Oscilloscope Isolation

I’m attempting to work on a PowerMax RV 80amp charger I need to use the Oscope on the pwm circuit but as it’s a mains power device what’s the best (safest) way to make measurements without blowing up components or the grounding practices that’s with probing mains voltage circuitry.
I have an isolation transformer can I plug the device under test in the IT or both the test equipment and DUT or just the scope.
Regardless of the isolated transformer, is it likely to still short the device or test equipment?


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2011-04-29 8:37 pm
A high voltage differential probe can be used with any scope.
The isolation is in the probe.

Is this a one time job, or is the equipment for an ongoing lab?
High voltage differential probes (safe for the AC line) are expensive.

Example of a suitable scope, CAT IV 600 V, 600Vrms isolation, up to 200V/div :
Fluke 120B Series Handheld Digital Oscilloscope | Fluke

Low cost HV probe examples:
CT4068: 35 MHz Differential Probe

You only need either the probe, or the scope, but not both.
In any event, please exercise extreme caution.
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2011-04-29 8:37 pm
It’s on ongoing lab, but I would want also a battery one, can you make any recommendations
on both types of devices?

Fluke Series Industrial ScopeMeter handheld Oscilloscopes:
Fluke 120B Handheld Digital Oscilloscope
Fluke 123B Handheld Digital Oscilloscope
FLUKE 124B Handheld Digital Oscilloscope
FLUKE 125B :Handheld Digital Oscilloscope

HV probes, I would get the Pico.
GDP-025 Instek 25 MHz Differential Probe New
CT4068: 35 MHz Differential Probe
TA042 1400 V active differential oscilloscope probe | Pico Technology
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2011-04-29 8:37 pm
The VDS-1022I (note the I) is a relatively inexpensive isolated USB scope. Of course, any battery powered scope will work. You don't need an expensive probe to measure deferentially. All you need is two matched probes and a 2-channel scope.

That USB scope is not suitable unless used with a high voltage differential probe, it is not isolated.
Any single ended probes used with an isolated scope must still have an appropriate voltage rating/attenuation factor.
Matched single ended probes must be used with an isolated scope and can cost as much as (or more than)
these HV differential probes. In our power electronics research lab we would never use single ended probes,
they are very dangerous around HV.
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2011-04-29 8:37 pm
With the two probes i will be able to take measurements on any parts of the primary (120vac) circuits,
what would be the settings, and is it standard across all scopes (digital or analog, brand or model)?

Absolutely not, you need an isolated differential probe for that.
All scopes are ground referenced, unless they run on batteries.
You cannot use any combination of unbalanced probes with a grounded scope for your purposes.
You can't fool around with mains voltage, this is extremely dangerous.
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Before going straight into AC, do some experimentation with DC. You'll see that touching one probe (generally channel 1) to the +B and the other probe to the ground, the scope acts like it would if you used only the ch1 probe. If you reverse the probe positions, you will see that the trace moves down (negative) just like a multimeter when you have the probes reversed. If you have the tutorial, TT8, item 8 covers this. If not, the following is what you need to do.

Two probes
Both scope inputs used
Input set to add
Both channels set to DC coupling
Both vertical amps set to the same voltage
Ch2 input set to invert
Bandwidth limited (works best for most measurements in car amps)
Trace aligned to the reference line on the scope's display
Ground leads for both probes connected together
Ch2 probe on source leg of the FET
Ch1 probe on gate leg of the FET
So, regardless of the BNC shielded connector which is ground, do i still have to use the ground clips connected together or can it be not used as the probes will be on the same ground from the test equipment?
Also with that initial setting, can i just measure across an input filter cap in a switch mode circuit thats energize, will i be able to probe from capacitor ground to virtually any point for measurements without any problems to the scope or components been probed?
You do not use the ground clips unless you find that connecting the clips from the two probes together gives a cleaner signal.

I don't know what the other user is going on about. You are not in any way connecting the scope ground to any live circuit. You're simply using the two probes to find the difference between two points when one channel is set to invert. You can touch two probes to the circuit without fear of doing any harm, just like you would one probe.

As a test on DC, after testing as suggested previously, place both probes on the same point. No matter where you place them (even both on B+), the trace will not deflect because there is no difference between the probes when on the same point.

As always, you may want to take precautions to insulate the exposed ground around the tip of the probe. Most probe kits have covers for when you're not using the hooks.


2008-07-27 3:32 am
You can use an isolation transformer to "float" the scope. Be aware that almost all scopes that are powered from the AC line, connect the scope probe ground clip to AC power ground. You also must disconnect the scope safety ground from the isolation transformer output safety ground (if it has one).

Back in the day it was the norm when working on HV equipment to float scopes. Be sure the isolation transformer chosen can handle any potential differential voltage.

Be aware you have created a very dangerous safety issue. If you accidentally touch any nearby equipment or anything metal connected to AC "ground" or neutral you are in big trouble.