Wherer is the leavkage voltage comming from  ? - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: June 10, 2018, 04:17:16 pm
content: in a circuit as shown the schematic is from the datasheet of the MOC3032M (https://www.mouser.com/ds/2/149/MOC3032M-1011226.pdf) I can read voltage between 0.8 to 1.2vac. Where does this voltage come from ? The Load is inductive and switched OFF.

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: June 10, 2018, 08:32:03 pm
content: where are you mesuring this voltage ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: June 10, 2018, 09:00:24 pm
content: The inductive load is a Microwave Oven Transformer (MOT). I am measuring at the primary terminals of the MOT.

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: June 10, 2018, 09:50:12 pm
content: SCR would be my guess . What is secondary connected to ?

author: Cerebus
author_rank: Super Contributor
timestamp: June 11, 2018, 12:56:15 am
content: Almost certainly capacitive coupling, often referred to as "ghost voltage", or leakage currents through the semiconductors.Check your layout for adjacent input/output leads, check the leakage current specs of your SCRs. The MOC 303x range are rated for a maximum 100nA leakage all on their own.Remember, if you're measuring with any modern multimeter the load impedance (of the meter) is going to be at least 10 meg, if on the low range of the meter it might be gigaohms. If you have a low input impedance voltage range use that to measure. 1.2 Vac into 10 meg is only 120 nA, a trivial current, into a 10G DMM input that would only require 120 pA of leakage current, equivalent to a stray capacitance of 265fF (0.265pF) at 60Hz.

author: anishkgt
author_rank: Frequent Contributor
timestamp: June 11, 2018, 04:34:05 am
content: Quote from: Cerebus on June 11, 2018, 12:56:15 amBeing a beginner i just did not get what you meant. The MOT is part of my spot welder project. I was trying to use this leakage current to my advantage as a way to sense when the secondary are shorted.I'll try to explain the whole idea behind this. The source of the signal is from the mains but parallel to Microwave Oven Transformer (MOT) with its secondary rewound with 3AWG cable for spot welding. When the MOT is OFF there is a voltage of 1.2vac at the primary and when the secondary are shorted a voltage 0.2vac. The MOT switch ON is controlled via two SCR in a back-to-back configuration. Basically a TRIAC. So the voltage at the primary is the leakage from the SCR's which am using it as an advantage to avoid a button.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 be fed into an Arduino to activate the MOT.My schematic for that so far is like this but i doubt his would work because the input voltage is very low

author: anishkgt
author_rank: Frequent Contributor
timestamp: June 11, 2018, 04:36:01 am
content: The opamp as a comparator works to compare the set voltage in the first with a 1v dc offset and the second opamp is just a follower.