Current transformer -> black box -> Propeller Chip

Jay Kickliter

Can anyone can point me to application notes, or good reference schematics, for filling in the black box?

I'm completely blind here, and even having a hard time deciding out what ratio of transformer to buy. My buddy and I tried making our own, but it didn't work very well. And we need a more profession looking product, since it will be used in a visible demonstration.

The best I could find is mention of an absolute value op-amp circuit. I suppose if the output is filtered, it could give a decent reading, but maybe that's not the best route to take at all. Any opinions?

We're going to need to read up to 15 amps in the primary. And we can't just hook a CT up to an multimeter, since we need to interface to the Prop, and need it to look professional, not hacked together. A shunt isn't an option, since this project is meant to simulate industrial applications, where the current will be much higher.

Again, not looking to for anyone do design it for us. But I've exhausted my google skills, and would like to stick to best practices, if there are any.

Thanks a lot

davejames

Jay,

Would these be suitable? I have a few if you like.

http://www.fwbell.com/PDF%20Documents/NT-Datasheet.pdf

DJ

Jay Kickliter

Ok, I finally found a circuit that suits my needs (attached). I'd never seen zener diodes attached anode to anode. A little googling later, it appears they are for protection when the primary is disconnected. But, the app note doesn't provide any values. Also, the app note says to pick a burden resistor as to keep the secondary voltage within 10 mA – 100 mA. Since the primary will be measuring current from a standard US outlet, I'm thinking I should allow for a secondary voltage of 100 mA with a 15 A primary current.

Questions:

If I follow their recommendation to keep the secondary voltage between 10 mA – and 100 mA, I won't be able to design the circuit to measure both normal household appliances and something like a space heater. Does anyone see the reasoning for for the 10 mA minimum? If I had to choose, I'd prefer to measure lower currents, but without damaging anything if a space heater is plugged in.

Whats a good zener to use in the circuit? And what about the diodes at the output of the opamps?

I've seen current transformers with a suitable aperture at many different turns rations. Should I go for high ration and large burden resistor, or a low ratio and small resistor?

Jay Kickliter

davejames wrote: »

Jay,

Would these be suitable? I have a few if you like.

http://www.fwbell.com/PDF%20Documents/NT-Datasheet.pdf

DJ

Unfortunately I can't tap the circuit, and need to use a current transformer. It looks to me like those devices are meant to for higher currents. Am I wrong?

sam_sam_sam

I have to ask the question

What happen when you have a attached anode to anode with two zener diodes like in the circuit that was posted

http://forums.parallax.com/attachment.php?attachmentid=78111&d=1297441540

I do not mean to high jack this post But ....?

davejames

...sorry, I misunderstood how much current you were anticipating, and what you meant by "current transformer".

Yes, the NT-50 is suitable for up to 50A...although F.W. Bell offers other units for less current.

DJ

Jay Kickliter

@Sam, that was one of my questions. It appears to protection from a voltage spike that may occur if the primary is suddenly disconnected.

Phil Pilgrim (PhiPi)

The way the anode-to-anode zeners work is this: if the voltage across them (of either polarity) exceeds V_zener + V_fwd the network will conduct and short the excess voltage. This is because one zener is conducting in the forward direction, while the other is conducting in reverse breakdown mode. Incidentally, the circuit will behave the same if the zeners are connected cathode-to-cathode.

-Phil

sam_sam_sam

Phil

Thanks for your reply

Do you have a simple safe circuit I can make to see how this works Thanks