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100:1 probe for measuring ripple in a tube amp power supply
Andreax1985:
Exactly. Anyway, let me be more precise. I measured my DC current at 320V DC with a multimeter. My scope input (Siglent 1004x-e) is rated for 400Vpk.
So the question is: is a 400Vpk rated input enough to withstand a full 320V DC signal in AC coupling mode?
DaJMasta:
When you're using a 1x probe, it's closer than I'd cut it but it's within spec. When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope. Just make sure your probe is rated well enough.
Andreax1985:
--- Quote from: DaJMasta on January 29, 2019, 04:18:10 am ---When you're using a 1x probe, it's closer than I'd cut it but it's within spec. When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope. Just make sure your probe is rated well enough.
--- End quote ---
Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?
DaJMasta:
--- Quote from: Andreax1985 on January 29, 2019, 04:25:49 am ---Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?
--- End quote ---
I don't know why you reask this or are so insistent, myself and several others have answered your question accurately and you just keep second guessing it. Unless you're expecting your ripple component to be more than 160V peak to peak on top of your 320VDC, then you are still within the spec of the frontend with a 1x probe (or just a BNC cable going straight into the front). If you just use a 10x probe, it has an internal 9M Ohm internally, so with the inbuilt voltage divider, the scope's frontend, at the BNC right where the AC cap can connect, only ever sees 1/10th of the voltage at the tip of the probe. That means that with a normal 10x probe, you're going to be more worried about the dielectric breakdown of the parts at the very tip and the clearances around the resistor string in the probe itself because of your input voltage (the probe's voltage rating) than the scope's frontend, because 4kV on the tip of a 10x probe is required before the frontend of your connected scope will see that 400V maximum voltage.
The answer really has nothing to do with the coupling mode - as the rating of the frontend should be for any coupling mode. It would say in the datasheet/user manual if the AC coupled input mode had a different maximum voltage.
rf-loop:
--- Quote from: Andreax1985 on January 29, 2019, 04:25:49 am ---
--- Quote from: DaJMasta on January 29, 2019, 04:18:10 am ---When you're using a 1x probe, it's closer than I'd cut it but it's within spec. When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope. Just make sure your probe is rated well enough.
--- End quote ---
Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?
--- End quote ---
Input rating is NOT 400V pk.
It is:
1 MΩ: ≤400 Vpk(DC + Peak AC <=10 kHz)
If this is difficult to understand and you are working with over 300V voltages I think it is not oscilloscope at all what you need first.
First you need some basic fundamentals study book. Yes I know reading is boring but...
but also not alone reading, all need also understand. But it is good you ask. This is wise.
But then you do not listen, you repeat and repeat same question without giving any more real facts about signals what you need measure. If you only tell 320V DC and you need measure ripple how you think it can answer anymore than example +320V + +80V is +400V (400Vpk) also -320V + -80V is -400V (400Vpk) You can even alternate between these as long as you stay below 10kHz (sine). (so <10kHz sine <800Vpk-pk (note small but important difference between pk and pk-pk) is accepted and with specification limits)
Without more details I think many peoples can repeat these answers infinitely.
You can connect 400V DC to scope input. But remember also what is DC. When you connect it, it is not DC at all. How long time after step you can call it DC. Think carefully this 10kHz. When you switch from 0V to 400V it depends this edge what is maximum frequency what there exist. So you can "slowly" (yes 10kHz is slow) rise it from 0V to 400V.. to +400 or to -400V (ref GND) and same when you change it from this voltage back to 0. You need stay below 10kHz if you are workin there near max limits.
(btw, pure DC is possible only in human imagination. In practice there is not DC at all but yes, perhaps 1pHz can think in practice as "DC" perhaps even nHz or uHz). ;)
But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc". What kind of ripple. What is this ripple level range and max freq.
Btw, as far as I know Siglent use 500V rated capacitors in front end these positions what need it.
(it can also see in this old SDS1202X-E "compensation issue" case)
But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc". What kind of ripple. What is this ripple level range and max freq.
If you afraid this internal AC coupling capacitor you can of course use external capacitor and then oscilloscope DC coupled. This give also one more advantage. You can select what is low frequency corner what you need instead of Siglent very low corner freq around 1.2-1.3Hz. (more low than many other scopes and if your DC have low f fluctuations then ripple measurement may be extremely difficult if you need look low levels.) Is is possible or useful to use scope DC coupled and external coupling capacitor?
What is this ripple (over this 320V) what you want measure. Ripple pk-pk level and something about freq range and about what V/div, mV/div range you need for ripple measurement?
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