O-scope probe ×100

[Behemot]

If I am not mistaken, it is just insulated high-voltage 10MΩ resistor inside the probe plus some tweaking capacitor on the BNC side, right?

So if I wanted to embed BNC for measuring AC line output of a device, I could just hook 10MΩ HV resistor in series on the center pin of the BNC connector inside the device and than connect it with BNC-BNC coaxial directly to O-scope? OFC the device must always be turned on AFTER conecting the cable because of the risk of electrocution, I am aware of that.

[stj]

Re: O-scope probe x100

wrong, it's a voltage divider.
so it has to have atleast 2 resistors.

[budm]

Re: O-scope probe x100

100:1 probe?. Your scope has input Z of 1M. It has to do with Voltage divider and compensation.
Please see the attached PDF.
Probe_Fundamentals-_Tektronix.pdf

[redwire]

Re: O-scope probe x100

Typically x10 probe has a 9MEG resistor and trimmer capacitor.

The problem you might have is the scope GND is connected to earth GND (via the line cord), so you will probably blow things up.
On-line SMPS have a bridge rectifier and if you work on the DC bus, connecting the scope GND to - will make smoke.

I would add a small fuse in series with the probe GND; Otherwise one mistake and you melt the traces inside the scope. Use a bigger (1/2W) 9M resistor good for 500V or more. You don't need the trimmer capacitor to view 100/120Hz stuff.

[stj]

Re: O-scope probe x100

You don't need the trimmer capacitor to view 100/120Hz stuff.

but you will for the 100-200Khz stuff entering the transformer.

[japlytic]

Re: O-scope probe x100

If you need to take measurements on the mains side, you need to use an active differential probe (which is appropriately rated for the application); it also minimizes the problem with common mode noise affecting the measurement.

From some older Pace satellite receiver service manuals, the floating of the oscilloscope ground was mentioned - don't do that under any circumstances!

[Behemot]

Re: O-scope probe x100

Typically x10 probe has a 9MEG resistor and trimmer capacitor.

The problem you might have is the scope GND is connected to earth GND (via the line cord), so you will probably blow things up.
On-line SMPS have a bridge rectifier and if you work on the DC bus, connecting the scope GND to - will make smoke.

I would add a small fuse in series with the probe GND; Otherwise one mistake and you melt the traces inside the scope. Use a bigger (1/2W) 9M resistor good for 500V or more. You don't need the trimmer capacitor to view 100/120Hz stuff.

Oh yeah, 9meg, sure, didn't realize at that hour, just got a quick idea and didn't think about the math thoroughly…thanks for pointing it out! But actually I now also realized it does not matter, I do not care about the absolute value, I won't be measuring any values on the X axis - I only need to look whether voltage is there or not. Fast. And than do measurements based on time, so Y axis.

No problem, what I want to scope is actually wall phase against earth ground and the whole device is earth grounded so no problem here. Otherwise I am well aware of shorting devices with earth ground on secondary… So the phase will be on the central pin, GND on the rim of the BNC as the chasis is grounded too. I will be only looking at this 50Hz phase (whether it is on or off) so if I don't even need the tuning capacitor, even better.

japlytic: yeah man I know differential probe is the best and I should use it for everything incl. actually measuring PSU secondary. But are you going to buy me any? I guess not? Two probes will be more expensive than the whole frelling O-scope. Unless they start making them for 1/10 of the current price, than I am interested but not buying any.

[budm]

Re: O-scope probe x100

Typically x10 probe has a 9MEG resistor and trimmer capacitor.

The problem you might have is the scope GND is connected to earth GND (via the line cord), so you will probably blow things up.
On-line SMPS have a bridge rectifier and if you work on the DC bus, connecting the scope GND to - will make smoke.

I would add a small fuse in series with the probe GND; Otherwise one mistake and you melt the traces inside the scope. Use a bigger (1/2W) 9M resistor good for 500V or more. You don't need the trimmer capacitor to view 100/120Hz stuff.

The scope itself has 1M input so with the add 1M (R2) in parallel with the Input Z of the scope will not make it 10:1, unless that drawing referring the R2 as the input Z of the scope.

[redwire]

Re: O-scope probe x100

It's a crappy diagram, R2 is the scope's input resistance and C2 is the probe cable-capacitance + scope input capacitance.
C1 is in the wrong place I think, because the probe compensation trimmer cap is usually inside the BNC.

[stj]

Re: O-scope probe x100

the probe compensation trimmer cap is usually inside the BNC.

not on any of mine - maybe on very expensive stuff though.