Is there a safe way to observe ripple current w O'scope?

[acheld]

I'm dealing with 120 Hz hum on a Blues Junior variant (cathode biased) I just built and would like to view and measure ripple on my HT rail.

My Fluke is a 117, and I don't think it has that cool graph function, so that won't work . . .   

My oscilloscope is a Hantek DSO2D10, which I like (and was not expensive), BUT right on the panel it warns it is CAT II (300V) limited.   I'm wondering:  is there a reasonably* safe way to view and measure ripple with a scope like this?

Edit:   So, looking at the probes, with the attenuation set at x 10 it says (on the probe itself) "600Vpk". So maybe it's within design parameters.

  * reasonably safe means that with care and some luck my scope won't die in a puff of magic smoke -- I'm not looking for a NASA level of safety.

[shooter]

the scope cannot deal with >300v regardless of what probe came deal with.


your call, me I bought a 100:1 probe




EDIT:  if you measure at the plate you'll "see" what the tube feels and stay <300v  (In most cases)

[Williamblake]

Sorry, wrong topic.

[judge74]

You can use a capacitor inline with your red probe to block the DC. Just use something like a .022 or a .047 taped or heat shrinked to a chopstick. You can probe around with that while the black is hooked to ground. It should just pass the ripple as AC and block the DC.

[PRR]

You can use a capacitor inline with your red probe to block the DC....

You still get a 300V spike. This may or may not do damage.

You want a resistor attenuator. Since you are reading a power rail, loading hardly matters. Since you are looking for 50Hz-500Hz, it does not need to be fancy compensated. Since you really don't care about microVolts, it can be 100:1 ratio. Since you don't need 1% precision, it can be 101:1 ratio.

I have used just this scheme to read car ignition systems spikes.

[judge74]

You can use a capacitor inline with your red probe to block the DC....

You still get a 300V spike. This may or may not do damage.

Maybe that’s why my scope stopped working

[acheld]

Interesting discussion.

I understand the logic behind a capacitor; I'd probably use a 1kV+ rated cap (given my shaky hands).

Not sure I understand the .gif with the voltage divider.   When I measured the resistance through one of my probes, I saw 100R when on the x1 setting, but almost 10M (!) on the x10 setting (Fluke meter, good battery, rechecked). 

What if I just created classic highpass filter  --  signal (in this case on the HT rail)--> cap --> probe WITH a 1k resistor to ground, in addition to the standard black wire to ground.   Use the x10 setting on the probe.  Would that reduce the spike I'd see when probing the HT?   

My scope says it can take 300VRMS . . .

[pdf64]

A 10M - 1k attenuator is going to reduce say 20V ripple to 2mV!

[HotBluePlates]

Not sure I understand the .gif with the voltage divider.   

Make a voltage divider of 2 resistors from B+ ("In") to Ground.  Connect your scope probe at the "Out" and the voltage is unconditionally safe (the signal size is reduced to 1kΩ/(100kΩ + 1kΩ) = 0.0099x the actual size).

Less work and less brain-strain than any other method.