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100:1 probe for measuring ripple in a tube amp power supply
Posted by Andreax1985 on 28 Jan, 2019 15:06 -
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?
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It is not clear whether you are saying that your (unidentified) scope already has AC mode, or whether you think you need a new probe to get AC coupling?
Ripple is typically a very small signal. Using a 100x probe will probably make the ripple nearly invisible.
You don't need a 100x probe unless you are trying to measure a very large signal (350V). But you are not asking about measuring the 350V. That should be measured with a DMM. You are asking about measuring the ripple which is a very small AC component resting on top of the 350V. You need a 1x or 10x probe with AC coupling. -
It is not clear whether you are saying that your (unidentified) scope already has AC mode, or whether you think you need a new probe to get AC coupling?
Ripple is typically a very small signal. Using a 100x probe will probably make the ripple nearly invisible.
You don't need a 100x probe unless you are trying to measure a very large signal (350V). But you are not asking about measuring the 350V. That should be measured with a DMM. You are asking about measuring the ripple which is a very small AC component resting on top of the 350V. You need a 1x or 10x probe with AC coupling.
My scope has AC coupling mode. As I said, I want to visualize ripple in the (high voltage) b+ stage of a tube amp (350V+). Of course the ripple component will (hopefully) be very small compared to the signal, but I think you are well aware that in AC coupling mode the DC component is not attenuated at all. So, if I use a cheap 100:1 passive probe for probing a 350+ V rectified signal in AC coupling mode I have good chances of damaging the scope. Hence, my question. -
you can use a Tektronix P6006 Probe from flea market , 10:1 , <600V
It is build for glowing scopes but useful with any 1M input BNC
Martin
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you can use a Tektronix P6006 Probe from flea market , 10:1 , <600V
It is build for glowing scopes but useful with any 1M input BNC
Martin
I already have 10:1 probes (CAT I, 600V), but perhaps I'm not being clear enough on what I think the problem is... -
I think you are well aware that in AC coupling mode the DC component is not attenuated at all. So, if I use a cheap 100:1 passive probe for probing a 350+ V rectified signal in AC coupling mode I have good chances of damaging the scope. Hence, my question.
Quite to the contrary, AC coupling completely "attenuates" the DC component 100%. That is its whole purpose. It completely blocks any DC component and allows only the AC signal through. That is exactly what you want when measuring the small AC ripple signal on top of the large DC signal. -
Quite to the contrary, AC coupling completely "attenuates" the DC component 100%. That is its whole purpose. It completely blocks any DC component and allows only the AC signal through. That is exactly what you want when measuring the small AC ripple signal on top of the large DC signal.
Yes, but the oscilloscope's AC coupling capacitor will charge to the voltage at the probe's tip, so that, if the signal has an high DC offset, the input voltage rating of the oscilloscope can be exceeded causing high voltage breakdown in the oscilloscope, possibly damaging it. -
You have not revealed what scope you are asking about. Do you have some scope that has no input protection? If the capacitor charges up to the 350V, that means it is doing its job with 350V DC at the tube amp end, and 0V DC at the oscilloscope end. That is exactly how AC coupling works.
Are you saying that the AC coupling capacitor in your mystery scope is not rated for 350V? If so, then simply add a high voltage capacitor between the amp and your 1x scope probe.
If you use DC coupling with a 100x probe, you will never see the ripple. -
You have not revealed what scope you are asking about. Do you have some scope that has no input protection? If the capacitor charges up to the 350V, that means it is doing its job with 350V DC at the tube amp end, and 0V DC at the oscilloscope end. That is exactly how AC coupling works.
Are you saying that the AC coupling capacitor in your mystery scope is not rated for 350V? If so, then simply add a high voltage capacitor between the amp and your 1x scope probe.
If you use DC coupling with a 100x probe, you will never see the ripple.
My scope is a Siglent 1104x-e, whose input is rated for 400V pk-pk. However, I'm only estimating the voltage on the amp b+ rails at 350V and I feel that my estimate is dangerously close to the limit of my scope (and I'm not even accounting for possible spikes). Hence my desire to operate with a slightly larger saftey margin. -
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 applying 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.
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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.
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. -
Sorry, not enough coffee this morning. Your B+ is DC, so it is just a straight 350V.
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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+ -
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.
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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.
Thanks! -
I've had spectacular ka-bangs come out of scopes
when I disconnected the probe and switched back from AC to DC coupling. This hard shorts the scope's input blocking cap as it discharges through the switch.
The 1MEG input resistor is in the scope, and AFTER the blocking cap when you are on AC-coupling. So the blocking cap charges right up to the full HVDC potential on B+ even though on x10. You can measure ripple at 350VDC but you have to be careful. X1 is very risky, I would not use it without a jig with a blocking cap and voltage divider and bleeder resistor.
Connecting the scope to the HVDC present is hard on the scope's input protection (clamp) as you are fast charging the input cap to the HVDC present. Disconnecting (and touching GND or anything else in the circuit) the blocking capacitor discharges through the scope's protection clamp which is also hard on the scope and can damage it.
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I've had spectacular ka-bangs come out of scopes
when I disconnected the probe and switched back from AC to DC coupling. This hard shorts the scope's input blocking cap as it discharges through the switch.
The 1MEG input resistor is in the scope, and AFTER the blocking cap when you are on AC-coupling. So the blocking cap charges right up to the full HVDC potential on B+ even though on x10. You can measure ripple at 350VDC but you have to be careful. X1 is very risky, I would not use it without a jig with a blocking cap and voltage divider and bleeder resistor.
Connecting the scope to the HVDC present is hard on the scope's input protection (clamp) as you are fast charging the input cap to the HVDC present. Disconnecting (and touching GND or anything else in the circuit) the blocking capacitor discharges through the scope's protection clamp which is also hard on the scope and can damage it.
Perhaps I could simply switch off the amp while leaving everything connected, so that the blocking capacitor slowly discharges through the amp resistors... -
That would work, but one last trap.
Keep in mind changing VOLTS/DIV switch does switch in and out divider resistors, so I would start and end (measurements) at high settings like 10V/DIV.
Don't do this (turn amp power on/off or connect probe) at sensitive settings like 50mV/DIV where the scope's front end is vulnerable.
Looking at B+ you will see mains ripple, rectifier switching noise and the DC voltage will bob up and down due to mains voltage moving around, as well as load from music playing. -
That would work, but one last trap.
Keep in mind changing VOLTS/DIV switch does switch in and out divider resistors, so I would start and end (measurements) at high settings like 10V/DIV.
Don't do this (turn amp power on/off or connect probe) at sensitive settings like 50mV/DIV where the scope's front end is vulnerable.
Looking at B+ you will see mains ripple, rectifier switching noise and the DC voltage will bob up and down due to mains voltage moving around, as well as load from music playing.
Thanks! My 10x probes are rated for 300Vrms CAT II, are they enough for 320V DC? (I measured the voltage with a multimeter) -
Here is my understanding of the issue.
No, the OP wants to look at low level ac "hum" riding on top of around 350v DC.
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 applying 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.
I will, however, address your comment:-
Peak to peak isn't a real thing with ac signals, as the positive & negative half cycles do not exist at the same time.
The highest voltage at any time is the peak voltage on any half cycle---with your values, about 495v.
What I believe the OP was worried about was that the coupling capacitor in circuit in the "ac coupled" mode could charge up to the full 350v DC voltage over approx 5t=CR.
Here, it is assumed that "R"is made up of the series resistance of the probe, in turn in series with the input impedance of the 'scope vertical input,(assuming the latter to be a pure resistance.).
This further assumes that the coupling capacitor's DC voltage rating is less than 350v.
Once the cap breaks down, possibly, all sorts of nasty stuff can happen.
As the 'scope is being used to look at low level ac, the volts/div will be set to a quite sensitive range,which will be overloaded by the 350v DC, with probable damage.
Against this, is the fact that, over many decades, people have used analog Oscilloscopes set to "ac coupling" to look at low level ac superimposed upon DC voltages of the same order as those the OP has.
If you have a fairly sophisticated analog 'scope, it will have adjustable "DC offset", which allows you to stay "DC coupled", but most of the analog 'scopes used for this sort of thing don't have that function.
If damage in the way anticipated was common, it would have become part of the "folklore" of the industry, but it hasn't.
Possibly this is more of a problem with DSOs.
I have read a lot of warnings about this as a problem, but I have not heard of a lot of people killing their DSOs in this fashion. -
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? -
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.
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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.
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? -
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?
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. -
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.
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?
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?