Filter AC ripple - Page 1
Forum section: electronics / beginners

author: vol.2
author_rank: Contributor
timestamp: February 01, 2021, 03:37:07 pm
content: HiI'm trying to figure out how to get rid of an AC ripple in a derived 178VDC supply.The gist of the circuit is that it starts off as a 63us AC pulse, and it goes to a rectifier diode followed by a cap and an inductor.What I'm seeing on the scope is that there is still a significant AC ripple getting through the components. I would estimate that the AC ripple is something on the order of 5.7us (perhaps harmonics), with slight spikes at 63us where the original AC pulse was.The 178VDC looks great when viewed on a scope or measure with a multimeter.I'm assuming an RC filter is out of the question (and not already in the circuit) because it would lower the voltage.Does anyone have an idea to remove this ripple without changing the voltage?Thanks

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2021, 07:19:05 pm
content: TV? 63 us is the line frequency, 5.7 us the HSYNC. I wouldn't expect that a bit of ripple causes problems.Think about the ground lead from your 'scope probe. Bad placement can give you exaggerated results on the screen.

author: bob91343
author_rank: Super Contributor
timestamp: February 01, 2021, 07:26:23 pm
content: Measuring ripple is not as simple as you think. I concur that the ground lead is important but there is more to it than that.

author: vol.2
author_rank: Contributor
timestamp: February 01, 2021, 08:45:40 pm
content: Thanks for your replies.Does anyone have any suggestions about my question? I am looking for a way to filter ripple from this power source. The details are all in the first post.Quote from: Benta on February 01, 2021, 07:19:05 pmYes.QuoteI'm tracking down an elusive and stubborn issue causing vertical bars in scan time. So it's less a matter of itbeingand issue, and more of a matter of eliminating it as a possible source of the issue. I see the brightness knob having a direct effect on the presence of the ripple (which is close to 1V pkpk). I highly suspect the bars are getting into scan time through the guns via the 178VDC. I'm looking for a way to confirm or deny it. Some kind of filter that mitigates the ~800mV ripple on the 178VDC would just about do it.QuoteThanks, yeah, I thought about that. I have a good ground on the set. I can see the ripple here, and it doesn't appear everywhere else.Quote from: bob91343 on February 01, 2021, 07:26:23 pmWell, I'm doing the best I can with it. As far as I can tell, I'm getting fairly clean results on my scope. I don't just get this same ripple current in all locations, it's specific to this test point.

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2021, 09:29:48 pm
content: 1 Vpp on a 178 V rail is next to nothing and certainly not your problem here.

author: vol.2
author_rank: Contributor
timestamp: February 01, 2021, 09:41:36 pm
content: Thanks for all the fascinating responses!I'm looking for a way to filter AC ripple on the 178VDC source that I'm seeing in order to rule out an issue I see with a device. If there is anyone reading this that might have a suggestion on how to do that I would be grateful!Thanks!Quote from: Benta on February 01, 2021, 09:29:48 pmThanks for the insight. I'll keep that in mind. If you had a suggestion about how I might go about building a basic filter for troubleshooting purposes, I would be grateful.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2021, 09:59:27 pm
content: If you really want to reduce it for test purposes, you could use 3 100uF or so capacitors, one from point 39 to ground, one from 6 to ground and one from S to ground, then break the link between 39 and 6 and put in a resistor. I assume the current is quite low, so use a resistor that only drops as much voltage as you're sure won't be missed, so for 10mA, 100R for 1 volt drop. This should drastically reduce the ripple that you measure. As Benta said, this is likely not your issue, but it will allow you to rule it out, as you wished. Keep in mind that ripple can be a function of load as well as input, what I've described will address both issues.

author: vol.2
author_rank: Contributor
timestamp: February 01, 2021, 10:07:38 pm
content: Quote from: bdunham7 on February 01, 2021, 09:59:27 pmThanks, I'll give that a shot.I'm to the point where I'm trying everything, as I've looked very hard for other more obvious issues.The main thing that drew me to this is that the ripple is effected directly by the G2 voltage. In fact, the ripple gets *much* larger when I put a raster on the set. It goes from about 1Vpkpk idle to 4V Pkpk or higher. (like 6V if I crank the raster up)It might not seem like much, but it's the closest thing I have to a smoking gun after many hours of troubleshooting, so I'm going to go ahead and rule it out no matter how much Bunta doubts me.No offense intended. I'm sure you are both more knowledgeable than I on these matters (which is why I came here in the first place), but I'm sure you can also relate to being stuck on something and wanting to cross things off a list.Whatever the issue is, I'm fairly convinced that it's a design flaw and that the cause will not be obvious and not seem likely.