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Can I measure current draw with my cheap multi meter?
- Thread starter nakman
- Start date
@DaveInDenver alright I'm gonna read that a few more times then decide if it's worth the effort... starting to feel like I'm splitting hairs here but appreciate all the tips. If nothing else I've confirmed that this fridge consumes more power both on and off, compared to my other Dometic fridge, as well as quantitatively validated that it'll indeed run the battery down if left plugged in. Funny how that's now a feature of all new fridges... I'm kinda missing that old Engel with the dial.
I'm losing this car for a few days now as Gavin is going to the mountains with his friends, but those boys will be another nice test of that fridge if nothing else. Right now it's on, full of pop, told him to leave it on all the time... between his driving and the 100w solar panel up top, I'd be surprised if he notices an issue.
I'm losing this car for a few days now as Gavin is going to the mountains with his friends, but those boys will be another nice test of that fridge if nothing else. Right now it's on, full of pop, told him to leave it on all the time... between his driving and the 100w solar panel up top, I'd be surprised if he notices an issue.
DaveInDenver
Rising Sun Ham Guru
@DaveInDenver please send more 0.2A mini fuses! 
I was doing some experimenting myself today with some misbehaving loads (liquids and stuff) and blew another meter fuse. I already have a bunch because they blow all-the-time. So I ask E, why can't the shunts be designed to handle a touch more current so I could have a 0.5A fuse for my 200mA circuit and a 15A fuse for my 10A circuit? Maybe I should just send it on a cheapo meter and see what happens? Bad idea?
I was doing some experimenting myself today with some misbehaving loads (liquids and stuff) and blew another meter fuse. I already have a bunch because they blow all-the-time. So I ask E, why can't the shunts be designed to handle a touch more current so I could have a 0.5A fuse for my 200mA circuit and a 15A fuse for my 10A circuit? Maybe I should just send it on a cheapo meter and see what happens? Bad idea?
DaveInDenver
Rising Sun Ham Guru
You brought up the fuse selection @AimCOTaco and thought you (and perhaps a couple of others) might be curious why they have special meter fuses. And why they're so dang easy to blow.
Not meaning to offend anyone but these cheap Chinese DMMs usually have plain AGC glass fuses and strictly speaking it's kind of a dangerous oversight.
How the fuse is done and the component selection plays heavily into the CAT III 1000 V, CAT IV 600 V ratings a DMM has. Not so much important for automotive work but is for even at household 120VAC and highly relevant when working in commercial and industrial switchgear and what-not when concerned with arc flash and shock. But even at 12V it's not completely out of the question since a fuse that creates an arc in an explosive environment is an obvious problem. So you may consider using ceramic body AGC instead of glass. Not that Toyota or anyone else did that with FJ40 fuses panels, though.
www.fluke.com
To your actual question, those shunts may tolerate different current than the rating. Fluke used an 11 amp and a 0.44 amp fuse in my DMM. The two shunts are intended for 10 amps and 400 mA but the specs say the high current shunt can tolerate 20 amps for 30 seconds and the low current one 600 mA for up to 18 hours.
This is an important specification because in some cases you're doing duty cycle measurement and the 10 amps is based on an average current delivered rather than instantaneous. This is particularly obvious in an RMS meter since since the peaks for 10A average are 14.14A. So the included fuse is designed to accommodate all of this. It'll blow fast but not so fast as to make typical electrician type measurements impossible. There's a lot of design choices made by the fuse guys.
It's kind of weird, though, that the sensing circuits can do things the fuse can't. If you read the spec closely you may be able to use different fuse values without damaging anything but just be aware you might also lose your safety ratings if you change fuse types. You know this but to be explicit, putting a 440mA fuse into a 3-1/2 digit, 200mA DMM isn't going to make it show up to 440mA either. It'll still be a max display of 199.9mA.
Not meaning to offend anyone but these cheap Chinese DMMs usually have plain AGC glass fuses and strictly speaking it's kind of a dangerous oversight.
How the fuse is done and the component selection plays heavily into the CAT III 1000 V, CAT IV 600 V ratings a DMM has. Not so much important for automotive work but is for even at household 120VAC and highly relevant when working in commercial and industrial switchgear and what-not when concerned with arc flash and shock. But even at 12V it's not completely out of the question since a fuse that creates an arc in an explosive environment is an obvious problem. So you may consider using ceramic body AGC instead of glass. Not that Toyota or anyone else did that with FJ40 fuses panels, though.
How to Choose the Correct Fuse for Your Tester | Fluke
Find out what the hidden dangers of making voltage and current measurements with a tester that does not have the fuse protection that was designed into the tester - dangers than can cause serious burns, and possibly even death.
www.fluke.com
To your actual question, those shunts may tolerate different current than the rating. Fluke used an 11 amp and a 0.44 amp fuse in my DMM. The two shunts are intended for 10 amps and 400 mA but the specs say the high current shunt can tolerate 20 amps for 30 seconds and the low current one 600 mA for up to 18 hours.
This is an important specification because in some cases you're doing duty cycle measurement and the 10 amps is based on an average current delivered rather than instantaneous. This is particularly obvious in an RMS meter since since the peaks for 10A average are 14.14A. So the included fuse is designed to accommodate all of this. It'll blow fast but not so fast as to make typical electrician type measurements impossible. There's a lot of design choices made by the fuse guys.
It's kind of weird, though, that the sensing circuits can do things the fuse can't. If you read the spec closely you may be able to use different fuse values without damaging anything but just be aware you might also lose your safety ratings if you change fuse types. You know this but to be explicit, putting a 440mA fuse into a 3-1/2 digit, 200mA DMM isn't going to make it show up to 440mA either. It'll still be a max display of 199.9mA.
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gungriffin
Rising Sun Member
@DaveInDenver Is this an instance that a clamp style meter would work better? I have one you can borrow if it would Tim. The only issue is that I don't know the exact resolution of what it can read. I bought it so I could measure the AC circuits in my fuse panel at my house. I was impressed at how precise the reading was on the circuit for my HVAC system. For $34 shipped at the moment, I am very impressed with it.
That looks like a bargain. What is the counts mentioned in the ad?
gungriffin
Rising Sun Member
I couldn't quite remember. Looking at the website it looks like the difference is the peak amps and voltage. The 4000 is 400 each and the 6000 is 600 each. I have had the 6000 for about a year. I paid about $45 for it a year ago. I believe the 6000 was only a few bucks more than the 4000 so I just sized up to the 600 instead. Given the current price with the 4000 being $20 after coupon, I would probably go that route now instead. The 6000 has proven to be surprisingly well built.
Edit The other item that is really nice about the 4000 meter is the included thermocouple for temperature readings.
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DaveInDenver
Rising Sun Ham Guru
Count(s) is a term we use to describe a digital meter's resolution. It's sometimes given in counts, it's sometimes given in digits.
A 2000 count meter shows 1999 as it's maximum value.
A 4000 count meter shows 3999
A 6000 count meter shows 5999
So in this case say you're measuring a value of tens of amps. The 2000 count on an example range (say 20A) can show zero up to 19.99 amps. The 6000 would give you a range of zero to 59.99 amps.
To show the same 59.99 amps the 2000 count would need a range of zero to 200A, thus showing 059.0 on it's display, So you lose one point of resolution to the right of the decimal. You only know the measurement to the tenths of amps (e.g. +/- 100mA) while the 6000 count will show you the current out to +/- 10mA since it has two digits right of the decimal.
Don't assume, though, just because one meter shows you more digits that it is accurate. It may be and having those decimal places allows it but doesn't mean the designers took advantage.
Not only can you learn a bunch here you can spend a bunch too! Ordering the one mentioned above.
DaveInDenver
Rising Sun Ham Guru
This meter is a good example of what I said in passing about resolution vs precision and accuracy. It gives you the option to pick 60A or 600A. In the comments one person mentioned the smallest he was able to measure is 1.75A.
So looking at the specs it says it can do a 6A range so it must try to auto range in the 60A setting. But reading the accuracy it says DC current is +/- 2.5% +5. Ignore the +5 for the moment and just focus on the percent. That 2.5% of 60A just happens to be 1.5A. So when you're at the lowest value of 60A range it will only be able to tell you a value within 1.5A. So that's would be the smallest value you'd expect to see. This is the point where the meter would drop to show zero based on the bit-width of the analog-to-digital converter.
Then you factor in the +5 part, which means plus 5 count. That means the lowest significant digit on a given range may fluctuate up to 5 digits. So if it's on a 60A range the values shown will be 00.00 up to 59.99. So 1.5A actual may show 00.00 or 00.05 up to 03.05 in theory. Even assuming that 1.5A is really it's minimum resolvable value on 60A it would still have a theoretical display of 01.45 up to 01.55 at that even if it did happen to have zero error (e.g. 0%).
This I know starts to really go off in the weeds about measurement, meters and analog-to-digital conversion.
Point is that, yes, this meter would work for Tim but since we're working in currents in the 5A region and want to see quiescent draws that are in 50mA to 100mA it would not really be a good choice unless you can get it to be in the 6A range.
In that case 2.5%+5 would resolve a value of +/-150mA+5 digits. So say you have 1.000A actual it would display 1.000 ideally but might show you 0.845 at the low end up to 1.155 on the high end of all its accuracy tolerances.
Yeah thanks for jumping in Ryan... if I read this correctly, it sounds like I'd be ok with the 4000 counts version? I've always wanted one of these things, and heck for $20 today sounds like the day. I only really care about truck stuff... so 12-13 volts, likely under 20 amps, just want to compare some fridges. Gonna add this one to the cart... https://www.amazon.com/dp/B0B9RKFJ2...0?ie=UTF8&psc=1&pd_rd_plhdr=t&aref=h7xf0KpOg0
Inukshuk
Rising Sun Member
I came here to post the clamp meter i have used to measure alternator output
Even cheaper. Works great for a year already
Inukshuk
Rising Sun Member
Be sure to buy the AC/DC model
Thanks for taking us through all the weeds Dave! Just what I was looking for!
The way I test fridge current with standard ranged DMM's is to get it cold first with a good load of sodas while wired on the 10A side, observe the peak current as the "compressor on load". Then, when it's all steady state cool, set the temp 15-20 degrees higher and leave it closed, this way you know the compressor won't kick for a long time so you can use the low range meter circuit to test "idle load".
If you're having a problem with start rush even with the compressor off you could use a jumper to bypass the meter as it starts then unclip to take readings.
I just watch the peak current these days and let the solar soak up the little standby current, but with an old edge-star digital/no radio fridge it's not a problem.
The way I test fridge current with standard ranged DMM's is to get it cold first with a good load of sodas while wired on the 10A side, observe the peak current as the "compressor on load". Then, when it's all steady state cool, set the temp 15-20 degrees higher and leave it closed, this way you know the compressor won't kick for a long time so you can use the low range meter circuit to test "idle load".
If you're having a problem with start rush even with the compressor off you could use a jumper to bypass the meter as it starts then unclip to take readings.
I just watch the peak current these days and let the solar soak up the little standby current, but with an old edge-star digital/no radio fridge it's not a problem.
Dudes, I think that amazon clamp is A/C current only. They need much more internal complexity to measure D/C so many clamps are A/C only...
DaveInDenver
Rising Sun Ham Guru
Yes! Look carefully at the description. Most handheld DMM with a shunt can do both but clamp style vary widely. Watch ranges close, too. A clamp style that can do 600A may say it can read small like 6A current but the likelihood it will do very wide ranges accurately is suspect, particularly on DC.
The one Tim linked has an AC only and AC/DC current.
https://www.amazon.com/dp/B0B9RKFJ2...0?ie=UTF8&psc=1&pd_rd_plhdr=t&aref=h7xf0KpOg0
The one Daniel shows first pops up on the AC model but there's others that are DC as he noted.
https://www.amazon.com/gp/product/B08MTY51R4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
The one Ryan linked is DC capable, the difference in models is 4000 vs 6000 count:
https://www.amazon.com/dp/B0CVXCFMLY/ref=twister_B0D7PDZNNG?_encoding=UTF8&psc=1
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Ok how about this guy for $32.65... https://www.amazon.com/dp/B075Z1GH5...trial&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWw&th=1
DaveInDenver
Rising Sun Ham Guru
Says it does DC current, so that ought to work. Don't see specs or a link for a manual to know any more than that.
A range of 800A is awful large for your use but it could be handy for checking starter, winch or alternator.
My general concern would it may not work well at small currents but lacking a test that's just a guess.
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