Measuring Average DC Current

Presumably your scope doesn't have an average function? Just doing it by eye will give a pretty good answer, depends how accurate you need to be I s'pose.
Sorry not much help, but it shows I care
Del

Thank you for the quick reply.

My scope is a Tektronix TDS 1012. It has an "Average" function in the Acquire menu, but that is only to average out noise on the incoming signal acquisition.

Would you know of any quality DMM (e.g., Fluke) that would be able to give me an average DC current measurement, even when the current fluctuates wildly over time?

Thank you.

'Fluke?' The cheapest of analog meters will do the job much better. They have an inbuilt averaging function.

Any specific model of Fluke you recommend?

I do have an analog multi-meter -- a KAISE SK-300. But it lacks an averaging function. As a result, the needle goes nuts when I put it in the circuit.

Hang a weight on the indicator pointer.

I have a TDS 210.
Can't you set the time base to nice and slow, capture the voltage across your resistor and measure it with the 'mean' function? I think you scope has the 'mean' function?
I dunno about any meters for doing it.
Del

If I push the AUTO SET button on my scope, it will give me the Mean. But something is not right when I do that. I've got a 1.0-ohm resistor (5W) between the ground input of an alarm and the ground of my power supply, with the scope probe attached to the alarm side, and the probe ground lead attached to the PSU ground. The alarm is being powered by my PSU at 12-volts. The alarm has a power-cycled 2-way RF transceiver that varies the current wildly. The scope reading on my resistor gives me a Mean of 28mVdc. Calculating, we have: I=E/R=0.028V/1.0-Ohm=0.028A However, the average current consumption according to the product manual should be 13.5mA. And I know the manual is correct. Hmmm.

C'mon, just refine what you are doing a bit.
If you have earthing probs there are ways round that, using 2 scope probes (or be very very naughty and temporarilly removing the scope earth... I never said that right?)
You are assuming the 1 ohm 5W resitor is 1 ohm which is very unlikely.
Thirdly, if you believe the product manual why are you measuring it????
I'd also suggest reading the scope manual carefully as there are probably much better ways than the auto set, you need to consider how long a time frame you can store and measure to the accuracy required.
It sounds like you are nearly there, just slow down, have a cuppa and get to know you your scope like it's your best mate... maybe take it out to dinner?
Del

Where is this going? Is it an attempt to disprove the manual? Why has the question arisen?

I was just giving one example. Sometimes I do know what the average current is, and I just want to measure and confirm it. For example, if I designed a product to draw an average of 1.5 mA, plus or minus 1.0mA, if I test a product off the assembly line and find it to be drawing an average of 10mA, I know something is wrong with it.

The reason for my post is because using a resistor and scope is fiddly, slow, and prone to error. And as I said I'm not getting accurate results using it. I'd much rather just take a DMM and throw it in series with one of the power lines of my circuit, and have it give me the average current reading after a few seconds. But none of the meters I current have do that.

So rather than just suggesting "any cheap analog meter will suffice," I would be interested in having an exact brand and model number that someone here has used personally to take average DC current measurements, in applications similar to mine.

Thanks

A current trip amplifier will do the job. Set the trip to the maximum acceptable current, say 4mA. If the switch changes state, then something is wrong.

Thank you for the idea. But I am also measuring unknown avg. current too, where a tip meter isn't what I would want.

All said, I want an easy way to insert a couple meter probes in series with one of the input power lines to my circuit and then get an accurate reading for avg. current consumption, measuring current between 0.1mA up to a max of about 35mA.

Good luck with that.

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I ended up having good luck. I bought a Fluke 8845A, which does precisely what I needed it to do:

http://en-us.fluke.com/products/bench-instruments/8845a-8846a-6-5-digit-precision-multimeters.html

Where's the fun in just buying a proper test instrument.

JDW,

This cheap meter comes with a software CD for loading on your PC. You can connect the meter to your PC, connect the leads to your work, and the software will record data over whatever period of time you want. I think it will do what you need.

WJMFIRE-

That Radio Shack meter looks like an inexpensive approach- one usually has to pay an order of magnitude higher or more to get a DMM with communications interface (or internal memory). What isn't available in the product review is the reading frequency- how many readings one can capture in a given period of time...

I have used a Fluke 89 (which died long before it should have, just like a number of other Fluke products I have owned recently, which is why I no longer consider Fluke a quality instrument in spite of the price. I currently use a Metrix 3281 (expensive- but one is partly paying for the French "haute couture", flip-phone styling), and there are others on the market more reasonably priced. The Metrix allows me to record a running average and notes the peaks and lows during a measurement session, but I am generally watching things that don't change rapidly. I'm not sure I would trust the reported average if the signal were changing more rapidly than the meter's sampling rate.

To get a valid average, one must be sampling at at least twice the highest frequency in the signal. If the signal is changing rapidly, the best approach might be to catch the data on the scope, then pass the readings to some standard statistics package to determine the average (I like NIST DataPlot or R for quick looks at the data).

You don't seem to understand the averaging function of your scope, and you probably set it wrong. First you have the sample number averaging e.g 16, 32, 64, 128 etc , but the sampling rate vs available sampling memory is also critical to get any useful result. For example you can't expect to sample at 100Mbps on 4k of memory and have any stable output on a slow signal. The rule of thumb is to fill your sampling memory with ENOUGH equivalent time MULTIPLES of the sampled signal period (or with as MANY of its changes as possible if it's not periodic). S.M.

I haven't read all the posts so I apologise if this has already been suggested.

What about a cheap DMM with RS232 interface

http://cpc.farnell.com/_/in06158/digital-multimeter-rs232/dp/IN06158

Take a hundred measurements: stick them in a spread sheet and do the averaging there. That way you get a record for each unit tested.

Maybe there is some confusion with the specifications - you want "the fastest" way but thats always the most expensive. So I think it should be a solution based on limitations as accuracy and money. But you dont tell us which error will be allowed.

If it was my application I would take one of the cheap LCD-panelmeters (about 10 EUR) made by voltcraft and others. They have an input of +-199mV or +-20mV and should be equipped with a dual-Slope-converter. These are very accurate, suppress mains frequencies (which is essential with these low signals) and follow directly the rising/ falling amplitudes and count them up.

Good luck, regards

Uwe

The easiest method will be to use a power supply with a built in ammeter. For the cited pulsed current draw condition, you will need a custom test jig to measure this. The easiest method to do this is to power the device under test (DUT) with only a charged capacitor for a fixed amount of time. This time interval should allow for an anticipated number (ten) of bursts of current draw. The size of the capacitor should be large enough for these ten bursts of a good circuit to not drop the capacitor voltage below the operating voltage of the DUT. Measure the capacitor voltage drop after this interval to determine the total charge used during this time interval and one now can easily calculate the average current draw.

There is several Fluke DMM that has min-max available. Read the min and max for average. I'm not sure about measurements that small(I've never tried myself).

Min-max measurements alone will not provide an average without knowing duty cycle and that these two results are the only states of current draw.

what I would do because I dont think there such a device because when measuring queseient current and the ON current would drive a meter offscale.

Wire a cro across shunt in the meter insert into circuit measure voltage peaks eg determin a threshold voltage to reject device for ON current. you will have to measure the meter shunt resistance eg 200 ua is about 1k the resistance, will decrease as current range goes up.

To measure the OFF current or standby decrease the meters range and put cro to higher senitivy to check the Off current eg determin the max voltage drop across the higher meter shunt value.

To give example I used to test data recorders that ran a RTC that was about 15 microamps measured on the 200uA scale but would sometimes read OL because the ON current is about 30 mA every 1 second. for 200uS.

Depending on your desired averaging interval (1 sec, 10 sec, 100 sec, etc.) buy a supercap from DigiKey to put across your 1 Ohm resistor. A 10F cap (589-1002-ND for about $4) will give you a 10 second time constant. The average voltage across the cap will be the current in Amps.