Author Topic: Safe way to scope the ripple on a 460 V DC power supply?  (Read 11425 times)

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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 #25 on: July 02, 2013, 01:44:20 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 02, 2013, 02:14:43 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 #26 on: July 02, 2013, 02:41:15 pm »
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 02, 2013, 03:52:32 pm 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 02, 2013, 02:48:29 pm »
large valued, (1-100F) high-voltage electrolytic capacitor

I hope that was a typo.
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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 02, 2013, 04:14:36 pm »
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 02, 2013, 08:47:05 pm »
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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