Author Topic: Safe way to scope the ripple on a 460 V DC power supply?  (Read 9993 times)

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Offline Chris Wilson

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Safe way to scope the ripple on a 460 V DC power supply?
« on: June 30, 2013, 02:20:45 am »
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?
Best regards,

                 Chris Wilson.
 

Offline G7PSK

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #1 on: June 30, 2013, 03:13:43 am »
I just use a 100 X probe, not sure if this is the correct way but I have been using one on 415 volt 3 phase supplies for years and neither I or the scope has come to harm. There are probes made for working in the KV range.
 

alm

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #2 on: June 30, 2013, 04:28:02 am »
Note that the scope will have a limit on the allowed DC voltage in AC coupled mode, regardless of probe attenuation. The DC impedance of the coupling cap is almost infinite, so the 9/99 MOhm series resistance will not provide much attenuation.
 

Offline ddavidebor

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Safe way to scope the ripple on a 460 V DC power supply?
« Reply #3 on: June 30, 2013, 04:32:34 am »
For the 460v you should use two probe for do a differential measurement.

Just push "chan. B invert" "dual channel" "a+b" on the scope and DON'T use the ground clip.
Davide Bortolami,
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Offline Paul Price

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #4 on: June 30, 2013, 10:24:32 am »
G74FK, 100x and 1000x probes are quite expensive and may not be so often used to justify their expense, over several hundred US.

DDavidebor, your method will likely damage the scope, possibly expose you to shock, and would also be very inaccurate due to grounding issues of your scope and the D.U.T.
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You can make fairly accurate low freq. ripple measurements by simply attaching a large valued, (1-10uF) high-voltage, low leakage capacitor to the voltages of interest to a 1 meg resistor to ground, where upon you attach the x10 scope probe, but you must be careful because you could damage your scope or your your probes if they have x10/x1 switches or you encounter instantaneous large changes in either HV supply voltage. For the 5KV test,  you might find having a high cap value >5KV capacitor around might not be so likely.

In either case you would have to make first some AC low-voltage measurement first to calibrate your readings.

Or you can make a test fixture DYI x10 and x1000 probe divider.

The good thing about making a DYI probe divider is that once you've made it and calibrated it, you can just take it out of your storage drawer and use it again to quickly make a HV measurement.

For the 400V or 5KV measurements, just use off the shelf common resistor values to make a sloppy ratio voltage divider:

For the 460V case, I'd make a simple 10/1 voltage divider using a 10 Meg 1W resistor in series with a 1Meg resistor and connect the top of the divider to the 400v point bottom of the 1M to ground. With a 460V supply this gives a peak amplitude of around 40-45V. You must use a 10x probe. You set the scope to 10V /div. You will see the ripple "riding" on the DC waveform and you do the math, you multiply the readings x by a calibration factor and you can easily see the ripple if it is large enough. If the ripple is small in amplitude, switch the input to AC coupling and you will be able to change the scope sensitivity to see the ripple.   This resistor only divider setup will work fine with low-freq 50-1000 Hz ripple. You can calibrate the voltage divider by making a known low voltage DC measurement and if you then learn the error and you can accurately apply this correction factor to your KV HV scope reading.

The 1.0 meg resistor is in parallel with the input resistance of your oscilloscope probe so the combination of these two resistances creates in effect an approx 11/1 divider. Once your know the exact attenuation factor with a low-voltage DC power supply or a square wave, you can apply this correction factor to the actual HV reading to get an accurate result. The purpose of the 1meg resistor is there to protect you and the scope under all test conditions from having more than about 45V applied to the probe tip input.

For higher frequency ripple meas. you will need to parallel a high-voltage 1p to 10pf 1000V variable capacitor across the 10meg resistor to compensate the approx x100 divider.  You always adj the compensation of the divider with a low-voltage squarewave for safety. You can make a makeshift 1-10 pf variable capacitor by fairly tightly twisting a pair of 1000v insulated solid or stranded wires together with a length of 3-in with one wire to each side of the 10meg. To calibrate the probe, connect this voltage divider to a low-voltage squarewave and carefully trim the length of the wire until you see a flat leading edge risetime with no overshoot. Then carefully separate the clipped wire pair ends a little to prevent arc-over when using the divider for measurement in the HV circuit.

Like the x100 probe, a x1000 probe is even more expensive.

When playing with high voltages, even at low currents and HV, you must take special care to attach the voltage divider securely while the HV supply is off and the HV discharged and always keep your hands away from the measuring area while the supply is powered up and you are making measurements.

I would make for a 4000V measurement  an approx 1000/1 divider using the same idea. In this case it would be 40Meg/39k divider. For ripple measurements where you just need a low-frequency ripple reading just use four 10meg/1W resistors in series with a 39k 1/2W to make a voltage divider for the test point for the 10x scope probe at the top of the 39k.  The 10meg input resistance of the scope x10 probe is in parallel with the 39K of the divider,  and a correction factor is necessary, but you have the convenience of  using just off the shelf easy to find values of resistors to make a quick, if somewhat odd division ratio voltage divider. In this case, the 5KV would not present more than 50V to your scope under any conditions.

If you are interested in accurate measurement of  high frequency (KHz to low MHz) ripple, you will need to bypass each 10meg with a twisted pair(1000V insulated/twisted wire) compensation handmade capacitor, and you will have to trim all three twisted pairs to match, so a little practice and some redoing may be necessary to get it right for a precise divider.

You can calibrate the x100 or x1000 voltage divider at DC by making a known low voltage DC measurement with the divider or use a known amplitude squarewave and you also can use the squarewave to adj the divider freq compensation. You can then accurately apply a correction factor to your KV HV scope reading. For instance, if the you make the 10/1 divider for the 460V case and find you need to multiply the reading by 1.28 to get it to measure the correct value with the low frequency (1KHz) squarewave or DC power supply, then you always multiply the reading on the scope by 1.28 to get the actual voltage. First make sure your probe is properly compensated before compensating the divider.

In either case, make sure the top of the voltage divider securely attached with an alligator clip or by tacking a solder connection, and also the low end the resistor divider securely mechanically and electrically with a clip to the circuit ground, along with your scope ground and always do this with the HV power supply off and after the power supply has discharged its output to zero.

Always be very sure your hands are away from the circuit when the HV power supply is on, and always make sure the bottom side of the low end divider resistor is securely grounded, along with the ground of your scope before applying the tip of a 10x probe to the top of the bottom resistor of the divider and powering up the supply to make a measurement.


You might ask, why a 1000/1 divider for the 5KV.  Because the HV supply you are measuring may only have a very low current output and and smaller resistance values may overload your HV power supply or your scope.
« Last Edit: June 30, 2013, 11:05:19 am by Paul Price »
 

Offline G7PSK

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #5 on: June 30, 2013, 07:41:33 pm »
I would not call £25-00 or so expensive, there are plenty of HV probes in this price range on e bay while they may no meet the frequency range claimed they will reach the voltages required by the OP.

http://www.ebay.co.uk/itm/T3020-2000V-2KV-100X-DC-20MHz-High-Voltage-Oscilloscope-Probe-/231005630475?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item35c902300b

http://www.ebay.co.uk/itm/New-100MHz-2000V-High-Voltage-Oscilloscope-Probes-P4100-LI-/261073363108?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3cc92f60a4

The 100 times probes that I have (2 off them) I got on ebay, they are both RS brand new old stock and I pid under £15-00 each.
« Last Edit: June 30, 2013, 07:47:38 pm by G7PSK »
 

Offline vk6zgo

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #6 on: June 30, 2013, 07:48:04 pm »
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?

Let's put the 5kV case to one side for a while.

Back in the day,before such a specification evinced squeals of terror as seems to be the reaction today,460V DC was just a little higher than the common HT values used on much valve (tube) equipment,& not at all uncommon.

An Oscilloscope which could not,in conjunction with a x10 probe, look at the anode of an Output stage in a high power amplifier would have been regarded as a pretty poor instrument.

That said,there are Analogue 'scopes & analogue 'scopes.

The old 1A5 plugin used in the Tek 545B etc,had a very good voltage offset control,so you could "null out" the DC component & look at the AC ripple at higher sensitivity---from memory,it could have done your job.

One of the plugins in my 7613  has a similar function,but I don't think its range is as good.

If you 'scope doesn't have this facility,you are stuck with "AC coupling".
Most of the time this works OK,but in some cases,there is still too much offset at very low volts/div settings.

In any case,check the specs of your 'scope,& especially your probe.
Tektronix ,HP,or Philips probes  normally have quite high ratings,but "El Cheapos" are not as trustworthy.

There is no reason whatsoever to hold the probe when you are testing-------that's what the clip is for!
Connect up the probe,switch your 'scope to a high volts/cm setting,turn on the supply.

If your probe/'scope doesn't show any sign of distress wind up the sensitivity,on AC coupling,( null out the DC  first if you have that facility & are on DC coupling)

Read the hum level.

All the above is presupposing that you are looking at the 460V at the output of a transformer type supply,or at the isolated output of a SMPS.
The high DC voltage present after the bridge rectifier in the non-isolated side of a SMPS is a different kettle of fish,as there is the complication of the direct connection of one side to (hopefully) the Mains Neutral.

For the 5kV case,the high value resistor voltage divider idea is the best--just remember that your 'scope & x10 probe look like 10MOhm in parallel with the bottom resistor.
In any case,you are usually interested in hum percentage,so a bit of absolute inaccuracy won't matter.
 

Offline Paul Price

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #7 on: June 30, 2013, 09:07:51 pm »
If you need to measure a HV power supply ripple, why would you want to wait for days or weeks to complete an auction on EBay, then wait for the shipping to it to you, when you can take a few resistors out of your drawers and get the job done.

That other thing, the cost, not everyone has that kind of money to throw around, plus pay for shipping and insurance and then wait for Customs and Home Land Security to allow it to be shipped to you, etc.

Plus your wife will likely suspect you've bought an intimate interest toy.
« Last Edit: June 30, 2013, 09:20:07 pm by Paul Price »
 

alm

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #8 on: July 01, 2013, 03:26:47 am »
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).

« Last Edit: July 01, 2013, 10:18:26 am by alm »
 

Offline vk6zgo

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #9 on: July 01, 2013, 01:40:09 pm »
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).



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

As such capacitors were widely used as screen bypasses in tube circuitry,with considerably lower values of series resistance,I can't see  any  great problem.

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: All of the above is only applicable to the 460V DC supply---the 5kV supply should use the external voltage divider suggested previously.
 

Offline digsys

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #10 on: July 01, 2013, 02:12:24 pm »
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.
Hello <tap> <tap> .. is this thing on?
 

Offline ftransform

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #11 on: July 01, 2013, 03:12:38 pm »
So did anyone here suggest the dave jones video yet? http://www.eevblog.com/2010/05/08/eevblog-85-high-voltage-oscilloscope-probe-design/

courtesy of doug ford.
 

Offline vk6zgo

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #12 on: July 01, 2013, 04:01:52 pm »
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.

R2 in alm's equivalent circuit is the series resistance of the X100 probe.

Radio Transmitters commonly used series strings of very high voltage rating resistors to give a reading of HT at around 10kV or more,built on large  specially made tagblocks,using red fibre or fibreglass  insulation around 12mm. thick.

Others still were even more over-engineered & used large porcelain standoffs.
These things lasted for years---no conformal coatings needed ! ;D
« Last Edit: July 01, 2013, 04:10:43 pm by vk6zgo »
 

Offline digsys

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #13 on: July 01, 2013, 04:28:52 pm »
The other problem with a resistive divider is that you ALSO divide the ripple that you're looking for. So a 1000:1 pretty much wipes
out it out. What we really need is a HV DC blocker / AC high pass probe.
Hello <tap> <tap> .. is this thing on?
 

Offline Paul Price

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #14 on: July 01, 2013, 10:22:57 pm »
Digsys says,"
The other problem with a resistive divider is that you ALSO divide the ripple that you're looking for. So a 1000:1 pretty much wipes
out it out. What we really need is a HV DC blocker / AC high pass probe."
------------------
It doesn't matter so much that the divider attenuates the ripple, that why we have oscilloscopes that can be set to 500uV/cm so with a 10x probe that's 5mv/cm. Is the OP really interested in uV of ripple?

Digsys says, "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."


I suggested four resistors, does anyone else want to improve an easy solution to the OP's request that is that maybe the goal of making just one measurement?

Does anyone think that everyone has a Tek Mainframe scope and a 1A5 plug in just for this measurement?

How about:

Give your butler $1000 US to run off to the city to buy a 7100 series oscilloscope and a 1A5 plugin to match just to make this measurement.

Gold plating the alligator clips to ensure an accurate reading and a good connection.

After giving each of the 10 series resistors a conformal coating, wrap them in HV shrink tubing and then dip the whole assembly in epoxy for good measure.

Have your resistor divider sent out to a certified calibration lab certified traceable to the National Bureau of Standards, along with you oscilloscope before making any measurements.

Hire two recently laid-off NASA Space Shuttle Quality Assurance Specialists to witness the test to sign it off as acceptable.

« Last Edit: July 01, 2013, 10:29:25 pm by Paul Price »
 

Offline robrenz

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #15 on: July 01, 2013, 11:02:38 pm »
Here is exactly what you want, basically made to do just what you want.

This is a preamble 1855 from before Lecroy took over so it fortunately does not have the added Lecroy interface stuff. It limits its output to the scope to +/-500mV so the scope is never overdriven. It has 1x and 10X attenuation and  1x and 10X gain setting and if not terminated into 50 Ohms you gain an additional 2X of gain. It also has a built in offset voltage supply of +/- 15.9999V with digital readout and setting. Adding the DCA100X probes for the 1855 differential preamp  Gives an additional 10x and 100x attenuation and matching to keep the CMRR high on top of the 10x attenuation available on the 1855. At 10x it boosts the common mode voltage to 1550V but is actually limited by the probes max voltage rating.

Amp manual

 10X-100X probe manual
« Last Edit: July 17, 2014, 04:01:33 am by robrenz »
 

Offline Chris Wilson

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #16 on: July 01, 2013, 11:40:33 pm »
Oh, dear, I seem to have opened a can of worms here, sorry! I thought it would elicit a simple answer or two, I never considered there'd be so much diversity of opinion. I have a Marconi encapsulated "brick" that was used in military high power valved transmitters for reading the anode voltage on a normal panel meter. It has an internal high voltage resistir network and is designed for 5 kV input. I could use that for looking at ripple on an HV supply I guess?

Back to the 460 volt supply, my main scope is an old Tek 7854 but I don't have a vast range of plug ins for it, certainly not a 7A13  In fact none go higher than 5V/ division on the amps.  I do have X10 probes. I'll experiment, playing safe with an isolation transformer and a variac, and see how things go. As people have said, 350 / 450 volts was commonplace on valve gear of not so long ago, and certainly during the manufacturing period of this scope, so I just need to think carefully what I am doing and act sensibly.  5kV of course is a different kettle of fish and any measurements will be done remotely with probes pre connected and just the PSU on off switch under my control and contact. Thanks everyone.
Best regards,

                 Chris Wilson.
 

alm

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #17 on: July 01, 2013, 11:52:17 pm »
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 0.046mA
Sure, but what's the voltage across R1 at this point?

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.
 

Offline vk6zgo

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #18 on: July 02, 2013, 06:58:09 pm »
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 0.046mA
Sure, 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! :D

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.

Out of interest,I had a look at the specs  of the two Tek plugins I have,plus my BWD 511.

7A12 (solid state input)

Maximum Input voltage
DC direct coupled
5mV/div &10 mV/div----------------------350V DC or DC+Peak AC at 1kHz or less
20mV/div to 5V/div -----------------------500V DC or DC+Peak AC at 1kHz or less
 -----------------------------------------------------------------------------------------------------------------------
AC ( Capacitive) coupled input
5mV/div &10 mV/div----------------------350V DC or DC+Peak AC at 1kHz or less
20mV/div to 5V/div -----------------------500V DC or DC+Peak AC


 -----------------------------------------------------------------------------------------------------------------------
7A18 (solid state input)

DC Coupled---------------------------------250volts.(DC+Peak AC);AC component 500volts peak to peak
                                                                                                                   maximum,1kHz or less

AC Coupled---------------------------------500volts.(DC+Peak AC);AC component 500volts peak to peak
                                                                                                                   maximum,1kHz or less

------------------------------------------------------------------------------------------------------------------------

BWD 511 (tube input circuit)
Max Input Voltage----------------------400V DC or 250V AC or 400V AC & DC p-p combined.
Common Terminal to ground--------+or-400v DC or 250V AC.
(The BWD is a bit on “the weird side of the Force” as its default is with its input floating)

From the above,it looks like the 7A18 would do the job OK for 460V.
 

Offline robrenz

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #19 on: July 02, 2013, 09:19:38 pm »
7A13 has the same limits but those are values with 10X probes.  On diff amps 10x probe matching is critical to maintain CMRR. That is why the 10X probes for the preamble amp has a tuning box with 16 pots/caps for adjusting/matching response at all frequencies. If you need any accuracy beware ;D

Offline Chris Wilson

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #20 on: July 02, 2013, 09:32:13 pm »
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).



I mentioned differential amps to an old pal of mine and he dropped off a gift today. A mint 7A13 for me! I need to read up the way to use this, but looks like I have the means for a proper check now! Thanks.
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alm

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #21 on: July 02, 2013, 09:42:30 pm »
The advantage of a diff amp is that you don't need AC coupling, so you can actually use an attenuating probe to increase the max voltage. This may not work for a normal scope with AC coupling.

From the above,it looks like the 7A18 would do the job OK for 460V.
It sounds like some scopes will barely able to do the 460 VDC measurements, and some won't (eg. the 465B I mentioned). The 7000 plugins should barely be able to do the measurements, although the 20 mV/div minimum of the 7A12 could be limiting, especially in combination with 10x/100x probes. Most 1x probes are not rated for 500 V and may damage the scope unless you pre-charge the AC coupling cap. I think many 10x probes may only go up to 350 VDC or so.

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.
 

Offline robrenz

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #22 on: July 02, 2013, 10:39:57 pm »
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.

You have my highest respect, so I question this knowing you are going to straighten me out.  Since the internal voltage offset in the amp is after attenuation (probe and amp combined) and the signal content of interest is still AC (ripple/noise) of probably broad frequency spectrum.  How is the attenuation/frequency response matching of the 10X probes not an issue just because we add a DC offset before it goes to the scopes screen vertical amp?

alm

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #23 on: July 02, 2013, 11:00:18 pm »
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.
 

Offline Chris Wilson

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #24 on: July 02, 2013, 11:08:49 pm »
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.

Can I read this as the 7A13 and a bog standard, El Cheapo 10X probe should be all I need to look at AC ripple on 460V DC then? Just want to be clear as to the hardware needs before getting too excited ;) Thanks!
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                 Chris Wilson.
 

Offline Paul Price

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #25 on: July 02, 2013, 11:44:20 pm »
Why make things complicated?

You can make very accurate ripple measurements by simply attaching a large valued, (1-100F) high-voltage electrolytic capacitor to the 460 voltage supply of interest to a 1 meg resistor to ground. Insert a 1K ohm in series with the 1meg  and capacitor common meas.  point. At the other side of the 1K resistor just connected, connect two 12V zener diodes in series to ground. Connect the zeners back to back, (anode to anode), one side to ground the other to the test point.

Note: if you use a very large valued electrolytic cap, it may take some seconds for this coupling capacitor to charge up through the zener protection network before you will see a clear oscilloscope reading.

These two zeners and the 1K connected together will protect your scope from any possible overload, so then just attach the x10 scope probe to junction of the 1k and the two zeners and at this test point you can safely measure the exact 460V supply ripple on your scope without any chance of damaging your scope. If the observed ripple waveform is clipped, you might know at that point your power supply needs new filter caps, otherwise add another pair of 12V zeners back to back in series with the two already to give you a +-24V Peak to Peak  dynamic range of absolutely safe measurement
« Last Edit: July 03, 2013, 12:14:43 am by Paul Price »
 

Offline robrenz

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #26 on: July 03, 2013, 12:41:15 am »
Never let respect keep you from considering that I may be wrong :).

Thanks alm,  After a reread of the comparison mode of the preamble manual I understand the difference now.  I was always using these in differential mode and never bothered to understand the comparison mode. 

Edit: You are losing isolation with the single probe but I dont think it matters for the OP's original need.

But the single ended probing will be subject to ambient noise that the differential mode would minimize IF the CMRR was good with the 10X probes.  If the OP's min. resolution needs are above the single ended noise it won't be an issue.
« Last Edit: July 03, 2013, 01:52:32 am by robrenz »
 

Offline c4757p

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #27 on: July 03, 2013, 12:48:29 am »
large valued, (1-100F) high-voltage electrolytic capacitor

I hope that was a typo.
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline Chris Wilson

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #28 on: July 03, 2013, 02:14:36 am »
large valued, (1-100F) high-voltage electrolytic capacitor

I hope that was a typo.


Gee, don't say that, I have it nearly half way in the shack now, and it's been a struggle with my bad back.... ;)
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Offline Paul Price

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Re: Safe way to scope the ripple on a 460 V DC power supply?
« Reply #29 on: July 03, 2013, 06:47:05 am »
Not a type for a modern 450V electrolytic capacitor, Nichicon has some pencil-thin round electrolytic capacitors >150 uF  and not taller than your little finger., for the ones rated at 5000V you may need to rent a tractor.
 


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