Measuring Ripple on 800 vdc Swinging Choke Linear supply. - Page 1
Forum section: electronics / beginners

author: K1JOS
author_rank: Regular Contributor
timestamp: February 28, 2015, 05:59:21 pm
content: For my ham radio projects, I recently recapped a 800vdc 350mA linear supply. This uses a swinging choke-capacitor filter circuit. I would be interested in seeing what the remaining ripple voltage is but for the very infrequent measurement I can't bring myself to invest in an expensive high voltage differential probe for my scope. I guess if I had a spare 800v secondary transformer to use as a reverse step-down I could feed the supply 800vdc output into the spare xfrm secondary and look at the waveform on the step-down and do some calculations. Are there any other methods that can be used safely?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2015, 06:05:34 pm
content: Choose a coupling capacitor large enough for the bandwidth required, precharge it, and measure across that?To go down to 60Hz within 10% of amplitude, for a 10x (10M) probe, you need more than 2.6nF. Probably 10nF would be fine.Connect capacitor from B+ to 1M resistor to GND.Turn on power supply.Attach probe across 1M resistor.Disconnect resistor (or, use a large enough capacitor that the 1M also doesn't matter), take measurement.Disconnect probe and reconnect resistor.Turn off power supply.Notice the sequence of events is important. If you added a TVS across the 1M resistor, you could protect your probe against more careless handling, but if you don't have 200V zeners on hand...Tim

author: K1JOS
author_rank: Regular Contributor
timestamp: March 01, 2015, 04:43:06 am
content: Why do i need to do the disconnects and measure the stored voltage on the cap? Couldn't I just measure the AC ripple voltage across the resistor? I put this into a SPICE software and to get the 60 Hz across the cap I would need a fairly large cap more like 5F. I know I must be missing something very basic here ... please help

author: vk6zgo
author_rank: Super Contributor
timestamp: March 01, 2015, 08:47:10 am
content: The old school way is to make up a voltage divider,so you can look at a voltagewithinthe rating of your Oscilloscope.This has to be appropriately insulated & use resistors of adequate voltage & wattage ratings.Hang the 'scope probe across the low value resistor at the very bottom of the voltage dividerObviously,the overall resistance must be high enough to not draw too much current from the supply,& the bottom resistor must be low in value compared to the input impedance of the x10 probe.The resistance values don't have to be incredibly accurate,as you can set your 'scope vertical gain to uncalibrated,adjust the display for an appropriate level in the DC setting, ( whatever voltage this is,call it 800v) then reduce the v/div setting.The ripple at this voltage will be in the same ratio to the DC reference as the ripple at the full voltage is to 800v DC.With some 'scopes,this can all be done on the DC setting,as they have enough "offset"that you can look at the ripple just by reducing v/div .With others,you will have to change the setting to AC.You could probably use a true RMS DMM across the bottom resistor,too.