuracy of the voltage that appears across it.  5% is quite a big variation in measurement - but if you are happy that an actual 200mA current is showing as something between 190mA and 210mA, then you're good to go.

If you want to be a lot more accurate, you can measure the actual resistance of your shunt and do some V=IR math - but you may need to consider temperature effects on the shunt resistance, especially if it gets noticeably warm.

When you get down to it, you can use ANY resistor as a shunt - providing the burden voltage is acceptable - and use V=IR to calculate the current.  Using resistors in the decadic sequence of 0.01, 0.1, 1, 10, etc means you can read values directly from the meter, so long as you pay attention to where the decimal point goes.

Thanks, that is good to know.  I'm going to try and get something precision.  The less terms I have to include in an equation the happier I'll be.  :-DD
Seriously though I have no need specifically for 100% accuracy so I could easily get a way with a 5% differential.  It would be just a nice to have thing.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 22, 2016, 04:46:14 am
Can someone check this post please. I did some searching on the suggested resistors and came up with the ones below.  I was looking at Newark because they also have the fuses in stock which Digikey was out on.  Basically the same offerings though.  I did not find .1R's rated to 10W.  The most was 7W, however the Caddock's are rated at 30W.  How do they get there?  Also should I only be looking for CSR because the Caddocks are not called current sensing resistors but I'm not sure of the difference.

CADDOCK  MP930-0.10-1%  Through Hole Resistor, Thick Film, Kool-Pak®, MP900 Series, 0.1 ohm, 30 W, ± 1%, 250 V, TO-220

CADDOCK  MP930-1.00-1%  Through Hole Resistor, Thick Film, Kool-Pak®, MP900 Series, 1 ohm, 30 W, ± 1%, 250 V, TO-220

OHMITE  15FR100E  Through Hole Current Sense Resistor, 10 Series, 0.1 ohm, 5 W, ± 1%, Axial Leaded, Wirewound
http://www.newark.com/ohmite/15fr100e/current-sense-resistor-0-1-ohm/dp/64K7926 (http://www.newark.com/ohmite/15fr100e/current-sense-resistor-0-1-ohm/dp/64K7926)
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Paul Moir on July 22, 2016, 05:00:36 am
Quote
How do they get there?

They get there by you magically keeping their case temperature at 25 degrees with your powdered unicorn heatsink.  Look on page 2 for the derating curve, and the line where it says 150 degrees max case temperature.

The nice thing is you can use a heatsink pretty easily with those resistors.  But you'll have to supply it.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient on July 22, 2016, 05:13:16 am
Quote
How do they get there?

They get there by you magically keeping their case temperature at 25 degrees with your powdered unicorn heatsink.  Look on page 2 for the derating curve, and the line where it says 150 degrees max case temperature.

The nice thing is you can use a heatsink pretty easily with those resistors.  But you'll have to supply it.

So more power more heat but it's not going into a stationary application for constant or long term monitoring application.  I'll be using it to measure with the DMM, though I'll still use a heat sink.  So these two Caddocks look okay for my needs?

Title: Re: Putting multimeter for current measure in series with circuit
Post by: mariush 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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: ez24 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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient 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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient 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!
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient 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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: mariush 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/ (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 (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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Brumby 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.   ;)
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient 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.
Title: Re: Putting multimeter for current measure in series with circuit
Post by: ez24 on July 23, 2016, 06:22:25 am
Be sure to get a HF IR meter, if you do not have one
Title: Re: Putting multimeter for current measure in series with circuit
Post by: Stuartambient 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.