Testing amperage draw with DMM

[punkykatt]

Happy New Year everyone. Hope you all had a safe one too.    Im thinking of building a test device (box with male and female 120vac receptacles with the hot leads connected to meter probe connectors for series connection of a ampmeter) to test amp draw of equipment ie:drill press, saws, hand drills, grinders, guitar amps, pa amps etc.  Will a DMM with  AC/DC amp function do the job? or do I have get a special ampmeter?    Thanks Punky

[PRR]

Why do you want to know amp-draw?

[punkykatt]

Curiosity,  Sometimes I just like to be anal about things. Having the ability to do it.

[John]

I've been looking for a way to measure amperage output of power trannies. Is there a nice simple way to figure out their current capacity?

[stingray_65]

I've found in general there are 2 differnt amps that I measure.

The first is small dc amps, milliamps DC like in a B+ rail.
that I can do with my DMM

tha second is power draw from big electric thing connected to the mains. like a motor or apliance.

These I use what's refered to as an amp clamp, it surrounds a mains wire and acts like a current transformer.

It's not real accurate, but it's acurate enough to let me know if I'm drawing more power than I should (due to bearings, belts or load)

[kagliostro]

A clamp meter is the more rational solution

there are also unexpensive clamp

http://chinagrabber.com/cheap-multimeter-electronic-tester-ac-dc-digital-clamp-meter---dm-84.aspx

but if you want build something in a box

use a shunt resistor and your DMM

http://www.rc-electronics-usa.com/current-shunt.html

http://en.wikipedia.org/wiki/Shunt_(electrical)

Kagliostro

[punkykatt]

Thank you Kagliostro for the great info. If I decide to go with the shunt resistor, would a (100A-100mV) resistor be enough or more that I need? The highest amp rating on any of my equipment is 15A.  I have seen smaller shunt resistors but they say DC only.

[kagliostro]

The shunt resistor can be used for AC or DC measure

I forgot to tell you that you can also use a Current Transformer

http://www.elkor.net/pdfs/AN0305-Current_Transformers.pdf

http://docs-europe.electrocomponents.com/webdocs/0eb1/0900766b80eb1ad7.pdf

http://www.elkor.net/pdfs/AN0305-Current_Transformers.pdf

someone more expert can tell you more

I used my last shunt resistor something like 40 years ago

my advice is a cheap clamp

Kagliostro

[punkykatt]

Thank you Kagliostro.  I will go the clamp on route.  Punky

[PRR]

I am wary of the "10 Amp" connections on most multimeters. The cheap ones are likely to fail on overload, with fire. My Fluke will blow a large strange fuse which is no longer made.

I would prefer to keep the high current out of the meter. Use a resistor shunt and measure voltage. Since modern DMMs will read very small voltages we may pick the resistor for small loss (and heat).

0.01 ohms will read 15 Amps as "0.15V" and 20A as "0.2V", simplifying mental conversion. Heat at 20 Amps is 4 Watts.

www.digikey.com has these for a buck and a half:
PWR4412-2SDR0100F
PWR4412-2SDR0100J

I do not know why they serve 5% and 1% at the same price.

Put 100K between the 0.01r resistor ends and the tip-jacks for your voltmeter, so the naked 120V is not exposed to fingers. This is a compromise between "safe shock" and accuracy.

Now the voltmeter. AC measurements can be wave-shape sensitive. True AC motors will pull near-sine current. PC power supplies and some universal (power drill) motors will pull huge spiky waves. Common AC voltmeters may not read the spikes correctly. Also... what do you want to know? Utility cost is the average of the low-passed wave. In commercial billing the V/I phase also matters. Line waveshape is a function of spikes. If you just want to know "roughly" what sucks more than what, it may not make any difference.

There IS a $40 device which reads current, voltage, computes billable power, and I think can integrate power and time so you know what an hour of casual sawing really pulls. Here's one: www.calacademy.org/teachers/lounge/?p=3379

[TubeGeek]

This is what I use around my shop:  http://www.thinkgeek.com/gadgets/travelpower/7657/

[punkykatt]

Thats better yet.  Thanks TubeGeek

[sluckey]

Im thinking of building a test device (box with male and female 120vac receptacles with the hot leads connected to meter probe connectors for series connection of a ampmeter) to test amp draw of equipment

OK, build it just as you say. And then... Put a 1 ohm resistor across the meter probe connectors. Now set your DMM to measure AC volts. The measured voltage converts directly to amperage, ie, 1VAC = 1A, or 2.5VAC = 2.5A, etc. This is the same principle as putting a 1 ohm resistor under the cathode of a tube to measure cathode current. Results should be good enough in an amp shop.

[PRR]

> Put a 1 ohm resistor across the meter probe connectors

He's looking to meter "equipment ie: drill press, saws, hand drills, grinders, guitar amps, pa amps etc.". If he has a strong table-saw and pushes a chunk of knotty oak hard, it will pull 25 Amps. With 1 ohm the 120V drops toward 95V and the 1-ohm's dissipation goes up toward 600 Watts.

Yes this could only be a short-term thing. A hard-pressed home saw will kick-out on overheat or on cellar fuse. This is just the kind of thing _I_ might want to know: is the saw weak or am I asking more current than my wiring is good for?

Adding another 1 ohm skews the situation somewhat. The saw will be weaker but it may appear to pull only an "OK" current instead of fuse-popping current.

And it seems that any of the proposed test items can, when pushed, pull enough current to smoke any 1-ohm resistor found in amp shops. If we have to buy, we also have the option to pick a lower-value resistor. I remember when <0.1r was large/custom, now 0.005r is standard stuff, 0.01r makes mental math easier.

> This is what I use around my shop:   http://www.thinkgeek.com/gadgets/travelpower/7657/

THAT'S the one I was trying to think of.

$25 is an excellent price. ThinkGeek are good folks.

For $25 I think pre-made is much safer than working with wall-wires.

The instant Volts Amps Watts VA PF readings are handy. But the KWH is wonderful. If you run a 100 Watt lamp for 10 hours it says 1KWH (1,000WH), so what? But put it on your table saw. It pulls say 300 Watts idling, 3,000 Watts on an oak knot.... are you paying for 300 or 3,000?? I think for any non-factory use, this meter will show a table-saw is a lot closer to 300 than to 3,000 KW per hour.

[punkykatt]

Received my Kill-A-Watt meter today.  Wow !!!  what a great tool to have. Works great!!  I dont think Im gonna hook it up to my table saw(draws 15A) and cut any oak boards with knots and burn this thing up .  Thank you all again for all the great info.   Punky

[PRR]

> I dont think Im gonna hook it up to my table saw(draws 15A) and cut any oak boards with knots

It won't die that easy. Anyway it's under warranty.

The saw may pull a full 15A for a second when spinning-up, but then will idle much less, maybe 3A maybe less if friction is low. Now push some pine at it-- 5A, 8A if you push it. Only hard work will pull the full 15A. And most popular-price table saws "can't" pull 15A for long: the non-industrial motor will heat and the thermal cut-out will trip.

[punkykatt]

Thank you PRR for that reply.  I consider this forum to be the best classroom I have ever been in, with the best instructors anyone could ever ask for.  Thank you Doug for such a wonderful forum.   Punky]