I need a clamp ammeter but it's for very occasional use and I'm trying to decide between a few cheap Chinese products. One has ranges of 2A, 20A & 200A while the others have 20A, 200A and
400/600/1000A.
In the unlikely event that I ever need the higher ranges, I can always borrow a meter from the power company, so I'm attracted to the one with the 2A range. But I have a question about the practicability of measuring down to a few mAs with a standard non-contact technique. Is it likely to be usefully practical or about as practical as, say, a milliohmmeter with a two-wire probe?
I'm not sure this is going to work out, but I just bought an AN8802 TRMS Clamp Multimeter 6000 Counts Temperature Auto Range AC/DC Ammeter for US $29.68.
Here are two ebay item numbers:
282421973048
332087546732
They claim AC and DC current measurement from 600uA to 1200A, True RMS readings, plus a bunch of AC and DC, capacitance, frequency, resistance capability. Here are the specs:
550V protection in resistance and capacitance ranges. Large LCD display,MAX display 6000 counts. Sample rate: 3 times per second. Backlight Data hold Polarity identification Low voltage indication
1200A high current measurement, low current measurement Auto power off Main measurements: AC/DC Voltage, AC/DC Current, Resistance, Capacitance, Diode and Continuity Testing, Temperature, Frequency and duty cycle.
Specifications:
+15)
5.0%+5)
Measurement mode: Double-integral style A/D transform Over range indication: OL Working environment: 0~40?, relative humidity < 80% Supply power: 3V (1.5V AA batteries * 2 Pcs)(Not included) Frequency: 1Hz to 10MHz Temperature: -20C to 1000C(-4? to 1832?) Capacitance: 10pF to 6000uF AC volts: 0.1mV to 750V DC volts: 0.1mV to 1000V AC current: 0.1uA to 1200A DC current: 0.1uA to 1200A Resistance: 0.1 to 60M Duty cycle:1% to 99% Product size: approx. 238*90*45mm/9.37*3.54*1.77'' Gross weight: approx. 286g
Note: Please allow 1-3mm differs due to manual measurement. Due to the different display and different light, the picture may not reflect the actual color of the item. Thanks for your understanding.
Included:
1 x Multimeter
2 x Test Leads
1 x Temperature Sensor Cable
1 x Storage Bag
1 x User Manual
Even though the uA range is printed on the case, I suspect the uA values should really be mA. Then it would keep the progression from 600 mA to 6A and 60A to 1200A.
I don't know how they claim to measure DC current, but if it's true, it will pay for itself quickly on problems with my car battery charging. If not, then True RMS and a wide current range is still very useful.
The specs are nice but it's way above my budget. What I'm looking at are in the US$6-7 range.
I once read a long time ago - long before the internet age - about a technique used by one manufacturer for making DC amp measurements with a clamp. It involved biasing the clamp's magnetic property with a multivibrator (discrete of course). The measured DC current shifted the operating point along the non-linear portion of the B-H curve. I don't remember what the claimed accuracy was but I get the impression that it was quite reasonable.
In addition to the DC issue, one should remember that the AC _current_ waveform can be far from sinusoid these days with all kinds of SCR, Triacs and SMPSs.
What do you want the meter to display and what is it actually displaying ? Ipeak/1.41, some quasi-peak or trueRMS ?
Current technology uses a hall device to measure the DC current using the clamp. For RMS, they might also have to sum in the AC component from the coil, depending on frequency response required.
If you look carefully at the specs and the front panel, you might conclude that the clamp has a lowest range of
60A FS and the test leads measure up to 6mA.
Would be more useful if the leaded range went two decades higher to 600mA.
Just verified that both my similar units limit at what would be 6mA on this one using the wired input.
The Tektronix P6042 current probe can do 1mA/division from DC to
50 MHz, but the size of the clamp will barely accept a #12 wire. The instrument is maybe 50 years old. It's a hall device.
Speff, do you have any idea how they could fix the zero drift?
I looked for a zero adjust on the AN8802 but couldn't see one. Googling for DC clampmeters shows several methods, but they all sound drifty. That's why I am a bit skeptical about their specs. If it actually does handle DC current, first thing will be to take it apart and find out how it works!
I bought 7 of these for $7.35: because I was tired of loaning my good clamp ammeter to friends, neighbors, customers, visitors, friends of visitors, etc. It would usually take weeks and ocassionally a burglary to arrange for it to be returned. I gave 5 of them to the worst offenders and haven't heard a word for about 2 months. That means either they are satisfied with the gift, or the meter exploded when they tried to use it. I haven't bothered to ask (or care).
The meters a junk, but adequate for the purpose. It takes 3ea AAA batteries which I guess is cheaper than the usual 9V battery. It does AC only on the clamp, not DC. It is NOT a true RMS meter and will produce weird results with non-sinusodial waveforms (which is just about everything I would want to measure).
The maximum current is 400 amp. If you ever find something that actually draws 400 amps, I would hesistate to use this meter to measure it. I don't know what might happen, and I don't want to find out. Use a better meter for high currents.
Mostly, I use these for much lower currents. 20A full scale is not sensitive enough. So, I use a 10 turn wire loop in series with an AC power plug/jack adapter to increase the sensitivity to 2A full scale. Radio Shack (Micronta) used to sell them.
Good luck and don't forget to buy a spare to loan (or sell) to your cheap friends.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
More info. The DC ammeter may be sensitive to the Earth's magnetic field as well as thermal drift. In this case, a zero adjust is necessary to cope with the varying position of the clamp jaws. So if a clampmeter advertises DC current capability, it needs to have a zero adjust somewhere. There is no other way to fix this problem.
REF:
3.2 Zero Adjustment
When in DC mode the display zero may change due to thermal shifts and other environmental conditions. An auto zero adjustment is provided. Proceed as follows to perform the adjustment:
Ensure that the instrument is away from the current carrying conductor and that the jaws are closed during the adjustment cycle.
Select the DC position of the power switch.
Use the auto zero button to zero the display if necessary.
The auto zero button can be used to null the effects of the earth's magnetic field on DC measurements.
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So when you have zeroed the clampmeter, the zero adjustment may change when you put the jaws around the cable. It may not be practical to put the clamp on the cable and stop the current flow to zero the reading.
There is another problem. If you zero the meter in the same position as when measuring, then any steel in the vicinity may change the reading when the clamp is moved to the cable.
Other references show the accuracy depends on centering the cable in the jaws. Apparently there can be a significant error if the cable is off- center and close to the jaws.
DC current measurements may turn out to be tricky.
Typically, a spot weld short enough to weld a small item is too short for the sample rate of the meter. Mine don't have display hold functions either.
Microwave transformer welders certainly have their place, but to do small stuff without overheating the attached pieces, like for battery tabs, you need a short pulse. For short pulses, the amount of energy actually delivered to the weld is strongly dependent on the where you left the core on the B-H curve at the last weld. To get useful repeatability, I had to count cycles of AC and start/stop at repeatable core magnetization positions. Even then, welds were reliably unreliable.
My real CD spot welder does a 7000 amp pulse. Measuring that with a scope proved difficult. When it fires, every scope on the bench triggers whenever it wants to.
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shows the output pulse and how they reset the core between hits.
Not always. I order quite a bit of electronics on eBay that allegedly is shipped from China. However, instead of the usual 2 to 4 weeks delivery, it arrives in a few days from a random US location. Apparently, some of the vendors have become big enough to warehouse their stuff in the US. I haven't checked, but my guess(tm) is that it is shipped from someone's garage or house, who has relatives in China.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
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