My circuit detects the zero cross on the AC sine wave and triggers two Thyristors in a back-to-back configuration which switches on a MOT(Microwave Oven Transformer) at the peak of the sine wave, which is the typical way to switch on inductive loads to avoid inrush currents. I also have two taps on the secondary to measure the voltage during a weld to calculate the Weld current. All these are initiated at the press of a button which am hoping to automate when the electrodes touch a conductive material like here, nickel strip
Just not sure how to do this as i am trying to sense the secondary when no current or voltage flows through it. If am correct it would be a bad idea to pass a small dc through the secondary so the opamp can sense a voltage drop when the electrodes make a contact because during a weld it would be a short or maybe in someway block that current during weld using an optocoupler or a power MOSFET or a TRIAC ?
Would anyone have a better idea to accomplish this in anyway.
Hi,
A schematic of sorts would help, as just telling us how you envisage the circuit can be interpreted/misinterpreted by all of us.
A picture is worth a thousand words.
Why do you need to sense the secondary when it is not powered up?
The sense contact is an optional feature to trigger the weld without the weld button. it doesn't have to be the secondary, i said that because that is where the electrodes are used for spot welding.
Hi,
Thanks for the schematic(s).
Sorry but it is what I call a search-a-word schematic,
Shows no signal flow.
For example
Your FTDI Interface connector is not in the same section as the UART?
That it is what it is directly connected to.
Where does the the TEMP/1.?? in the NTC section go, and to how many different places?
Sorry but it might be okay if you are doing a PCB and it will be interpreted by program (nice job by the way) and I know its prototype, but for trouble shooting its a pain.
I was tiding up the schematic and those '?' seems to be popping up every where and not sure how to get rid off.
By the way how accurate would this approach be to sense an electrode contact?
I just checked the OFF-State voltage at the thyristor and it read 1.05VAC and when the secondary was shorted it was 0.5VAC.
I could add a comparator in parallel to monitor the voltage variation and use the out of the op-amp to trigger the thyristor.
anishkgt:
By the way how accurate would this approach be to sense an electrode contact?
I just checked the OFF-State voltage at the thyristor and it read 1.05VAC and when the secondary was shorted it was 0.5VAC.
Voltage between what two points on the circuit?
Tom..
anishkgt:
I was tiding up the schematic and those '?' seems to be popping up every where and not sure how to get rid off.
By the way how accurate would this approach be to sense an electrode contact?
I just checked the OFF-State voltage at the thyristor and it read 1.05VAC and when the secondary was shorted it was 0.5VAC.
I could add a comparator in parallel to monitor the voltage variation and use the out of the op-amp to trigger the thyristor.
I was hoping to use it as a comparator. would that be ok ? what would be the best comparator for this LM741 ? and would it be ok to use 1N4001 diodes ?
