Author Topic: Putting multimeter for current measure in series with circuit  (Read 5738 times)

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Offline Stuartambient

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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?

« Last Edit: July 21, 2016, 05:28:12 am by Stuartambient »
 

Offline ProBang2

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Re: Putting multimeter for current measure in series with circuit
« Reply #1 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.
 

Offline Paul Moir

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Re: Putting multimeter for current measure in series with circuit
« Reply #2 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.
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #3 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?
« Last Edit: July 21, 2016, 05:28:45 am by Stuartambient »
 

Offline Kilrah

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Re: Putting multimeter for current measure in series with circuit
« Reply #4 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...
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #5 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. 
 

Offline Kilrah

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Re: Putting multimeter for current measure in series with circuit
« Reply #6 on: July 21, 2016, 05:42:13 am »
Would probably learn more if we knew what your "lamp" is.
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #7 on: July 21, 2016, 05:47:44 am »
T 1 ¾ Bi-Pin Base Incandescent Lamps
Specification    Value
Terminal Type   WIRE TERMINAL
Additional Feature2   T1 3/4 bi-pin
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
 

Offline klunkerbus

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Re: Putting multimeter for current measure in series with circuit
« Reply #8 on: July 21, 2016, 06:02:41 am »
...The lamp is rated at 20mA...

FYI - the datasheet on  your Jameco link says 200 mA.
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #9 on: July 21, 2016, 06:13:15 am »
...The lamp is rated at 20mA...

FYI - the datasheet on  your Jameco link says 200 mA.

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. 
 

Offline KL27x

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Re: Putting multimeter for current measure in series with circuit
« Reply #10 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.
« Last Edit: July 21, 2016, 06:15:16 am by KL27x »
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #11 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. 


« Last Edit: July 21, 2016, 06:49:32 am by Stuartambient »
 

Offline Kilrah

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Re: Putting multimeter for current measure in series with circuit
« Reply #12 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.
« Last Edit: July 21, 2016, 06:25:11 am by Kilrah »
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #13 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.
 

Online tautech

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Re: Putting multimeter for current measure in series with circuit
« Reply #14 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.
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Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #15 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.
 

Offline Paul Moir

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Re: Putting multimeter for current measure in series with circuit
« Reply #16 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.
 
 
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Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #17 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?
 

Offline alsetalokin4017

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Re: Putting multimeter for current measure in series with circuit
« Reply #18 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.
The easiest person to fool is yourself. -- Richard Feynman
 

Online Brumby

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Re: Putting multimeter for current measure in series with circuit
« Reply #19 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.
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #20 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.
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #21 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. 
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #22 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
http://www.newark.com/caddock/mp930-0-10-1/current-sense-resistor-0-1-ohm/dp/02H2374

CADDOCK  MP930-1.00-1%  Through Hole Resistor, Thick Film, Kool-Pak®, MP900 Series, 1 ohm, 30 W, ± 1%, 250 V, TO-220
http://www.newark.com/caddock/mp930-1-00-1/current-sense-resistor-1-ohm-30w/dp/02H2377

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
 

Offline Paul Moir

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Re: Putting multimeter for current measure in series with circuit
« Reply #23 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.
 

Offline Stuartambient

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Re: Putting multimeter for current measure in series with circuit
« Reply #24 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?

 


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