EEVblog Electronics Community Forum
Electronics => Metrology => Topic started by: 001 on December 04, 2017, 03:24:41 pm
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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? :-//
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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.
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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.
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take a look at AN-118 from Linear Technology: http://www.linear.com/designtools/app_notes.php (http://www.linear.com/designtools/app_notes.php)
Please READ IT ALL !!!