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1 ohm load across leads when multimeter is measuring current in the Ma range?
Posted by
Light Science
on 24 Oct, 2023 03:09
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Before you knock my dmm's, I know. I have an agilent bench dmm on the way, but for now, my crap kaiweets dmm's agree at least, but both of them exhibit this... thing.
When I was monitoring a 500 milliamp current, I ran the load through my meter in the milliamp range, but noticed the power supply was showing a higher resting voltage. I moved the meter to the amp range, and the leads, and everything was fine.
I used the other meter to measure the resistance on the probes basically there is 1 ohm of resistance between the probes in the milliamp range, but not the amp range. Is this normal? Both meters had it and it's annoying. How can I measure milliamp currents if it is going to drop a 1 ohm resistor in the path? Please tell me this is just something with my shit dmm's?
Thank you!
This is my first post! But I've been watching for 13 years.
Christian
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That's perfectly normal. Any amp, milliamp, microamp, meter is going to have some series resistance. The lower the range, the higher the resistance will be.
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#2 Reply
Posted by
EEVblog
on 24 Oct, 2023 03:32
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That's called burden voltage. All meters have it, good or bad quality.
It's a combination of the shunt resistor and the fuse resistance.
Well done on noticing it!, many people are completely unaware of it.
That's why I designed the uCurrent project, it's designed to minimise this.
Also my 121GW multimeter has a Low Burden voltage amplifier in it.
https://www.eevblog.com/projects/ucurrent/Read the article from Silicon Chip magazine here:
https://eevblog.com/files/uCurrentArticle.pdfAlso:
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#3 Reply
Posted by
ejeffrey
on 24 Oct, 2023 03:42
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Generally the sense resistor will be sized to have a few hundred millivolts dropped at the maximum current for the range as that provides the best sensitivity for the ADC. The exception is the amps range which is usually something like 10 mOhm to limit power dissipation.
If you don't care about losing some resolution moving up to a higher current range is usually an easy fix.
Other approaches are to measure voltage after the current meter. If you are using a power supply with sense leads, connect them after the current shunt. You can use an amplifier like the uCurrent adapter to get better resolution with a lower sense resistance.
Hall sensors are sort of the ultimate in low burden current measurement but tend to be low accuracy and low sensitivity.
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#4 Reply
Posted by
TimFox
on 24 Oct, 2023 03:46
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Another method, not useful in the 500 mA (note correct spelling) range is to use a transimpedance amplifier (op amp with feedback resistor) that can have negligible voltage across its input.
This is what is found in, for example, Keithley electrometers.
The op amp has to supply the feedback current to balance the input current, which is why it can be used for low currents (nA up to several mA), but is not practicable for 500 mA.
Otherwise, the meter must have a finite burden resistance, as described above.
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#5 Reply
Posted by
bdunham7
on 24 Oct, 2023 05:04
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When I was monitoring a 500 milliamp current, I ran the load through my meter in the milliamp range, but noticed the power supply was showing a higher resting voltage. I moved the meter to the amp range, and the leads, and everything was fine.
I used the other meter to measure the resistance on the probes basically there is 1 ohm of resistance between the probes in the milliamp range, but not the amp range. Is this normal? Both meters had it and it's annoying. How can I measure milliamp currents if it is going to drop a 1 ohm resistor in the path? Please tell me this is just something with my shit dmm's?
You may be disappointed when you get your Agilent bench DMM depending on the model. A 34401A, for example, will have a 5 ohm shunt plus the fuse in the 100mA range. The 1A range will be a bit better with a 0.1R shunt and fuse, but still about 0.4R overall. Better meters with a 10A range and the typical modern fuse may have lower resistances, like 0.05R total or less. Your Kaiweets may simply lack any fuse in the 10A range and thus have a reasonably low burden as well. To measure 500mA with low burden, just switch to a higher range, like 3A, 5A or 10A depending on the meter. You'll lose a digit or so of precision, but if you have a good 4.5-6.5 digit meter you can still get a pretty good reading.
As mentioned, some meters do have an active input system that reduces the burden voltage. These meters don't really appear to have a fixed resistance, more of a constant burden voltage over the range. These are typically only available in the lower ranges. Meters with this feature include the EEVBlog 121GW, Fluke 850xA series, Fluke 8808A and my handy Keithley 414A Picoammeter, just to name a few.
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#6 Reply
Posted by
Kleinstein
on 24 Oct, 2023 09:55
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Some burden voltage is normal, as the meter need enough voltage drop to measure. How much depends on the meter and some DMM, especially those with autoranging and a 5000 or 6000 count display may have a rather high burden (e.g. 500 mV range at full scale). Part of the burden is from the resistance of the fuse, that can be an issue when close to the limit of the fuse. How much voltage is lost depends on the meter and range - some use 1 shunt for 2 ranges and this way have a low burden in one range and a relatively high burden in the next. This is expecially the case with autoranging.
Even with an active (transimpedance amplifier) current measurement system there is still a relatively fixed resistance, e.g. from the fuse and wires and possibly other protection / filtering. The resistance is however usually smaller. There is an additional offset voltage and thus current flow with shorted terminals, but this usually small (e.g. < 1 mV).
AFAIK the Sigilen SDM3045 /3055 also use this for the lower range(s ?). Because of the extra power needed it is usually not found in battery operated meters - so not so sure about the EEVBlog 121GW. AFAIK this meter uses low value shunts and an extra amplifier stage and thus way gets relatively low burden voltage also for the higher current ranges.
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Thank you so much!
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The man himself
Thank you for the reply!
I just found the 121gw a few days ago.
What a meter! A buddy of mine is gifting me a Brymen 789 because I fixed his tube amp and he's not interested in this glorious world.
Thank you Dave, truly, for everything. You've helped me get through some long weeks.
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Very interesting. Burden voltage isn't a foreign idea to me, but higher resistance on lower ranges seems backwards, because it will have such a large effect. Thank you for the reply!
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#10 Reply
Posted by
wasedadoc
on 24 Oct, 2023 15:46
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Please learn to get your units correct.
M means one million. 1,000,000
m means one thousandth. 0.001
A is the unit of current. Not a.
So you should have written mA, not Ma.
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I do have the 34401a on the way, but I am really needing that for accurate voltage in tubes, or "valves" and a number of other things. Hopefully this Brymen 789 that is coming my way will be of service. At least I can trust it. Thank you!
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Very interesting.
For me, if I was charging a battery in the constant current mode, the voltage will shoot up so high if running through the meter, it will make accurate charging impossible.
Ya'll have given me a lot to think about. I want to design a little circuit that counteracts the drop. Just for fun.
Do I need to @ the user I am replying to? I don't see my replies linked to the users, despite hitting "reply" on that users post.
I know, I should RTFM of the forum, I was just too excited to wait. 38 year old man with a family giddy over electronics and connecting with like minded people... (Shaking my head)
Thank you!
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Yes, you are right, I need to correct my units. My phone autocorrect was being a pain. Coming from the IT world I am well versed in similar unit/capitalization rules, but did a poor job here. I will correct in the future. Thank you for the check.
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#14 Reply
Posted by
bdunham7
on 24 Oct, 2023 15:56
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Do I need to @ the user I am replying to? I don't see my replies linked to the users, despite hitting "reply" on that users post.
You can use @, but the norm here is to press the "quote" button instead of reply, then the post you are replying to will appear as a quote, which you can edit or trim if you like. This is what I've done here.
Your new 789 will have the exact same issue in the 600mA range (>1R total burden resistance) but should work fine for 500mA in the 6A range.
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#15 Reply
Posted by
IanB
on 24 Oct, 2023 16:04
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If I am measuring 500 mA, I generally would not use the mA range of a meter. There is a risk of blowing the fuse if you accidentally let the current get too high (ask me how I know).
The series resistance of the meter in mA and µA ranges is not always a problem because often the source impedance of the current is much higher than the meter itself (consider that an ideal voltage source has zero impedance, and an ideal current source has infinite impedance).
In the case of a constant current source, for example for charging a battery, the source impedance will be close to the ideal infinite value.
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Very interesting. Burden voltage isn't a foreign idea to me, but higher resistance on lower ranges seems backwards, because it will have such a large effect. Thank you for the reply!
Most meters use a resistor to turn the current into a voltage drop which is what they are actually measuring with the ADC. Therefore it makes sense that they use higher resistances for lower current ranges. Ohms law:
200uA thru 1K
= 200mV
200mA thru 1
= 200mV
2A thru 0.1
= 200mV
etc
Then they just move the decimal point on the display and add a uA, mA, A symbol as needed.
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Do I need to @ the user I am replying to? I don't see my replies linked to the users, despite hitting "reply" on that users post.
You can use @, but the norm here is to press the "quote" button instead of reply, then the post you are replying to will appear as a quote, which you can edit or trim if you like. This is what I've done here.
Your new 789 will have the exact same issue in the 600mA range (>1R total burden resistance) but should work fine for 500mA in the 6A range.
Okay cool. Thank you. I am such a forum noob..
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If I am measuring 500 mA, I generally would not use the mA range of a meter. There is a risk of blowing the fuse if you accidentally let the current get too high (ask me how I know).
The series resistance of the meter in mA and µA ranges is not always a problem because often the source impedance of the current is much higher than the meter itself (consider that an ideal voltage source has zero impedance, and an ideal current source has infinite impedance).
In the case of a constant current source, for example for charging a battery, the source impedance will be close to the ideal infinite value.
Very interesting. I didn't even think about the source impedance. I will save and think about your reply. Thank you.
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Generally the sense resistor will be sized to have a few hundred millivolts dropped at the maximum current for the range as that provides the best sensitivity for the ADC. The exception is the amps range which is usually something like 10 mOhm to limit power dissipation.
If you don't care about losing some resolution moving up to a higher current range is usually an easy fix.
Other approaches are to measure voltage after the current meter. If you are using a power supply with sense leads, connect them after the current shunt. You can use an amplifier like the uCurrent adapter to get better resolution with a lower sense resistance.
Hall sensors are sort of the ultimate in low burden current measurement but tend to be low accuracy and low sensitivity.
Heck yeah! That is a good idea.
My next challenge is to either filter the output of my crap bench psu for switching noise, or eventually replace it. I started with a bunch of junk stuff before this was a hobby for me, so I could accomplish some minor things, but now that stuff is all getting sold and replaced with much better stuff. When working with audio circuits, currently, I have to run them from battery to avoid the noise in the signal, which is really really not ideal for bigger power stuff. But, building a filter would be fun, if it worked. Don't even know if a good bench psu wouldn't have the same switching noise. (Not to jam another topic in. Can be ignored.)
Thank you!
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#21 Reply
Posted by
TimFox
on 24 Oct, 2023 16:46
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Very interesting. Burden voltage isn't a foreign idea to me, but higher resistance on lower ranges seems backwards, because it will have such a large effect. Thank you for the reply!
Most meters use a resistor to turn the current into a voltage drop which is what they are actually measuring with the ADC. Therefore it makes sense that they use higher resistances for lower current ranges. Ohms law:
200uA thru 1K = 200mV
200mA thru 1 = 200mV
2A thru 0.1 = 200mV
etc
Then they just move the decimal point on the display and add a uA, mA, A symbol as needed.
Ahh. Yes, that does make sense. Interesting how they don't basically have some tiny version of a clamp meter inside them. So much to learn!
DC-responding clamp meters take up a lot of room: see "flux gate" for the usual method.
The -
hp- 428A
https://usermanual.wiki/Manual/HP428A.387943453 is a classic: the actual flux gate is not very large, but it requires a reasonable amount of support circuitry.
Its most sensitive full-scale range is 3 mA fs.
It has no burden voltage, but inserts < 500 nH in series with the wire being clamped.
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#23 Reply
Posted by
EEVblog
on 26 Oct, 2023 01:29
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Thank you for the reply!
I just found the 121gw a few days ago.
What a meter! A buddy of mine is gifting me a Brymen 789 because I fixed his tube amp and he's not interested in this glorious world.
Nice! The 789 is a solid meter.