Accuracy of clamp on ammeters?

[devnull]

I've been looking for a suitable variac for working on SS amps, and see there are some with meters, some without. Naturally, the ones with meters are considerably more expensive, although those meters would be nice. I'd like to know just what is being fed to the amp, and was wondering if the clamp on style ammeters are accurate enough to provide a reasonable measurement.

John

[markn2wae]

The specs. on a given unit would tell you the accuracy.

I haven't used a clamp on ammeter extensively but I do know the proper and improper way to measure AC current draw to a device.

Note, unless the instruction tell you otherwise, you can't just clamp the thing around a (intact) line cord and measure the current draw, you need to separate one conductor (wire) from the rest and clamp around that to get a proper reading.

If you "clamp" the whole group of 3 wires (hot, Neutral and Ground), you are reading the difference between the current going out to the device and coming from it, and unless you have a fault that is diverting the current, you may read little or nothing (this is what a ground fault interrupter does incidentally)

I have seen electricians test a whole metal conduit, most likely to see if the conduit is carrying an un-desired current through a fault or mis-wiring error.

There are fixtures for doing this, that have an AC plug on one end and a socket on the other, with a square "loop" in the center that you clamp into to measure the current.

To make your own, just "take apart" the 3 wires of a line cord at the center, separating them WITH their insulation intact so you can clamp onto any one of the 3 wires you choose to measure.

There may be other "tips and tricks" to using these devices, but I just know the basics.

mark T.

[jaymanaa]

Modern day clamp ons are very accurate.

[ken kantor]

This here is the thing!

http://www.p3international.com/produ.../P4400-CE.html

Measures voltage, current, Watts, VA, power factor, etc. Costs about $20.

I have clamp-on and in-line meters, even a Tek current probe for my scope. But Killawatt is the go to device for most work. When I first got mine, I ran around the house testing everything I could think of!

Now my standard bench setup is variac -> lightbulb fixture -> killawatt -> DUT.

-k

[MOPAR3]

A clamp on meter is used to measure larger currents. Not sure how accurate it wiil be to measure say like a tube amps draw. My solid state amp only draws 500 ma at idle. You need to check the specs of the test device in question and see what its resolution is. If your measureing less than 10amps or so you can do it directly through your meter. You'd need a special cord so you could put your meter in series with the load. This can be dangerous if not set up properly and it would probably be more accurate to buy the metered variac.

[devnull]

Thanks for the insight into what a clamp on ammeter is about! I'm in the process of gathering up gear, and am definitely coming into this from a beginner's perspective. I found a Tektronix 2246 recently that was very reasonably priced, and have been searching for the necessary components to enable me to work on amplifiers.

Thanks for the general description of how these devices work, as I have absolutely no knowledge of them whatsoever. It has been a matter of doing research on everything, and trying to determine what my needs are. I have a few books on using oscilloscopes that I bought last year when entertaining the idea of buying one.

Troubleshooting with the Oscilloscope (fourth edition) by Robert Gordon Middleton
How to Use Oscilloscopes and Other Test Equipment by R. A. Penfold
High-power Audio Amplifier Construction Manual by G. Randy Sloan

I also find myself over at the Elliott Sound Products web site quite often. There seems to be some great information in those pages.

Here's a pic of the oscilloscope I have:



I'll get a variac and the clamp on that Ken suggested (thanks Ken!) next, and am looking for a capable, but inexpensive signal generator. What might be the frequency range any of you would recommend? I need to read further on this subject to make sure I get what I need. I haven't really looked into this for the most part. There are a lot of old signal generators out there in the world, providing plenty of options, although only some of them are going to work for my requirements.

All I've done up to this point with the oscilloscope is check the probes I have, which are two Tek X10 probes, and three ITT Pomona X10 probes with a lot of various accessories, such as various tip covers and replacement probe tips. One of the covers looks like it will come in handy as it is an extension that has a spring loaded hook, enabling the probe to be attached. After doing the probe tests I found one of the Pomona probes to be DOA. I'll have to look at it to see if there is anything obvious, but considering I got them for $25 including shipping, I'm not too worried about it.

Actually, I'm in need of a project. I saw a small Velleman amplifier that Fisherdude built that looks interesting, and could provide a great test platform for getting used to the gear I have. Presently the only thing I have that is broken is a Carver M1.5t amp, and that one had someone poking around inside that I'm not convinced should have been doing so. I have the service manual for it, but I'll leave that one alone for the time being.

Huh...I was just looking at Velleman's web site, and they do have quite a selection of various kits available. Even some high power tube amplifiers, although I'm not really interested in tubes. They seem like what Heathkit was from the '50s through the '70s.

I appreciate everyone's input! Thanks!

John

[Blast]

Your 2246 scope is a blast from the past! I repaired many VCR's with that model throughout the 90's! I had all the common settings saved in memory.

[33&athird]

Very cool scope, you can't go wrong with a tektronix...should last you for many years. If you want something cool to do with a scope, connect one channel to one channel of an audio signal, and channel 2 of the scope to the other. Connect the ground to the audio ground. This will display exactly what the signal looks like. Not only is it a cool show, you can play around with settings and figure how to operate it. Plus, it's free...and as long as you use reasonable ac voltage, you should not run a risk of damaging anything.

I like velleman kits as well, nice variety, good prices, and neat results.

I wish I had information about clamp-on ammeters, but I honestly have no idea how they work, I guess i'm old-fashioned, in that I perfer standard meters. Though I will admit the features on that oscilloscope are awfully nifty.

Either way, you are on the right track to an excellent work bench!...good luck, I hope you enjoy it .

[whoaru99]

I have a Fluke i1010 current clamp accessory. 1A-600A AC, 1A-1000A DC, 2%+0.5A is what the spec says.

If lower current is your thing, there is something like the i30 which is up to 30A AC or DC, with 1% +/- 2mA.

So, they can be pretty good given the convenience.

The Kill-A-Watt is a pretty nifty device, but it doesn't have very fast response to get a peak/inrush/glitch type reading.

[ken kantor]

http://www.eurekaspot.com/webdata/appnote/1603.PDF

IMO, the Tek 2246 is one of the finest scopes ever made. Yeah, higher freqs are sometimes needed these days and digital capture helps for complex signals... still it's often my 2246a or 2247 that gets used for design or repair work. Congrats! (For tube amp use, you are probably going to want a couple of x100 probes, or a differential input box).

A computer and external "sound card" can make a very flexible audio generator. If you prefer a stand-alone unit, there are a few ways to go. I would avoid the cheapie, $100, "function generators" with lots of bells and whistles. They can do tricks, but the waveforms are usually not very clean.

Here's a simple, stepped audio generator that is very useful. It's primitive, but the sine wave is very clean, (typically better than 0.005% THD) This can be good if you are measuring or listening for distortion.

http://www.allspectrum.com/store/pro...roducts_id=733

Next step up, (and my recommendation), would be:

http://www.testequipmentdepot.com/in...-809and810.htm

then...

http://www.proaudio.com/product_info...oducts_id=4833

Finally, if you really want to look at dynamic amp power, do complex sweeps and that kind of thing:

http://www.rigolna.com/products_dg10X2.aspx

For a learning kit, here's a fun and useful one:

http://www.qkits.com/serv/qkits/vell...ages/k4301.asp

-k

[ken kantor]

Yes, you are right: the Kill-A-Watt is too slow to see transients. And it's not really going to replace a probe for serious work where you have to watch the actual waveform. But, you gotta admit it's fun and useful to be able to monitor the line voltage, power consumption and VA so easily... I love watching the power consumption to my amp as I play guitar, or my stereo as I mess with the tone controls. Or my phone charger, or my my electric drill, or my soldering iron, or... yeah, I guess I am addicted...

-k

[westend]

Timely stuff. I just acquired a Textronik 454 from another AK'er. I have been too busy with projects to sit down and really see what it can do Thanks for all the links, Ken.

Devnull, I have a Fluke 36 clamper and it does a fairly good job with current, mostly used for residential wiring projects. Your scope is going to outperform it by a long shot for electronic work.
If you're looking for an amp build, you could join the F5 party. There are now more than a half dozen AK'ers that have built or are building these Nelson Pass amps. If you're looking for a simpler project, there are a lot of chip amp projects out there. I can't comment on any but the Tripath or T-amp, it is an amazing little amp. Since it is powered by 12v., there are a number of ways to power it and the amp board is very small. There have been thousands of builders and modders that have used the Tripath amp board so it is well supported and information is plentiful. Good luck with your new scope and your amp projects.

[whoaru99]

Originally Posted by ken kantor

Yes, you are right: the Kill-A-Watt is too slow to see transients. And it's not really going to replace a probe for serious work where you have to watch the actual waveform. But, you gotta admit it's fun and useful to be able to monitor the line voltage, power consumption and VA so easily... I love watching the power consumption to my amp as I play guitar, or my stereo as I mess with the tone controls. Or my phone charger, or my my electric drill, or my soldering iron, or... yeah, I guess I am addicted...

-k

Yeah, I was surprised to see how much my computer was using just sitting idle when I got my Kill-A-Watt. Caused me to change the power management settings and, as I recall, the power consumption dropped from low triple digits down to something like 27 watts when it went into standby. Might be able to go even further with hibernation, but my computer doesn't seem to like that so I don't use it.

[Fisherdude]

While clamp-ons are extremely useful devices, it's my understanding that they were never intended to measure currents in the 10s-100s of milliamps range.

Measuring the draw of an AC compressor under load, absolutely.

[whoaru99]

Some of Flukes smaller ones are good for that range. The i30 I mentioned says useable range 5mA to 30A, with +/- 1mA resolution.

Of course, with an electronic circuit, it may not be possible to use it due to sheer physical size and circuit layout regardless of range and accuracy of the tool.