AC Clamp Meter measuring DC

[drewh]

Hi all,

Great forum! Have read posts from time to time but only registering now.

I have a quick question about my AC Clamp Meter. I wanted to measure the current my pool Chlorinator cell was drawing from the power unit - its rated at 7V DC / 30 A. Seeing as I only had the AC clamp meter I gave it a try to see, and it read the current fine (about 14.5A, it's several years old now).

I'm slightly confused by this, the current generated from the power unit would be DC would it not (probably full wave rectification?), so how is it reading it? Just taking the top peak each time?

I have an intermediate knowledge of electronics but haven't really looked into this side of things at all.

 

[Lucien Nunes]

There are two common clamp meter technologies -transformer and Hall effect. If it is a transformer type, it will not read steady DC at all, but might give an (incorrect) indication of the AC component* of an unsmoothed rectified DC supply which yours might be. If it has a Hall effect sensor, it will read DC but probably isn't calibrated to do so and might indicate anything, maybe approximately the mean, unlikely to be RMS.

*consider the average value of the current as the DC component and the 'ripple' superimposed on that as a separate AC component.

 

[drewh]

Thanks Lucien, great advice. I'm 99% sure it's a current transformer type (can see the ferrite iron in the split ring & obv only measures AC). So are you saying if the unit power supply has a reservoir cap to smooth the rectified DC supply the clamp meter wouldn't be able to read the ripple at all? More than likely is just half or full wave rectified, for the application being a pool cell more precision probably isn't needed.

Would this reading be a relatively close approximation? I don't need exact, just an idea. I.e. it might be reading the rectified peak? My main reason for doing the exercise is to see the condition of the cell - rated 30A when new and one that is almost gone draws very little current.

 

[Lucien Nunes]

If it is a transformer type I'm afraid the indication doesn't have much numerical significance because the instrument is working well outside its parameters. You could have 1A DC with 15A ripple or 50A DC with 15A ripple and it would read the same, in fact possibly less with the higher current. Importantly, it is only calibrated for a sine wave, and the ripple is anything but sinusoidal. We can take a guess at the waveform - and hence the crest factor - of the ripple voltage because we can guess how it's rectified, but not that of the ripple current as we don't know the (probably quite non-linear) characteristics of the cell.

I think you'll have to blag a DC meter. If my life depended on it I can think of a number of roundabout ways to take the measurement but none that are sensible enough to suggest!

 

[drewh]

Ok so no chance of getting even an approx measurement using it - plan B! Thanks for all the excellent info Lucien, so interesting.

Out of interest, what were the roundabout ways you were thinking? I can guarantee you I won't do them, but would just like to process them in my head to see where you're coming from. Would one involve measuring the resistance across the wire from the cell to where it meets the power unit, then use ohms to approximate the current? Again, I won't do this - just out of interest.

May as well buy a DC clamp meter in due course... would you think this one - http://www.jaycar.com.au/productView.asp?ID=QM1566 - looks good? True rms is the main thing.

 

[Engineer54]

May as well buy a DC clamp meter in due course... would you think this one - 1000A True RMS AC/DC Clamp Meter - Jaycar Electronics - looks good? True rms is the main thing.

Do you really need an Amp Clamp that will read AC / DC current to a 1000A??

I take it you are located in Australia, so unsure if the model you are thinking about is a reasonable price or not. This unit is more of a multi function unit than a simple Amp Clamp. I can't see the point in other functions on an Amp Clamp, unless the Clamp meter measures KW/KVA etc...

I can't see a brand/manufacturers name anywhere on the above link only a model number, so of an unknown quality!!

 

[drewh]

No I don't, but at least with Jaycar the 600A ones are only average vs true RMS. It's def worth getting a true rms one isn't it? I'll see if I can find a cheaper one but still true rms.

PS yes I'm in Australia.

 

[Lucien Nunes]

You mention true RMS. Assuming that this is unsmoothed current, which we haven't proven, the effective current acting in the cell to produce electrolysis is the average, not the RMS. Because we don't know the waveform we can't convert one value to the other. Moving coil (analogue) meters are average-responding but scaled in RMS, so provided you allow for the form-factor of a sine wave (for which it is calibrated) the waveform doesn't matter in this application - you will get a correct result. A genuine average-responding digital will be similar. In practice the difference might not be that great.

Yes you could use one of the leads as a shunt and measure voltage across it (subject to above) but its resistance is probably too low to measure with a typical meter. You would have to rely on its CSA being accurate or inject a known current from a DC source to calibrate it.

But first, how do you know your clamp meter isn't responding to DC? The core of a Hall effect pickup is ferrite too so that's not a reliable guide. I'd clamp a car battery cable with the lights on and engine off, to give a pure DC current in the same ballpark as the cell current, to prove the point.

 

[drewh]

Do you really need an Amp Clamp that will read AC / DC current to a 1000A??

I take it you are located in Australia, so unsure if the model you are thinking about is a reasonable price or not. This unit is more of a multi function unit than a simple Amp Clamp. I can't see the point in other functions on an Amp Clamp, unless the Clamp meter measures KW/KVA etc...

I can't see a brand/manufacturers name anywhere on the above link only a model number, so of an unknown quality!!

It's a digitech which is Jaycars own brand, but has been established for 15 years here & from what I've read is regarded well (know electricians that use it). But you are absolutely right, I don't need all the multi function stuff, I have a multimeter that does that. The only real brand that comes up when searching is Fluke, which I know is very good but usually very expensive too going by the multimeters. Would you know a model number off hand for say a 200A true rms AC/DC clamp meter? It would be unlikely I'd ever need to measure anything close to 200A really, so whatever the minimum there is that is also true RMS (I suppose too not being true rms may not really matter for my uses...).

 

[drewh]

You mention true RMS. Assuming that this is unsmoothed current, which we haven't proven, the effective current acting in the cell to produce electrolysis is the average, not the RMS. Because we don't know the waveform we can't convert one value to the other. Moving coil (analogue) meters are average-responding but scaled in RMS, so provided you allow for the form-factor of a sine wave (for which it is calibrated) the waveform doesn't matter in this application - you will get a correct result. A genuine average-responding digital will be similar. In practice the difference might not be that great.

Yes you could use one of the leads as a shunt and measure voltage across it (subject to above) but its resistance is probably too low to measure with a typical meter. You would have to rely on its CSA being accurate or inject a known current from a DC source to calibrate it.

But first, how do you know your clamp meter isn't responding to DC? The core of a Hall effect pickup is ferrite too so that's not a reliable guide. I'd clamp a car battery cable with the lights on and engine off, to give a pure DC current in the same ballpark as the cell current, to prove the point.

Thanks Lucien for the ongoing help & interest.

Even to measure the voltage or resistance across it (assuming a meter could read it), there's no real way to get to the contact on the cell side as its screwed into a housing and sealed on the outer side. Housing fills with water & then the cell draws current as it produces electrolysis. So those methods sadly wouldn't work anyway.

Thanks for the suggestion, I tested it on the car battery with headlights on - doesn't read anything. It read a 0.42A current that stays on it but no change when the headlights come on, so it obv can't pick up a direct DC current as you said (so no Hall effect sensor?).

Interesting though, when I use the clamp meter on the cell wiring, it starts at 0 and as the cell fills with water (takes 1-2 mins to be completely submerged), the current detected increases in a very uniform way before settling around 14.50 but continually moving around that point (up to 15, sometimes down to 14, all gradual movements). Does this tell you anything about the possible DC source? Could there be anyway this reading is actually approximately close - it must be reading something from the DC source whether a ripple or the peaks or? I actually have an old oscilloscope & could use that to see what the voltage waveform was like, though would the 30A current be an issue?

I know I could just buy a DC clamp meter but given I wouldn't use it often, if I could save $120 for the time being it would be a better outcome. Down the track I'd get a AC/DC clamp meter for sure.

Once again appreciate your help Lucien & am inspired by your knowledge of electronics! Already learnt a lot.

 

[drewh]

Just an update - heard back from Crystal Clear, the guy said power source is half wave and not smoothed, so it (AC clamp meter) would give a reading.

Would that mean what I'm getting is pretty accurate? It just ignored that it's missing the negative swing as the positive wave peaks would resemble an AC waveform?

 

[PEG]

Hi,not knowing the set up you describe,is it not possible to measure current on the AC feed side? If that is how your DC is derived. Even if you had to separate the unit from other items supplied,this would give an indication of consumption. I picked up a small,AC/DC clamp meter at a show last year,and by referencing against dedicated units,it proved to be quite accurate.It was not expensive,so it lives in my truck,i will post the make and model shortly.:conehead: ;Just had a look,it is a GMC-1 PROSyS,Model CP41 and it measures up to 40A trms.

 

[drewh]

Hi PEG, thanks for that recc I will have a look at it, 40A is more than enough for what I'd ever measure with it.

I did measure the current draw on the AC feed side, it was 1.06A. Only prob is I don't know what a new salt cell would/should draw but that seems a fairly 'healthy' amount..? (remembering we're 240V here in Australia).

 

[Engineer54]

Hi PEG, thanks for that recc I will have a look at it, 40A is more than enough for what I'd ever measure with it.

I did measure the current draw on the AC feed side, it was 1.06A. Only prob is I don't know what a new salt cell would/should draw but that seems a fairly 'healthy' amount..? (remembering we're 240V here in Australia).

Same as the UK then...lol!! The 230 Volts that's bandied about here and in Europe is just a fictitious value to suit harmonisation with our European cousins, whose nominal voltage by the way, also still remains 220 Volts. Confused, so is everyone else!! lol!!

Have a look at the top left of this page (forum sponsors) you'll find quite a few suitable Amp Clamp models that are reasonably priced that will suit your needs. Not sure about the shipping costs to Oz mind, but at least you'll have makes and model numbers to look up on Oz websites!! lol!!

 

[Besoeker]

As a diagnostic tool.....

Using an AC clip on ammeter was a quick check on the input current to a three phase rectifier bridge.
The currents ought to be balanced but now and again you'd find that it was sticky to open the jaws. That was the presence of DC indicating that one of the limbs of the bridge wasn't conducting.