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| 100:1 probe for measuring ripple in a tube amp power supply |
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| Hydron:
The OP is 100% correct to be thinking of the 400V rating - coupling mode DOES matter and AC coupling will negate any probe scaling factor for DC input for most probe/frontend designs. He/she understands a subtle gotcha in the way the AC coupling mode works and is asking a reasonable question rather than trying to start an argument. I'd also agree with the suggestion above to use DC coupling and an external capacitor at the tip however. |
| Andreax1985:
--- Quote from: rf-loop on January 29, 2019, 06:52:40 am --- --- 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? --- End quote --- Thanks for the 500V bit of info on the coupling capacitor rating. As for the rest I can't be precise because I can't know before I probe the circuit. But since what I want to probe is a classic power supply stage in a tube (audio) amp I think ripple will be very low compared to the Dc signal (so no worries about it's magnitude). Also, I expect it to be at 100hz frequency as typical in this kind of applications. So I have no worries related to its frequency either. |
| capt bullshot:
--- Quote from: Andreax1985 on January 29, 2019, 07:21:37 am ---Thanks for the 500V bit of info on the coupling capacitor rating. As for the rest I can't be precise because I can't know before I probe the circuit. But since what I want to probe is a classic power supply stage in a tube (audio) amp I think ripple will be very low compared to the Dc signal (so no worries about it's magnitude). Also, I expect it to be at 100hz frequency as typical in this kind of applications. So I have no worries related to its frequency either. --- End quote --- A 10:1 300Vrms CAT II probe is enough for your measurement, including quite a good deal of margin. This is because 300Vrms equals to 420Vpk, and CAT II includes headroom for overvoltage. And your DC source is of limited energy (discharge of the filter cap, gives you a loud bang when shorted, but no violent explosion). I agree to the others, a 10:1 probe is safer than a 1:1 probe here (for measuring at 320V DC) and will give you enough display resolution as one expects the ripple voltage in the "some volts" range here. The internal resistance of the 10:1 probe will do a good job of protecting the scope's input. The general strategy for measuring ripple on a supply rail would be: Set the scope to a high enough voltage / div scale, to cover the DC voltage, set coupling to DC. Connect the probe, turn on the DUT (in any order you like) Watch the signal for plausibility, check it's not exceeding the range. As long as you can display the signal in DC coupling, one can assume it's safe to switch to AC coupling. Change to AC coupling and increase sensitivity (less volts / div) until you can watch the ripple. Disconnect and discharge the AC coupling capacitor by setting the scope to DC coupling again. This procedure takes care of safely charging / discharging the scope's AC coupling capacitor. Usually these steps are skipped, because most general purpose scope's inputs are robust enough to not require them. Especially when you're using a probe (even the 1:1 ones should have some internal losses to dampen the charge / discharge currents) and operating at typical supply voltages up to 20V. For your 320V, I'd recommend to be more careful in general, since a charged capacitor at 320V bites you and your electronics. A charged AC coupling capacitor (from measuring B+) can damage small signal semiconductors if you move the probe from B+ to a small signal node without discharging the AC coupling capacitor, even when using a 10:1 probe. |
| Wolfgang:
--- Quote from: Andreax1985 on January 28, 2019, 03:06:34 pm ---Hi, I wish to measure b+ ripple in a tube amp. Since the b+ rails are at 350V, I was considering buying a 100:1 passive probe for my scope (rated for 400V input). Nevertheless, here is my doubt: I'll need to use the scope in AC coupling mode to clearly see the ripple. This means that I cannot safely use a cheap 100Mohm 100:1 probe. I'm on a budget and I can't afford a differential probe. Could you suggest me a suitable 100:1 probe which does not give issues when used in AC coupling mode? --- End quote --- This is how I did it (to measure ripple in an 500V HV power supply): - a took a *big* foil cap (10uF@1000V) and connected one end to the PSU and grounded the other end. At the grounded side it looks like a resistor (e.g., 10kOhm) and a pair of antiparallel diodes (1N4007) that are bridged by a switch, now closed. - I connected the load to the power supply - I power up the PSU. The cap now charges up thru the closed ground switch - I open the ground switch. AC now appears across the 10kOhm resistor and can be measured. Lower bandwidth limit is ca. 10Hz. - Performing measurements - Afterwards, close the ground switch again - Disconnect scope - Power down the PSU - After coupling cap is empty, disconnect it - Done |
| trobbins:
The B+ ripple in a valve amp may well get to 5-10Vrms for a capacitor input filter, especially with a valve diode. Be very careful to check if the power supply uses a choke input filter, as the diode output voltage that connects to the choke will be a large DC+AC level. Most valve amp people just get a nice high voltage cap of about 10nF and place it between the B+ and a 10:1 probe, and use DC coupling mode to ensure the probe and scope parts are all ground referenced. Clipping the cap/probe to the test point, and then energising the amp, and then de-energising, allows the interface cap to charge up and down more sedately and safely. A 10:1 probe presents 10Meg loading, which is usually high enough not to noticeably alter voltage rail levels in the preamp sections of an amp (whereas 1Meg loading can have a noticeable influence). |
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