I've done some DC projects but am starting to get into AC/DC converters and ultrasonics. An isolation transformer is needed for the device under test but is one also required for the oscilloscope? It would seem to me one is needed for the oscilloscope as well as the DUT

 

is one also required for the oscilloscope? It would seem to me one is needed for the oscilloscope as well as the DUT

The oscilloscope power is isolated from the line.
Its only connection to the grid is the 3rd prong safety ground which is usually connected to the oscilloscope signal ground.
So you don't need separate isolation for the oscilloscope as long as the DUT is isolated.

 

Not unless the scope is an old one channel scope. If scope is dual, with add function, then isolation for scope should not be necessary.

There are always exceptions.

 

Not unless the scope is an old one channel scope. If scope is dual, with add function, then isolation for scope should not be necessary.

That's if he wants to do unisolated line-voltage measurements.
If the DUT is isolated, such restrictions don't apply.

 

The scope is new and the circuit to be checked is an ultrasonic generator circuit board. The board's frequency can be adjusted to the resonant frequency of the 40kHz transducer

Something else I would like to do is measure the board's output. Voltage and frequency is no problem but amps might be. What do you think the output amps will be? Do you think it'll pop a 10A fuse?

 

What do you think the output amps will be? Do you think it'll pop a 10A fuse?

10A fuse of what?

 

I've done some DC projects but am starting to get into AC/DC converters and ultrasonics. An isolation transformer is needed for the device under test but is one also required for the oscilloscope? It would seem to me one is needed for the oscilloscope as well as the DUT

Be very, very careful that you understand what you are doing as regards grounding and your oscilloscope in AC environments. Chances are, your ground in your scope is common among your probes. This means that grounding one probe, is as good as grounding 2 probes if you are using 2 probes in a circuit. But more importantly, if you use just one ground on a probe, and not one on both probes, you prevent yourself from accidentally creating a short-circuit condition through your probes' ground-circuits and the oscilloscope ground-plane in an AC environment.

It's easy to do. I did it once. BANG!! And I have remembered that lesson. Your probes can be more expensive than your scope, because at the end of they day, it is _their_ interpretation of the voltage signal that your scope sees, so they are very carefully crafted. I was fortunate, I had a spare probe, and the scope was not damaged.

 

I sure hope you understand(and just not "heard"), how dangerous the ground lead on the scope probe can be.

It can burn you up. Study, but more importantly understand AC, before you play with it.

Isolating a scope is a thing of the past. One can measure non isolated circuits without isolating a scope with modern equipment and probes.

I have had to isolate a scope many times. It's a dangerous procedure. And no longer necessary.

Certain rare situations and rare conditions may require rare exceptions in any endeavor.

 

"This means that grounding one probe, is as good as grounding 2 probes if you are using 2 probes in a circuit."

Only for low frequencies. At high frequency even a probe with its ground lead connected to circuit ground as close as possible to the point being probed will frequently show rubbish due to the inductance of the ground lead and resonance between the lead and the capacitance of the probe tip. A ground connected via a completely separate probe can corrupt signals at even moderate frequency. If you use active probes with tip capacitance on the order of a picofarad, you can get away with somewhat more sloppy grounding. For ordinary passive probes a tip grounder is often necessary.

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With the device under test operated from a good isolation transformer, the circuit is fully "floating" with respect to AC mains and you won't get a shock if you are grounded and touch any single point in the circuit. That is entirely changed as soon as the scope's ground is connected. Now the entire circuit must be regarded the same as it would be without the isolation transformer - as being at AC mains "hot" voltage and capable of delivering a lethal shock. That is, every single part of the circuit must be considered to be "hot." Lots of things in the circuit may be at a voltage that is safe, but to treat it that way without carefully considering every single move is to invite injury or death. Injuries often result not directly from a shock, but from reaction to it. You might jerk your hand back and punch yourself in the face or stick a scope probe in your eye. You might rip a big hole in the back of your hand as it encounters a sharp edge. You might fall off a stool and bash your head on the floor. You might involuntarily fling the equipment across the room (I know someone who threw a big heavy power supply through a window). If you normally work on low voltage circuity, which is what most people in electronics do these days, it is easy to get complacent about shock hazards.

If you are talking about measuring the output current of the ultrasonic generator with a meter, be sure to check the specifications of the meter. 40 kHz may be high enough that the meter could be quite inaccurate. Also be sure that the circuit will tolerate operating without the transducer connected in case you do blow a meter fuse or otherwise accidentally open the circuit. 10 A into an ultrasonic transducer is a lot if it isn't just a brief pulse. I assume this is some sort of large ultrasonic cleaner or a welder.

 

crustchow
I was concerned about burning the fuse on my DMM. It really isn't that big a deal if it burns. My real concern was running the circuit board without a load when the fuse fail. It would only run for a few seconds until it could be turned off.

I'll only be using one scope probe to adjust the frequency of the board. Having an isolated power supply for the DUT and have a scope probe grounded to earth seemed like you could still ground the DUT.

This project is a small ultrasonic cleaner. Nothing industrial or high amperage