Author Topic: How to measure HV DC power supply ripple voltage?  (Read 1249 times)

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001

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How to measure HV DC power supply ripple voltage?
« on: December 04, 2017, 03:24:41 pm »
Power Supply rated as 480volts 180mA DC
Ripple voltage (saw waveform) range about tens millivolts
How to measure ripple voltage and not destroy any gear?
« Last Edit: December 04, 2017, 03:48:54 pm by 001 »

e61_phil

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Re: How to measure HV power supply ripple voltage?
« Reply #1 on: December 04, 2017, 03:32:03 pm »
With AC-coupling.

Be careful, most scopes have a DC high-impedance if you switch them to AC-coupling. In this case, the scope will see the full DC voltage even if you use a 1:100 probe.

I've build an AC-coupler to measure noise on voltages of up to 10kV. The design is very simple: In front is a coupling capacitor followed by a 9Meg resistor. A variable capacitor is used to trimm the capacities. This gives a flat frequency response from below 1Hz to 10MHz, which is way more than I needed.
« Last Edit: December 04, 2017, 07:10:26 pm by e61_phil »

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David Hess

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Re: How to measure HV power supply ripple voltage?
« Reply #2 on: December 04, 2017, 07:07:26 pm »
Be careful, most scopes has a DC high-impedance if you switch them to AC-coupling. In this case, the scope will see the full DC voltage even if you use a 1:100 probe.

They are rare now but good x100 probes include an internal shunt preventing this problem.  They can be identified by having a specified input resistance which is less than 100 megohms.  A good example is the old Tektronix P6009 x100 probe which has a 10 megohm input resistance.

But a standard dividing probe is the wrong way to make this measurement.  Do what e61_phil suggests and make an external high voltage DC block with the needed frequency response.
« Last Edit: December 05, 2017, 01:54:59 am by David Hess »

eliocor

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Re: How to measure HV DC power supply ripple voltage?
« Reply #3 on: December 05, 2017, 12:07:14 am »
take a look at AN-118 from Linear Technology: http://www.linear.com/designtools/app_notes.php