Power supply filter capacitor sanity checks/advice - Page 1
Forum section: electronics / beginners

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 16, 2021, 05:03:45 am
content: So I have a DP30V5A-L power supply module in the mail and need something to power it. I know general advice is to buy a supply or use an old laptop brick, which I have, but also in the junk pile is an old satellite dish steering box (H bridge motor driver and some smarts) or whatever they're called that yielded a ~100VA (based on the 100W input rating) power transformer with 12 and 24V windings, push button 3.15A circuit breaker on the 24V winding (which drove the motor), beefy bridge rectifier, and a case, so I'm going with that. The idea being to have a simple transformer, rectifier and cap supply feeding the module, maybe add a second supply later running from the 12V winding.I've arrived at 10-15kuF being good enough for the cap, a couple volts of ripple at full load (where it will likely never be, 75 watts is a ton for what I mostly mess with), but how high of a ripple rating will I need for the cap to survive that? And am I going to have to do anything about inrush with a cap that big?http://www.skillbank.co.uk/psu/ripple.htmclaims double the load current is a safe assumption, so ~6 amps to be safe, right? Something likehttps://www.mouser.com/ProductDetail/EPCOS-TDK/B41231B6129M000/?qs=sGAEpiMZZMvwFf0viD3Y3YZmIOvBM3ji5hKpiSwBQWM%3Dorhttps://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/SLPX153M050H4P3/?qs=sGAEpiMZZMvwFf0viD3Y3ROGmxq83YAGxJncOUCHPyY%3Dshould do? Again, it won't normally be seeing this much load (I know running a cap right at it's ratings is not great for lifespan), just want the ability without smoking something. Am I missing some better way of doing this that doesn't involve a Pi filter with a giant choke or power hungry resistor?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 17, 2021, 09:48:12 am
content: If you use a torroidal transformer rather than a standard type, the inrush will be significantly higher.A current limiter is a simple devcie that will take the thump out and redcuce blown fuses.A pair (or more) of smaller caps in paralell will boost the ripple current handling overall. But this may increase the inrush.Withour knowing the maximum ripple you can allow and the motor rating and typehttps://product.tdk.com/info/en/products/protection/current/ntc-limiter/index.htmlits impossible to make a sensible calculation.seehttps://www.mouser.co.uk/Circuit-Protection/Thermistors/Inrush-Current-Limiters/_/N-axfxjhttps://product.tdk.com/info/en/products/protection/current/ntc-limiter/index.htmlOh and put some bleeder reistors on your caps!

author: wizard69
author_rank: Super Contributor
timestamp: February 17, 2021, 11:43:36 am
content: If you are going to build something why not make sure it has an acceptable ripple voltage over its entire operating range. You say you may not use the entire range of the supply today but you really can't say that will be the case tomorrow or next year. So I'd refactor the cap size to operate properly over a reasonable voltage range.

author: srb1954
author_rank: Super Contributor
timestamp: February 17, 2021, 12:02:23 pm
content: Quote from: BrokenYugo on February 16, 2021, 05:03:45 amIf the 24V winding had a 3.15A circuit breaker on the winding then the current is probably rated at no more than 2.5A continuous. Such a winding would really only be good for around 40W DC output.A general rule of thumb is that the transformer VA rating should be 1.5x to 1.8x the required DC output in Watts. If you are looking for 75W DC output then you really want a transformer rated in the 110-135VA range. To get a more accurate figure you need to do a more detailed analysis or circuit simulation taking into account all factors such the transformer voltage regulation, rectifier voltage drops and smoothing capacitor parameters.