CT or other means of monitoring an overloaded circuit

Bryan Warden

I am looking for an cheap means of monitoring the current of a 110v 1 HP AC motor that I want to start and stop using basic stamp. I have foud some very small Current Transformers that are board mountable but there are dangers associated with them if the wrong person got to tinkering with my project (you never know) the ct's can be lethal. So if anyone has done anything like this with· ct or what ever I would appreciate you help. THanks..

Leon

Put a warning label near them.

Leon

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Suzuki SV1000S motorcycle

Franklin

Encapsulate your circuit in epoxy. I had another idea but I will save that for April 1.

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- Stephen

yarisboy

Take a look at hall effect current sensors from Allegro Microsystems. I'm using part no. ACS758xCB. They have built in isolation protecting your BS2.

Clock Loop

If you use a microcontroller, just use rctime.

Voltage changes with current, so rctime should show that your current has changed.

dev/null

You can't use Stamp with 110 volts!!! And if you step it down, you'd have to rectify the AC first to use RCTIME. It's not correct to say that current changes with voltage. It's the resistive (load) element that changes, which changes the current while the voltage is constant.

Parallax also has a hall effect sensor. You generally don't need opto isolation if you use a hall effect sensor, since it's physically detached from the sensing current, but it's a good safety measure.

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Don't worry. Be happy

Clock Loop

dev/null said...
It's not correct to say that current changes with voltage. It's the resistive (load) element that changes, which changes the current while the voltage is constant.

Cmon, its all relative!

Ohms law states that if you change either voltage, current, or resistance, ONE of the three must change with it, Period. Try it with a motor, lock the rotor(simulating load/current increase), and tell me that your voltage doesn't change.

dev/null

There are controlled voltage sources (like a LED were voltage is controlled by current) but mostly there are independent voltage sources (like a batteries or mains supply). Independent voltage sources have near-zero internal impedance, and so are not affected by the current running in the circuit. Mains electricity (the topic of this thread) typically have 0.5 ohms of internal impedance. There CAN be voltage drop in a mains network if the load is too high. This happens in poor countries and in remote areas.

In your example there is so much power dissipated that your power source is drained so quickly it will show up on a voltmeter.

In any case, you cannot measure mains current by measuring a resulting DC voltage or current in the Stamp. A Hall effect sensor or a current transformer is the only way to go.

Clock said...
ONE of the three must change with it

Yes, and it's the current that changes not the voltage (keep to the topic of the thread, we are talking about mains voltage)!

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Don't worry. Be happy

Clock Loop

dev/null said...
we are talking about mains voltage!

oops. never mind. much different situation when you have direct mains. perhaps the words "overloaded circuit" threw me off.

kwinn

Actually, if the load on a motor changes then both the current and voltage to the motor will change. For a motor connected to the mains the voltage will change less than the current, but there is resistance in the circuit between the power source and motor, so changes in current will result in changes in voltage.