What is the accepted safe way to measure the ripple on a 460 V DC power supplie's output please, using an analogue scope?
And for future reference, on supplies that are in the 3 to 5 kV output voltage range?
Yes, the 1A5 or 7A13 would be perfect for this purpose. They offered a DC-coupled input to a differential amplifier, and would allow you to use a stable DC reference voltage for the other input of the amplifier, essentially subtracting a variable DC level from the signal. You could use this technique with any differential amplifier (even a scope in subtract mode), but you would need a clean DC voltage as reference signal. You can use attenuating probes and the scope's vertical amplifier to scale the signal, since you don't care about the exact DC level (you can measure that with a DMM).
Switching a scope to AC coupling while connected to a high DC voltage, or connecting the probe to the DC voltage with the scope set to AC coupling, will send a surge current through the cap while it is being charged. Many analog scopes allow you to pre-charge the cap if you set the input coupling to ground. A large-value resistor is then inserted in series with the AC-coupling cap to reduce this current. You first switch it to ground coupling, wait for the cap to charge, and then switch it to AC coupling.
To anyone suggesting AC-coupling with 100x/1000x probes, what does the input circuit look like in this mode? This is what a simplified schematic of an analog scope (shown in the blue box) and the 100x probe (compensation circuitry omitted) might look like. At DC, what is the voltage across R2, and what is the voltage across C1, assuming a decent quality cap? Some probes and scopes might have a resistor in parallel with the input, but many analog scopes will not, and neither do some 100x probes (for example the DX 100x probe with its 100 MOhm input impedance).
Even with the 460VDC, I'd split R2 into at least 2, ideally 3 resistors !! Even if you find one that takes the full V-Rating !!
But DON'T divide each rating by 3 (if 3 resistors), make sure if ONE fails, the others well and truly cover it.
Same goes for 5KV, AT LEAST 5-6 HV Resistors. Maybe even a conformal coating. There are also coating for Voltage isolation.
As soon as the cap has charged up after approx. 5CR,it will have the full 460V across it,which a 600V working capacitor should have no major problems with.
1000V wkg would give plenty of margin,& would not be hard for a 'scope manufacturer to provide.
Surge current?
With your equivalent circuit & a X1 probe,the maximum value of this current would be 0.46mA in the case of a 460V DC supply.
For a X10 probe,it would be 0.046mA
Analogue Oscilloscopes considerably less advanced than the 545B/1A5 or 7000 series/7A13 were used for many years in troubleshooting of tube equipment,without any problems,although "AC coupling" was the only way they could observe small AC levels superimposed upon high DC voltages.
As soon as the cap has charged up after approx. 5CR,it will have the full 460V across it,which a 600V working capacitor should have no major problems with.
1000V wkg would give plenty of margin,& would not be hard for a 'scope manufacturer to provide.Check the rating of the scope. The max DC rating on AC coupling is typically much lower than 1 kV. It's not just the cap, the switches/relays will also have to withstand the 480 V. For example, the Tek 465B is only rated for 250 VDC. Since the DC level does not get attenuated in my example, you can't just multiply this by x10 or x100 if you use an attenuating probe. Of course you can easily get a higher rated cap with an external DC blocking cap.Surge current?
With your equivalent circuit & a X1 probe,the maximum value of this current would be 0.46mA in the case of a 460V DC supply.
For a X10 probe,it would be 0.046mASure, but what's the voltage across R1 at this point?
Well,by Ohm's Law:
In the case of the X1 the voltage across R1 will be the full 460V ,
& for the X10 case it will be 46V
For a X100 probe it will be 4.6V
Fairly obviously,it wouldn't be the best idea to use a X1 probe!Analogue Oscilloscopes considerably less advanced than the 545B/1A5 or 7000 series/7A13 were used for many years in troubleshooting of tube equipment,without any problems,although "AC coupling" was the only way they could observe small AC levels superimposed upon high DC voltages.Note that the old tube scopes were considerably more tolerant to high voltages than the later solid state designs, however.
Yes, the 1A5 or 7A13 would be perfect for this purpose. They offered a DC-coupled input to a differential amplifier, and would allow you to use a stable DC reference voltage for the other input of the amplifier, essentially subtracting a variable DC level from the signal. You could use this technique with any differential amplifier (even a scope in subtract mode), but you would need a clean DC voltage as reference signal. You can use attenuating probes and the scope's vertical amplifier to scale the signal, since you don't care about the exact DC level (you can measure that with a DMM).
Switching a scope to AC coupling while connected to a high DC voltage, or connecting the probe to the DC voltage with the scope set to AC coupling, will send a surge current through the cap while it is being charged. Many analog scopes allow you to pre-charge the cap if you set the input coupling to ground. A large-value resistor is then inserted in series with the AC-coupling cap to reduce this current. You first switch it to ground coupling, wait for the cap to charge, and then switch it to AC coupling.
To anyone suggesting AC-coupling with 100x/1000x probes, what does the input circuit look like in this mode? This is what a simplified schematic of an analog scope (shown in the blue box) and the 100x probe (compensation circuitry omitted) might look like. At DC, what is the voltage across R2, and what is the voltage across C1, assuming a decent quality cap? Some probes and scopes might have a resistor in parallel with the input, but many analog scopes will not, and neither do some 100x probes (for example the DX 100x probe with its 100 MOhm input impedance).
From the above,it looks like the 7A18 would do the job OK for 460V.
On diff amps 10x probe matching is critical to maintain CMRR.
On diff amps 10x probe matching is critical to maintain CMRR.Except when you use the internal voltage source to supply DC to one if the inputs. Then there's nothing to match.
Never let respect keep you from considering that I may be wrong .
If the 7A13 is used in comparison mode (looks like the Preamble amp works the same), one of the inputs is connected to Vc, the DC comparison voltage. This means that the amp is being used in single-ended mode as far as AC is concerned, and that only one probe is being used. This means that matching between the probes is irrelevant. The only thing you might want to match is DC attenuation, but since Vc is variable, it will only affect accuracy, not CMRR (just tweak the knob until the trace is centered). I see no reason to use special probes like the Tek P6055 unless you want Vc accuracy beyond the accuracy of standard 10x probes.
This is of course completely different for differential operation, when matching of the two probes across the bandwidth is critical for good CMRR.