[Solved] Analog PSU rebuild: go for highest ripple current rating? - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: August 06, 2021, 05:01:44 pm
content: I just bought 50 1000uf caps off Mouser the other night and realized this morning I selected caps with just under 1A tolerance for ripple current. Argh!For an analog / non switching power supply (three individually fused AC lines from the transformer, two go into voltage regulators, one has its voltage quadrupled)Unfortunately i already removed the old caps. Should i new ones in, measure the ripple on a scope, and change the caps again if they exceed the ratings and hope they don't give the filter caps more than 1A of ripple? Or should i go back and order the 1000uf's with a much higher ripple current?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2021, 05:08:14 pm
content: for mains frequency use, some 1 A of ripple current rating is normally sufficient for a 1000 µF cap. Usually one has a ripple current about has hiogh as the DC current and 1 A more want some 2-5 mF of capacitance to kepp the ripple voltage at a reasonable level. The number depends on the voltage - lower valtage wants less ripple and this more capacitance.The ripple current may be higher with the voltage multiplier - so here one may want better caps. Still it depends on the circuit how large the ripple current really is - the specs are usually for RMS, not peak current.

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2021, 05:12:45 pm
content: If you need a detailed calculation, I recommend the freeware "PSUD 2" from Duncan Labs.https://www.duncanamps.com/psud2/It calculates actual ripple current through the filter capacitors, as well as rectifier current, DC voltages, and ripple voltages.(Note that the transformer must be characterized by open-circuit secondary voltage and equivalent series resistance. The instructions tell you how to deal with that.)

author: Mp3
author_rank: Frequent Contributor
timestamp: August 06, 2021, 05:18:43 pm
content: Quote from: Kleinstein on August 06, 2021, 05:08:14 pmAha, that is a handy rule of thumb, I will keep it in mind for future cap selection if I don't want to go calculate or simulate a circuit.The higher voltage caps used by the voltage multiplier, I bought some Vishay's that are rated something crazy like 125C and 10,000 hours, so I'm sure those will be fine.I also didn't even think to consider RMS ripple of my circuit so thank you.Quote from: TimFox on August 06, 2021, 05:12:45 pmNever heard of this but this is about to be my new favorite app!!Thank you very much for that link!

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2021, 05:31:09 pm
content: When using the software, be sure to choose the relevant parameter from the available results, e.g. rms current through the filter caps but peak-to-peak voltage at the output. Also, allow some elapsed time in the computation. The load can be a resistor or a current or a stepped current. In some circuits, the constant current load gives bad results during initial turn-on, but that can be ignored.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2021, 06:31:38 pm
content: Alternative to the special calculations /simulations, one can also do a simulation with the normal spice SW (e.g. LTspice or Tina). Just make shure to include the transformer parasitic effects like resistance at leat approximate.For a PSU / regulator design a simulation is a good idea anyway, not just for the ripple, but also the stability.

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2021, 06:52:49 pm
content: I believe PSUD2 uses Spice as the calculation engine. I find its user interface useful, and it includes lots of device models, including some vacuum tube rectifiers.It is limited to transformer-driven single-phase sinusoidal rectifiers, of the common configurations at (selectable) 50 or 60 Hz.