Attaching Oscilloscope to Welder

I am going to use my oscilloscope for check some of the circuitry in my old, broken welder. My scope has 3 possible connect points for channel 1 (or channel 2) - the probe, an alligator clip near the end of the probe, and chassis ground from the front panel.

What is the purpose of the alligator clip near the end of the probe? What do I hook it to when I am probing inside the welder?

Are there other safety tips to keep from frying myself or the scope?



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I'm not sure what you will learn, given your questions. Be careful.

Steve Smith

JohnP wrote:

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Steve Smith

If you don't know this then the displayed waveforms are going to be a real mystery. The alligator clip is a ground clip and is normally connected to the circuit's common point if you are measuring voltages with respect to the common. The common is often the chassis and may also be at ground potential or not referenced to ground at all.

Normally the clip is also connected to the chassis of the oscilloscope and the chassis of the oscilloscope is connected to earth ground via the 3-pin line cord. So the alligator clip is at ground potential.

Somtimes this can cause problems if the device being tested has a common with a voltage on it relative to ground. When you place the ground clip on it you will be placing a short from the common to ground and this can be bad. Normally though most commons are either at ground or floating relative to ground so it isn't a problem especially if the device is powered via a transformer.

Some people will get around a grounding problem by floating the oscilloscope chassis by using one of those cheater plugs that don't have a ground pin. This can be very dangerous because touching the oscilloscope can give you a nasty shock.

A better way around the ground reference problem should you ever have one is: if you have 2 channels you can use the probe on channel 1 as the signal probe and connect the probe of channel 2 to the common or other point in the circuit NOT using the alligator clips on either probe (they usually come off). There is usually a setting on the vertical amplifier section of the scope so you can display the difference between channel 1 and channel 2 and this is the desired signal.

If you are going to troubleshoot an inverter board you'll need lots of luck if there are no obvious signs of failure (smoke, fried traces, exploded parts). An inverter is loaded with feedback loops and a fault in any part of the circuit will cause abnormal readings in the other parts of the circuit. Just because you find something peculiar it doesn't mean you are near the cause. Billh

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Ah, something I can answer. The "alligator clip near the end of the probe" is a ground. A ground that is physically close to your test point will provide a more accurate picture when viewing high frequency signals. I dont think there is anything in a welder that is considered "high frequency" in the scope world.

Most of the time I dont use it because I spend most of my time looking at low frequency signals and the ground on the front of the scope is good enough.

Depending on your scope probe you may be able to remove the alligator clip. I usually do so that it does not accidentily touch something what would "let the smoke out" as we say. Remember its is at ground potential.

You should remember that welders are like computers. They both work on smoke and mirrors. If you do something to let the smoke out they stop working:-)

You should follow the old TV repair rule. Use only one hand and put the other one in your pocket. There are a lot of very big capacitors in there. Even with the power off they can bite you. Remember, the purpose of a capacitor is to store charge. If the circuit was not designed right or if your fault is in this area, the capacitors may be charged.

Also look out for floating voltages or isolated power planes. This means that there may be more than one ground. A circuit may be a 5 volt circuit but its voltage reference is floating at 120 relative to the ground you picked.

Rule two, dont work alone.

good luck, let us know what you find Al

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John: The other people who have responded to your question have good points. I thought I might add a couple more:

In some welders such as TIG machines, there are voltages possibly high enough to damage your oscilloscope as well as yourself. The "high frequency" for starting an arc without physical contact of the electrode and workpiece is really a high-frequency, high-voltage superimposed on the main welding electrical supply. That high voltage is enough to create an arc-starting spark between the electrode and workpiece like the spark that jumps the gap in automotive spark plugs. Oscilloscopes will usually withstand only a few hundred volts across the input terminals without damage. I believe the "high frequency" to be in the thousands of volts.

An old teacher and long-time friend of mine told me the saying this way: "Keep your left hand in your back pocket." That way you greatly reduce the possibility of getting shocked through your chest from one hand to the other and if you do get shocked, the current is more likely to run down the right side of your body through your feet and away from your heart.

I do this kind of electrical tinkering quite often and I enjoy learning about the pieces I test. The oscilloscope is a wonderful voltmeter that really lets you see what the electrical signals are doing. If you can find someone familiar with oscilloscope operation, that might speed up your project with this welder.

As usual, be careful, don't rush, have fun and good luck.

Andy S.

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What kind of welder, and whats broken about it? Buzzbox, inverter, what ?

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Mr Wizzard

Get yourself a good isolation transformer for the scope and be CAREFUL what you hook that ground to!

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Rossco in Oshawa

Take off the alligator clips.

Hook one probe to the circuit ground.

Hook the other probe to whatever you want to look at.

Select the proper voltage range.

Push the ADD button and the INVERT if your scope does not have a differential input button.

Have at it.

Using the ADD and INVERT buttons together gives a differential input. If your scope doesn't have these, I am not sure I would use it to check a high energy circuit.

John I used to do this stuff for a living

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You might also consider using a special HV probe that was designed to test TVs. The flyback in a tv is around 20,000v.


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