AC current measure

Hi,

For a small project I need to measure the current through an AC lightbulb. I am using a microcontroller with AD converter and a triac to control it.

Any suggestions for the current sensor?

Reply to
Pietje Bell
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for example: # RS Stock no. 189-5477 # Manufacturer Honeywell # Manufact. part no. CSNE151

But since you are using a triac you may need to do an RMS conversion in your software or use a true RMS ic to get a meaningfull result

martin

Reply to
martin griffith

First, define current. I am serious. Peak, RMS, etc. Then consider its relation to things you can measure, such as instantaneous voltage across a resistor, bearing in mind the actual waveform on the device. You might want to measure the actual light output.

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Reply to
CBFalconer

How about wrapping a coil or wire around the flex to the lightbulb and using use the current induced in the coil to produce a voltage across a resistor? Then you can use an A/D to measure the voltage and do some maths to relate it to current in the flex. Of course this would be an AC voltage so rather than trying to convert to DC maybe just take lots of measurements with the A/D and use the peak.

This is a method I'd been mulling over using so I'd be interested to see what the wise people on this ng have to say about whether it has a hope of working.

Reply to
Tom Lucas

Light bulb like a 60 or 100 W one at 220 V AC, well, just use a 1 Ohm resistor to get a mV per milliamp, do some amplification and if the MCU can sample this every few microseconds you are done whether you need current, power etc.

Dimiter

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Reply to
Didi

That will work, but the sampl> How about wrapping a coil or wire around the flex to the

"Wise people" say it will work, only they call it a "current transformer" ;-) It also provides galvanic isolation, unlike the previous (also working) approach. They are commonly implemented as transformers on a toroidal core. The only thing to keep is mind is selecting a core/wire-loop-count that will not saturate under the expected measurement range.

Reply to
Roberto Waltman

Like these:

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More info:

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Reply to
Roberto Waltman

"Tom Lucas" wrote: ( about current transformers )

No.

To the OP: For a one of a kind project, you could improvise using any old transformer core, specially if you are willing to spend the time calibrating it for your particular application. (Doesn't mater if the core is not toroidal, doesn't mater if the transfer function is not linear, doesn't mother if the field leaks, etc.)

Reply to
Roberto Waltman

And frequency.

Reply to
Everett M. Greene

Another way is to use a series resistor and attach a thremistor to measure the temperature rise. This way gives you isolation and true rms cheaply, with care you can get within 5%.

Reply to
cbarn24050

snipped-for-privacy@aol.com wrote: (current sensing)

That's a good idea. How would you factor out ambient temperature? A separate sensor?

Reply to
Roberto Waltman

There are several ways around it, depends on the application.

Reply to
cbarn24050

I'm sure you're aware that this method gives you the longterm integral of the current, rather than anything more direct.

Re current transformers etc: also beware that inrush current is high. Ensure that the A/D system input is clamped to prevent it going outside the rails - easy enough with an inverting op-amp as a 1st-stage buffer.

Steve

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Reply to
Steve at fivetrees

When working with mains voltage, unless there are pressing cost concern and a gurantee that a user cannot touch the circuitry, you should alway use isolation. So far I have only seen suggestions of current transformer in this thread, but you can also use Hall effect sensors. Take a look a the ACS7xx range from Allegro.

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The hall effect sensor and the signal conditioning circuitry are al within an IC package and is reasonably priced.

-Aubrey

Reply to
antedeluvian

It doesnt have to be that long term, time constants of around 1 sec are easily done. How useful it is depends on what your doing with the measurement.

Reply to
cbarn24050

Thanks all for your suggestions!

Reply to
Pietje Bell

And don't open-circuit the secondary of a current transformer that's under load unless you want to see spitzen sparken. Short-circuiting the secondary is OK.

Reply to
Everett M. Greene

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