Detect voltage variation

[anishkgt]

Hi all,

What would be the simplest way to detect a AC voltage variation from a load OFF state to the point it switch ON. The load here is a Microwave oven transformer with the secondary rewound with 3AWG wire. The mot is switched on via two SCR in a back to back configuration. To make it simple to understand a TRIAC would suitable.

So when the MOT is OFF I can see a voltage of 1.2vac at the primary and when the secondary are shorted a voltage of .2vac is seen. I just need to detect this change in voltage.

What would be a suitable design to achieve this ? Would an OPAMP like MCP6281 in a deferential configuration be helpful.


Theoretically this would be it



But that did not work. Just gave a square wave. I was hoping somebody could guide me here on how to achieve this.


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Equipments: DSO104z, Hakko FX888D, Brymen BM869s

[ramonest]

I'm not sure on what you want to measure, but i think your time constant is really high.

[anishkgt]

I'm not sure on what you want to measure, but i think your time constant is really high.

I don’t  need to measure but detect the voltage drop at primary when the secondary of the MOT are shorted. This is when the MOT is not powered. This voltage is probably the leakage current from the SCR into the primary of the MOT.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

[anishkgt]

It works as an autodetect for a spot welder when the electrode are in contact avoiding the use of a switch.

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.

[anishkgt]

I was thinking of a differential opamp design where the non-inverting would be connected to voltage divider to output 1.2v dc(the open circuit voltage of the MOT at the secondary) and the inverting via a 1Meg 1W resistor. when the voltage at inverting goes low that would do something at the output and that would be sensed by the ADC of a MCU.

[anishkgt]

So this what am trying to achieve but how do i get the out to go to GND when the voltage at the input is 1.2vac. other wise the output is 5v. I get a square wave when at 1.2vac.

[anishkgt]

yet to try out on a board but from the looks of it, i think this would work

[oldway]

What is the mains voltage? 110 or 230V?

1) in your diagram, thyristors T1 and T2 are inverted (anode / cathode)

2) The mot primary is not isolated from the electrical mains. The voltage detection signal must therefore be isolated by a transformer or an optocoupler.

2) the leakage current of the SCR's is very low, the voltage you measure at the primary is probably due to the snubber rc.

Why not increase this snubber to get a higher voltage than 1.2V?

If the voltage is greater than 5Vac, you could use a very simple circuit with a rectifier bridge, a zener diode, a capacitor, a darlington transistor and an opto coupler.

3) It should be remembered that the voltage detector will receive full mains voltage (110 or 230V) when the thyristors are conducting. It is therefore necessary that this detector can undergo large overvoltages without being damaged.

4) The snubber acts as an AC power source limited in intensity. It is therefore necessary to use only a very low current so as not to drop the voltage.at the terminals of the primary of the transformer.

[anishkgt]

Mains can be selected between 120 or 230v. Yes i just noticed the SCRs are inverted and i've changed, Thank you. I am not sure how to workout the snubber network. I got this from the TRIAC driver datasheet.

In the circuit attached C22 filters the mains high voltage when available and the zener diode D8 would keep it 4.7v.

[anishkgt]

on second the the neutral is connected to the opamp GND but i forgot add it to the schematic in my previous post.

Would the attached be ok so as to separate the neutral from the GND or would this be short ? Should the neutral be connected to the PCB GND as well ?