Products > Test Equipment
100:1 probe for measuring ripple in a tube amp power supply
Andreax1985:
--- Quote from: MaxFrister on January 28, 2019, 03:55:44 pm ---Here is my understanding of the issue.
A 10:1 probe is a complicated 1/10 voltage divider.
You want to apply a 350V AC RMS voltage and look at the ripple.
That means you are apply 350*2*sqrt(2) AC peek-to-peek at the probe tip, or 990V -- Let's call it 1KV.
Then, 1/10 of 1KV is 100V -- well bellow the scopes spec of 400V.
p.s., make sure that there is no way the ground clip on the probe can be disconnected while probing high voltages.
--- End quote ---
Let's say that IF the DC component of my signal is 350V and I'm using AC coupling mode, the capacitor in the scope (rated for 400v) will charge at 350V. But if I'm wrong or if a voltage spike takes place (how likely is it?) then I'm blowing the scope.
MaxFrister:
Sorry, not enough coffee this morning. Your B+ is DC, so it is just a straight 350V.
MaxFrister:
Your scope input will only see 35V.
Just check that that probe is rated for the DC voltage. Someone already mentioned that probes from older scopes were all rated for 500V+
capt bullshot:
As pointed out, the AC mode of the scope places the full DC voltage across the scope's AC coupling capacitor, regardless of the probe ratio or rating.
If you are unsure about the max. rating of this capacitor (which is expected to include the specified max. voltage of the scope alone plus some headroom), leave the scope in DC coupling and place a suitable capacitor between the B+ rail and the probe tip.
So if you scope is rated for 400V max. input voltage, this would be safe to use on 350V DC in AC coupling mode with any probe. Usually one wants to use a 1:1 probe here for best display resolution of the ripple voltage.
Andreax1985:
--- Quote from: capt bullshot on January 28, 2019, 04:37:13 pm ---As pointed out, the AC mode of the scope places the full DC voltage across the scope's AC coupling capacitor, regardless of the probe ratio or rating.
If you are unsure about the max. rating of this capacitor (which is expected to include the specified max. voltage of the scope alone plus some headroom), leave the scope in DC coupling and place a suitable capacitor between the B+ rail and the probe tip.
So if you scope is rated for 400V max. input voltage, this would be safe to use on 350V DC in AC coupling mode with any probe. Usually one wants to use a 1:1 probe here for best display resolution of the ripple voltage.
--- End quote ---
Thanks!
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