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:: Current Sensing :: HELP!!

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suby786

New Member
Ive created a really interesting project for my final year, wireless stage light controller, using a PC with an application i have programmed using VB.NET, talking to 2 receivers via FM wireless link (900MHz band)...

Each reciever has a pic 16f877a that is doing a great job at controlling four light outputs (through 4 triacs)..

I have 2 way comms too so if 1 reciever goes down, the VB app will update almost instantly, now i want to find out the status of each bulb, so current sensing is required... now is where i need some advice...

Firstly i want to know if a bulb has blown, or if its been disconnected, i.e. NO CURRENT FLOW when it has been commanded...

Also i want to know how much current is being taken, to work out what power rating the bulb is... P = I V where I will be read by the A/D and V will be 240V AC...

There are a few ways to do this, but i cant get the basis of it..

Current Transofrmer/Transducer, most of them that are avail start from 1A, nd a 40w bulb only produces 0.17A, i could wrap the primary around at least 6 times to get a reading?

I dont mind making my own toroid=, i have tried but the oscilliscope reads SPIKES @ 100Hz, which i dont understand why spikes and not an ac waveform, i hoped to rectify the output and send this voltage to the A/D but i dont know how too

HELPPPP!!!
 

k7elp60

Active Member
You could put a resistor in series with each lamp, and use an opto isolator between the resistor and your monitoring circuitry.
 

kchriste

New Member
Forum Supporter
Current Transofrmer/Transducer, most of them that are avail start from 1A, nd a 40w bulb only produces 0.17A, i could wrap the primary around at least 6 times to get a reading?
Yes, that would give you a greater output voltage for a given lamp current.
I dont mind making my own toroid=, i have tried but the oscilliscope reads SPIKES @ 100Hz, which i dont understand why spikes and not an ac waveform
I suspect your line frequency is at 50Hz and the 100Hz spikes come from your TRIACs switching. Try it with a bulb wired directly to the mains with no TRIAC; the spikes should not be there. I suspect you need to use a zero crossing detector to fire your TRIACs at the correct moment and thus avoid/reduce the spiking. Failing that, you'll need a low pass filter before the rectifier so that the spikes don't interfere with your current readings.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi suby,

As kchriste points out the 100Hz 'spikes' are caused by the triacs turning on.

Are you also controlling the light intensity of each light?

Why do you want to use the PIC's adc to detect a blown/open lamp.
There are easier ways to do that.

Lets know about the lamp intensity, it could affect the rectification circuitry

EricG
 
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mneary

New Member
Yes the spikes are caused by the triac turning on. The reason these are spikes and not big AC waves is that your current transformer only passes the higher frequencies.

You have two possible solutions.

1) Since the spikes will go away when there's no lamp, detect the spikes.
2) Put a resistive load on the current transformer and amplify its output.
 

suby786

New Member
hey dudes

thanks for the replies,

I already have zero crossing circuitry, and the ciruite works a treat...

the toroid is situtaed on the output, and is not due to the triac turning on, thats the trigger signal, i am checking the ACTUAL bulb connection, so there should be a 240V AC wave ( if i check @ 100% then i would have a full wave)

APPARENTLY the spikes i am experiencing are due to STAURATION, i am using a too high value of resistor across the toroid, i was using 330 ohms, apparently 1 ohm with 10 windings should be good..

What easier ways are there to detect how much current?

To check the current, i would need to command the triac to FULLY ON... then take the ADC reading, and then use the measurement to work out the POWER OF THAT BULB... that is my aim, if i read 0V when i command 100%, then i know i have A BLOWN/DISCONNECTED bulb...
 

Hero999

Banned
Why not use a pre-made current transformer and amplify it? It's bound to give better results than building one.

I thought that core saturation would be caused by putting too lower value resistor across the transformer winding. Perhaps you've got too many turns therefore it's saturating anyway.

You could also consider using 3V 1.2VA - 3VA mains transormer as a current sensing transformer by putting the low voltage secondary in series with the bulb but be warned the mains primary will give mains voltage so you'll need to conect it to your chip via a divider.
 

suby786

New Member
i have tried so many variations, ive got from 1 turn to 20 turns... still the same crappy signal...

Ive gone from 1 Ohm to 1000 Ohms (using a precision POT) and no luck, the output signal changes but it looks so wrong
 

kchriste

New Member
Forum Supporter
the toroid is situtaed on the output, and is not due to the triac turning on, thats the trigger signal, i am checking the ACTUAL bulb connection,
The spikes are caused by the fact that the triac doesn't turn on until apx plus/minus 3-4V is across it. This occurs around the zero crossing point. You might try a snubber circuit across the triac to see if that helps.
 

suby786

New Member
are you confusing the spikes with the OUTPUT of the triac?

The triac circuitry is perfect, no spikes on the output, when i command 100%, the output is a FULL COMPLETE SINE WAVE...

I do not understand your apx +/- thing... what do u mean?

The triac im using is SNUBBERLESS - does not need that extra circuitry..

My spikes are occuring on the wire thats WRAPPED AROUND THE CORE... which is ISOLATED from the main output... i can put some pics of what im experiencing on the oscilliscope... any1 would like to see?
 

kchriste

New Member
Forum Supporter
i can put some pics of what im experiencing on the oscilliscope... any1 would like to see?
Yes. Show us a dual trace from both the output of the current sensing coil and the voltage accross the associated lamp if possible.
I do not understand your apx +/- thing... what do u mean?
I mean that no TRIAC switches on at 0 volts. They all require some voltage across the main terminals before they fire regardless if the gate is already being triggered.
 
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