Measure RMS current (non-sinusoidal)

"Hammy"

** Is your scope grounded to the AC earth pin or not ???

Is it a hand held one ?

** The idea is workable - but quite unsafe done your way.

Use a small step down transformer ( say 12 volt to 120 volt ) in reverse to isolate the voltage across the 0.1 ohms from the AC supply.

Thus will be OK for any steady AC current readings - once calibrated.

However, inrush surges drawn by iron core transformers are ( surprisingly ) not AC - but consist of a brief string of current pulses all of the same polarity. The first one is by far the largest.

So you cannot transformer couple the event without distorting the waveform.

....... Phil

The scope is a bench top model (Earth grounded).

Yes I know. I'll exercise the necessary precautions. L & N PCB traces will both be insulated and amply spaced, terminations heatshrinked; the only exposed wiring at mains potential will be the resistors leads .One goal is to measure inrush so my leads will be connected before power is applied.

I considered using a transformer for isolation as you suggest but I know it will affect the accuracy.

I've done similar setups in school; input testing of a flyback, motor current, single three phase transformers etc. but I can't remember where the ground lead was clipped or if at all (it's been two years). We almost always used sampling resistors and a differential probe or scope setup as I explained.

Thanks for your assistance.

That looks OK.

Move the sense resistor into the N side to reduce the common-mode voltage. "Differential" scopes just subtract the channel signals and won't be very good at extracting a small difference from two big voltages. And/Or do like Phil suggests, and use a signal isolation transformer.

John

If you have a good DSO, then the measurement is probably good, but it must be the type that calculates the RMS between two cursors using a mathematical root-mean-square algorithm. I do this exact measurement for sub-cycle and multi-cycle pulses in my circuit breaker testing equipment. We use a Rogowski coil (air-core inductor as magnetic field sensor), which can be calibrated to show the current waveform to an accuracy of better than 1%, if properly integrated. You can make a crude coil by wrapping one of your mains leads around an air-core choke. You'll probably get about 0.1 mV/ampere, so you need a good preamp. This method is more suited to very high current measurement. Also look into Hall Effect transducers, or a DC clamp-on current probe.

I have software that can show the waveform of a powerline current pulse, and give readings of true RMS and peak current and time between two cursors. The hardware is proprietary (but I could sell you an old prototype board). If you can save the waveform in the format I use for my system, you can use my software for analysis.

More information on my website

and the software can be downloaded at

Paul

** For God's sake - buy yourself a Hall effect sensor.

Made by Lem Heme, Honeywell and others.

Just add a small +/- 15 volt DC supply and pass one of the AC wires through the hole and you are away for AC and DC.

DC to 100 kHz bandwidth - very accurate - very safe.

100 amp peak current capability even in the cheapest models.

....... Phil

Absolutely. You're going to have a very difficult time separating out the line neutral voltage from the measurement, and it really isn't a good idea or safe to put a scope probe on the line. It usually results in very large ground loop currents, and the scope won't like it (if it survives).

In addition to Mr. Allison's sources for current sensors, try Allegro for an inexpensive solution that has total isolation and an adequate

50KHz bandwidth:

The ACS750 operates off a single +5V supply. It's typically accurate to within several percent, more than adequate for most applications. If you AC couple the sensor output, you don't have to worry about zero drift, and can easily get an accurate measurement of inrush current. And by the way, inrush current is usually measured in peak amps. I'm not sure why you'd want an RMS reading for a one shot event at turn- on.

Good luck Chris

Thanks for the advice.

I know what inrush is.You didnt read my original post.I also want RMS to determine whether I can use a $4.00 relay or a $15.00 dollar relay,but thanks for the suggestion.I'll look into it.

You might be going the long way around here, as you're using a relay to switch a transformer. Absent arcing, relay contacts are rated according to the surface heating that will occur as current is passing through the contacts.

Since you're using full-wave rectification, you should size your transformer at about 1.2 times the DC current you're going to be using. So, let's say you've got a dual 6VAC secondary, and are shooting for an unregulated 8VDC at 1 amp at the output. That would mean you should size your 12VDC transformer so it can supply 1.2 amps AC. For this application, a 3A relay contact should do, because the relay contacts have a thermal mass which should keep surface temperature well below melting point for a turn-on surge -- except for one thing.

Assuming you're switching the transformer primary, you're switching an inductive load here. And when you interrupt current to the transformer primary, you're going to be getting a good arc, which will pit the contacts. When that happens, they don't fit together as well, so you will probably end up with very excessive heating over a small fraction of the original contact surface after a few close/open cycles, which will eventually weld the contacts together. This is about the worst thing that can happen.

Look for a relay which has a motor load rating. That usually means the contacts are farther apart, with a heavier spring to pull the contacts apart quicker. This also means you're going to need a lot more coil current here.

Since you didn't mention how much current you're switching, it would be difficult to size up a relay. But just offhand, if you're driving the coil with a DC voltage, a Potter&Brumfield T73-series relay or a less expensive equivalent might be a good, relatively inexpensive place to start. I've used them successfully to switch 60Hz power supply transformer primaries successfully several times with no problems. Coil power is about 1/2 watt.

Good luck Chris

snip

Hi Chris your last post wasn't picked up by my news server, but I read it in an alternative site. But here's an explanation of what I'm doing. Hopefully they don't drop this, my news server has been screwy the past couple of days.

I'm building a LM317 12Vdc @ 0.150mA PSU. My transformer is a 14VAC

4.4VA model. This is to be used to power three different devices that are separated from the PSU by 20 to 100ft.For reasons I won't bore you with its desirable to keep the PSU in a remote location, hence the need for the relay, to switch the PSU on using a control signal generated by a battery operated cct.

I'll be using a "latching" type relay. I will also have TVS from L to N and from L and N to ground. Would this not be adequate? The other reason I want the measurement is for sizing of the "power entry module" which includes an EMI filter, and fusing. I know there are rules of thumbs and semi-accurate methods for estimation "schade curves", simulation. But I like to measure and be as sure as I can be with what I have available, before I leave something plugged into an outlet for extended periods of time.

I've been reading application literature from the relay manufactures, and the TVS I'm using is what they recommend. I would have this irregardless of the relay. Are there additional measures that I can use?

"Hammy

** It not safe or AFAIK legal to connect a TVS to the local safety ground or exposed metal within an appliance - if the TVS fails it may send the safety ground live.

BTW:

You have seriously wasted our time with your misleading Q that turns out to be about a miniscule 4.4 VA tranny.

Strewth !! What inrush surge ???

How the HELL did you expect to measure * 360 uV * across that 0.01 ohms resistor ??

....... Phil

You idiot the fuse would blow if it fails short!

Straight from OMRON

So this is illegal is it?

I never stated anywhere that I was building a MW supply.I still think its good to measure inrush no matter the supply power.

Correct I should have thought it through more,but it was the general idea,I didnt run through a detailed calculati of component values.

Diferential Y type Caps are also connected to ground is this also illegal.Better recall all EMI/RFI filters.

Scratch differential off my above post.

"Hammy" "Phil Allison"

** Yep - it is very unsafe.

One cannot connect devices that are prone to fail short circuit from either AC line to the LOCAL safety ground in an appliance. MOVs are not permitted.

The only component that is generally allowed is a class Y cap.

** Then you are an even bigger IDIOT. ** Was MORE than an tad misleading to the readers of this NG.

Invalidates much of the advice given.

....... Phil

"Hammy"

** No - class Y caps are the exception as they can withstand very high pulse voltages and are not likely to ever fail short circuit.

However, the value used must not allow more than a very small current to flow to ground - usually less than 1 mA AC.

....... Phil