Probe spot welding pulses - Page 1
Forum section: electronics / beginners

author: BinaryBits
author_rank: Contributor
timestamp: June 02, 2023, 11:15:16 am
content: So i like to know how long the spot weld pulses are on the different pulse time settings (or "gears" as the Chinese call them). But im not too familiar with oscilloscopes, so im not totally sure how i go about doing that?The current output of spot welding pulses are normally more than 100 Amps, will the scopes input impedance limit the many amps from reaching the scope, or how does it work? The voltage is around 12V.I saw a video of somebody triggering the spot weld pulse against a 5 Watt resistor while the scope probes also were connected to the resistor, so if thats the way to do it, what resistance value and wattage should i use on the resistor?And what settings should i have on the scope and the probe, 10X ?I just want to make sure that i don´t damage the scope somehow?Thanks in advance for some suggestion on how this is done?

author: Manul
author_rank: Super Contributor
timestamp: June 02, 2023, 12:17:26 pm
content: Based on your question, you seem to not be fully comfortable with fundamentals. Also we don't know the insides of your welder. In that case, I suggest that you first try the most safe approach, which is to observe the pulses in a non-contact way. You short the probe by attaching the ground clip to the tip, making a small loop. Now pass welder pulses through some wire and a 12V car bulb or a high power resistor of around 2.2 - 4.7 Ohms as a load. If you run a wire through the loop of scope probe or even bring it near, you should be able to observe the start and end of pulses as short spikes on your scope. Leave it at 10X and expect to go as low as 10mV per division. I've never tried it myself with a welder and I don't know much about welders, but it should work in theory. At least it's safe for your scope.

author: BinaryBits
author_rank: Contributor
timestamp: June 03, 2023, 10:29:41 am
content: Thanks for your suggestion, unfortunately it didnt work for me, the scope didnt trigger at all. Maybe i did something wrong? I ran one of the spot welder pen wires inside the probe loop and triggered the spot welder pulse on a 12V car light bulb.I guess its doable the direct contact way since i have seen people do it on Youtube, but i dont know which scope settings they are using, or resistor values, etc.I think most battery powered spot welders works about the same way, at least on a basic level.

author: Manul
author_rank: Super Contributor
timestamp: June 03, 2023, 11:25:13 am
content: It's not clear what does it mean "didn't trigger", because you don't say how you set up the trigger. Anyway, at first you could just set the scope trigger to "Auto", set like 10mV per division, and look on a running trace if you see anything going on when you activate the welder. If you see something, but very low amplitude, you can take a piece of thin wire and wind a sensing coil with 10 or 20 turns, not just 1. That will increase the amplitude of signal by a lot.Regarding the direct connection, yes, it certainly can be done, but I don't want to be responsible by suggesting how to do it. Because if by any chance your welder output is not isolated or some momentary arcing produces high voltage, it might not be safe for a scope. I would myself either use a loop / coil approach described above, use a differential probe or check things really well and include at least rudimentary protection on the scope input before attempting. Personally I prefer to be on a safe side whenever possible.

author: BinaryBits
author_rank: Contributor
timestamp: June 07, 2023, 07:17:40 am
content: Yes i agree its better to be on the safe side. Thanks, i will try to do it the way you described.