Using scope for non ground referenced voltage measurements?

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treez

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I am supplying a 5W SMPS from an isolated bench PSU that is mains powered.

The SMPS has a sense resistor in the high side of its DC input.....One sense resistor terminal is at 5V, and the other at about 4.9V.

Can i put a scope probe across this sense resistor, even though this resistor has no terminal connected to ground?

Here is the scope that will be used...(TBS1000).........

https://www.tek.com/oscilloscope/tbs1000-digital-storage-oscilloscope
 


yes, u can put a scope probe across this sense resistor.
The important note is that when u use the second channel of your scope, u must connect the reference of the probe (the crocodile) to the crocodile of the 1st probe or u can left it unconnected (as there are internally connected together), and so, the second probe measuring will have the same reference of the first probe.

The problem will happened if u connect the the crocodile of each probe to two separate parts which have a voltage difference between them, and then u make a short circuit across that voltage difference.

Regards,
Hz
 
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The cautions mentioned in the reference article are valid. If the scope you are using has an "A-B" mode, you can connect two probes to the resistor. This technique works as long as the common mode range of the scope is not exceeded. Another possiblility is to construct a differential amplifier using, for example the Analog Devices AD624, and connect the output of the diff amp to one channel of the scope. You must observe the common mode limits of the differentiao amplifier if you try this
 
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A home made diff probe using an in amp sounds a great idea. -since diff probes cost £1000+.

....it would need all the trimmings though, and that would make things interesting...i bet a diff probe circuit isnt strightforward.
 

Your question isn't very clear, but i understand that you're talking about measuring at mains connected SMPS, means > 300 V common mode voltage. In this case, high voltage differential probes and supplying the device unter test through a safety transformers are suitable options.

A 25 MHz high voltage differential probe is available for about 200 GBP, not 1000. See e.g. https://uk.farnell.com/pico-technology/ta041/probe-active-differential-powered/dp/1667343

Common mode interferences respectively probe common mode rejection can be a problem with low voltages measurements though. Coping with it is a matter of designers craftmanship, there's no easy way to achieve it.
 
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i am running the smps from the output of a bench PSU that has an isolated output set at 5V..........so surely i dont need isolation transformers etc?

Surely its no different than if i were running my smps from a 5V battery?
 

That's much easier then. If the supply output is actually isolated, grounding the 5V side for the measuremet should be considered. Differential measurement with two (equal) probes and A-B math is another simple way. Various active probes may be used according to bandwidth and common mode rejection requirements.

Or use a standard instrumentation amplifier.
 
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Thanks, regarding A-B method, the link in #3 disfavours this.....round about page 14,15
 

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