Detecting OFF state of an AC Load

Question

Hi All,

I am trying to switch on a LOAD from its OFF state by detecting a change in the voltage. Here the load is a MOT, the voltage at the primary is 1VAC when OFF.

My idea is to switch ON the MOT by detecting the voltage change at the primary when the Secondary are shorted. I can see a voltage drop of .5VAC when shorted. How can i monitor this voltage change with an Arduino.

My solution to the problem is shown below.

The Live of mains is connected to the MOT through the LIVE_LOAD in the schematic. During the OFF state the Optocoupler is triggered so that the 1VAC is seen by the Precision rectifier LT1078 and is rectified and send to the Analog pin A0 on the Arduino. With a code i could detect the voltage change and use it trigger the MOT again and at the same disable the optocoupler to break the circuit to the LT1078. Basically eliminating the need to press a switch two switch ON the MOT.

is there a better way to get this done. I was thinking of using the basic 1N4007 to convert the 1VAC to dc but that would be a problem when the MOT is switched ON allowing the mains 240VAC to flow through the DIODE.

dougw · Answer

1.3 V rms rectified is 1.8 Vdc minus the drop in a lightly loaded Schottky diode of 0.3 V is 1.5 V dc peak. The zener on the Schottky diode output is just to clamp the signal so it doesn't exceed your arduino power supply when the ac voltage goes to 220. You could use 1N4007's for this if you want - just 2 in series clamps the voltage to 1.4 volts dc.

shabaz · Answer

The question (and the circuit) is not understandable I think : ( Could you rephrase the question, or draw a block diagram? The circuit doesn't make sense.

fmilburn · Answer

In simplest form, I believe Douglas Wong was suggesting something like this: D1 is a Schottky which is used in order to minimize the voltage drop to around 0.3V and serves as a half wave rectifier. D2 is a Zener which should be specified at a voltage which will protect the Arduino (i.e. clamp). R1 is high resistance, say 100k, so as to minimize current. The value of C1 is chosen to smooth out the voltage where it is sensed. It goes without saying that all components should be able to handle the maximum voltage at Vin, but there, I said it anyway. dougw , if I got this wrong I would be appreciative of your comments for my own education.

dougw · Answer

This is what I was thinking fmilburn except as genebren points out it needs a current limiting resistor directly before or directly after D1 and of course R1 needs to have much higher resistance than the current limiter. Thanks Frank and Gene.

luislabmo · Answer

You may try a Non-Invasive current sensor to detect an ON/OFF state of an AC load with an Arduino.

dougw · Answer

Its getting better but C4 still would need to be a high voltage part. If you take R2 away (open circuit), short out R1 (and remove it), and make R4 at least 100K then C4 can be a low voltage part.

mcb1 · Answer

pressing the switch 300 times or a foot switch would be........a little uncomfortable Ah yes that does make sense now. Sometimes having to explain what you're doing can help with a path towards a solution. So to give you an alternative option. .... Some of the generators use a small DC current on the output side to detect when the load is connected and ON. It then fires up the generator. You could adopt something similar ..ie detect the cell (or load) and then apply the mains. The last time I used a spot welder was way back at school. The welding arms were sprung loaded and once it had applied clamping force to hold the pieces together, continued pressure on the handle applied the power. Obviously there is some mechanical force clamping the two pieces together, which you can't avoid. Mark

anishkgt · Answer

Just another version, would this be any good if not wrong ? At 240vac At 1vac

dougw · Answer

Obviously you need to be careful connecting directly to 220V and presumably you have the high voltage implications figured out and how to connect the low voltage supply - as in the whole circuit and its power supply needs to be enclosed with no possibility of human contact.

You can limit the high voltage on the sense line with a zener diode, if you don't want to disconnect it.

A Schottky diode is enough to rectify the sense signal, followed by an RC filter.

The current limiting resistor can be higher than 100K.

anishkgt · Answer

So i've been simulating this, sharing graphs At 240vac At 1vac

dougw · Answer

The cap in your circuit would need to be gigantic to survive high voltage. If you use fmilburn circuit with a 100K resistor in front of D1 and make R1 1meg you can use a small low voltage cap.

anishkgt · Answer

I am sensing the voltage at the primary of the MOT when it is OFF not when the MOT is ON. The actual voltage of 1vac is the leakage current from the SCR from which the MOT is connected to.

When i said "shorted secondary" i meant as in placing the electrodes on a conducting surface like nickel strips for spot welding.

here is the schematic to better understand

genebren · Answer

Frank, I think that there needs to be a resistor following the diode and before the zener (there needs to be something to safely drop the excess voltage).

anishkgt · Answer

Thanks mark, am already aware of those optocouplers and one of them is used in the schematic as well. I'll try to explain the whole idea behind this, Lets assume we have a push button switch to switch ON the spot welder. We press it and that switches ON the MOT and we spot weld nickel tabs to 18650 cells. All looks good and Hunky-Dorey. Now lets say you have to build a battery pack of 300 cells and pressing the switch 300 times or a foot switch would be........a little uncomfortable or your legs get exhausted while pressing the pedal 300 times to spot weld those cells. Here we are replacing the 'SWITCH' by sensing when both electrodes makes a contacts on the nickel tabs. When it does current is drawn from the MOT and that draw will show a voltage drop at the primary and that voltage-drop is something that am trying to sense which would fed into an Arduino to activate the MOT. Hope that's clear. Sorry am not very good at explaining things but i guess would have helped you understand the idea.

genebren · Answer

George, It looks from your simulations that you have a clearly detectable difference between 'ON' and 'OFF'. This should work. Gene

anishkgt · Answer

Obviously not. The MOT is driven by the MOC3052. The MOT is driven by a pair of SCR in a Back-to-back configuration.

anishkgt · Answer

dougw wrote:

Obviously you need to be careful connecting directly to 220V and presumably you have the high voltage implications figured out and how to connect the low voltage supply - as in the whole circuit and its power supply needs to be enclosed with no possibility of human contact.
You can limit the high voltage on the sense line with a zener diode, if you don't want to disconnect it.
A Schottky diode is enough to rectify the sense signal, followed by an RC filter.
The current limiting resistor can be higher than 100K.

Thanks for the concern. yes am well cautious about the mains supply and working with them.

The least nominal voltage for a zerner diode is 1.8v which is quite high for the application. The OFF state voltage is only about 1.3vac max and i presume they are to be placed in series with the LIVE_OUT. A schottky would have a voltage drop as well.

I guess the initial design would be ideal as the voltage drop across the diode is also compensated and being an opamp with slew rate of 0.1v/us i should get quick response as well. I could omit the capacitor as that would take time to charge during voltage variation and maybe the diode as well to cut cost or may be both. All i need is to monitor ANY change in the voltage so without the diode and the cap they should be ok.

anishkgt · Answer

dougw wrote: Its getting better but C4 still would need to be a high voltage part. Yes it is 10uf 400V. But removing R2 would give a bit of a distortion on the wave form and it would get even worse without R1.

mcb1 · Answer

here is the schematic to better understand That provides much more information thanks. I'm still wondering why you need to detect the OFF state when you obviously have the opposite ie ON. They are a very interesting OptoCoupler. MOC3052SR2M OPTOCOUPLER TRIAC OUT 6-SMD Fairchild Optoelectronics Group datasheet pdf data sheet FREE from datasheetz.co… There is a delay between the LED and the Triac turning OFF, which if not handled correctly, could result in a voltage larger than expected at R36. If you're trying to somehow control the current to the spot welder then there has to be some better methods. Cheers Mark

Detecting OFF state of an AC Load

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