Hi everyone, First time poster here.

Anyway, I have a fairly new AC clamp meter (Kobalt AC TRMS cheapo from Lowes) and I was trying to measure the current draw from a 3-fan setup that I have on my porch. The three fans are all plugged in to an extension cord and I am measuring between the extension cord and the receptacle (a standard 15A GFI protected receptacle) using the wire splitter that came with the meter. According to the motor data I should be seeing about 1.3A AC from each fan, for a total of around 3.9A. The meter is reading 39A though. So I did some research on the forums here and saw a couple of threads pointing out that the line splitter for a clamp meter sometimes has a 10X side and a 1X side. That is indeed the case for the line splitter that I have, so I went back to re-measure using the 1X side of the splitter.

However, I'm reading 39A on the 1X side AND on the 10X side. For a sanity check, I plugged a phone charger in to the splitter and measured the current. The charger should draw max 0.7A AC, and it was reading 3.0A on the 1X side AND the 10X side of the splitter. So not exactly 10X during that measurement, more like 4X-5X

I'm okay with the splitter not actually having a 10X measurement side, but I'm wondering why a phone charger is giving 5X current that is expected, but the fans are giving exactly 10X the current expected. Does this have something to do with the fan motors being a highly inductive load? I believe they are pretty standard PSC motors. Is this meter just junk? Am I using it wrong somehow? Any help would be appreciated.

 

Actually, now that I think about it, I think I solved this myself.

After examining the line splitter very closely, although one distinct side of it is labeled “10X,” I’m thinking the whole line splitter is a 10X device, no matter which way you measure it. As a result, I’m measuring exactly 10X on the fans, and the reason I’m only measuring 5X on the phone charger is that it is a SMPS that is using less current than *its* max. Phone chargers may not be a reliable test vehicle because, you know, USB standards…

Tl:dr I’m thinking the line splitter is 10X on both sides and now I have to build a patch cable that is a true 1X device. Thanks Obama!

 

Hi papaweely,

I noticed you mentioned that your phone charger should draw a maximum of 0.7A AC. I wanted to point out that this seems unusually high for a phone charger. For example, if you're on a 220V supply, 0.7A would translate to about 154 watts (220V * 0.7A), and for a 110V supply, it would still mean around 77 watts (110V * 0.7A). Phone chargers typically draw less than this. Could you double-check this rating or share how you arrived at the 0.7A expectation?

 

Hi papaweely,

I noticed you mentioned that your phone charger should draw a maximum of 0.7A AC. I wanted to point out that this seems unusually high for a phone charger. For example, if you're on a 220V supply, 0.7A would translate to about 154 watts (220V * 0.7A), and for a 110V supply, it would still mean around 77 watts (110V * 0.7A). Phone chargers typically draw less than this. Could you double-check this rating or share how you arrived at the 0.7A expectation?

I got that number directly off the charger itself. Picture attached.

 

I got that number directly off the charger itself. Picture attached.

That’s interesting. Your charger’s actual power output is 25W, so the input current should be less than 0.7A. Assuming 80% efficiency, the input current at 100V AC would be around 0.31A, which aligns with your measurement and suggests the clamp meter is accurate.

I don't think the harmonics of the SMPS are responsible for such a large difference (0.31A vs. 0.7A).

However, I do have a question: Why is the charger labeled with a 0.7A input current? I’ve noticed that other 25W chargers also tend to have similar 0.7A or 0.6A markings. It could be related to the internal fuse rating, though I’m not entirely sure.

 

That’s interesting. Your charger’s actual power output is 25W, so the input current should be less than 0.7A. Assuming 80% efficiency, the input current at 100V AC would be around 0.31A, which aligns with your measurement and suggests the clamp meter is accurate.

I don't think the harmonics of the SMPS are responsible for such a large difference (0.31A vs. 0.7A).

However, I do have a question: Why is the charger labeled with a 0.7A input current? I’ve noticed that other 25W chargers also tend to have similar 0.7A or 0.6A markings. It could be related to the internal fuse rating, though I’m not entirely sure.

I don't think the switching frequency or harmonics contribute much to that current rating either. However, I'm thinking that, especially in a cheap SMPS like this, the larger rating of 0.7A may be due to the inrush current which can easily be double the operating current. So, I'm thinking maybe they list that max current rating because it is possible for the supply to draw that much, albeit for a very short time, and if you had a fast acting fuse in the circuit it may blow during the inrush cycle even though it would be fine during regular operation.

According to this hypothesis, we should see that cheap SMPS circuits such as this one have a max current rating around double that of the operating current, whereas a more expensive SMPS would have a max current rating closer to the actual operating current because of a slow-start circuit being included. Just a guess on my part though.

 

Some regulatory agencies make the equipment makers to stipulate the maximum current drain under any circumstance. Circumstances which may include the inrush current or overload conditions.