"High ripple" electrolytics? Suitable for general PSU usage?

[Chris Wilson]

I have a box of new electrolytics a friend donated. There are some 47uF 350 volt electrolytics that the bag states as "high ripple". Can I use these to replace some 30 year old plus generic ones on a power supply board for a transceiver? Thanks.

[SeanB]

If the originals were 10 - 68uF and the voltage is 200 - 350V and they physically will fit in the position then yes. They will be higher ripple rated and will run cooler than the originals.

Only caveat is if the supply has Selenium rectifier stacks or valve rectifiers, then you will need to place a series resistor in the lead from the rectifier to the capacitor, generally something in the order of 10-22R 5W, current depending, to reduce the ripple current through the rectifier to a lower level. These old rectifiers rely on the capacitor being a high resistance to limit the current to a safe level, so then you need extra resistance with the modern low ESR unit. Only needed on the first capacitor, as then you will find that further on there is sufficient series choke or resistance built in to the circuit.

[Chris Wilson]

Thanks SeanB. They are replacing existing 47uF ones, so value isn't changing. The rectifiers are silicon, so I should be good to go with these then. Good reply, learnt something else again from this, thanks again.

[FenderBender]

I think High Ripple typically goes hand in hand with Low ESR. So because the capacitor resists less current (Low ESR), then it coincidently produces less heat and thus it has a higher Ripple Current rating.

Usually getting these isn't an issue unless it is being used in a circuit where impedance is absolutely critical.

[SeanB]

Old electrolytic's and high voltage are tricky.  Only good thing is that the tolerances are so big that you can go a value or three up or down without problems. The ones you have are probably meant for SMPS input capacitors, and if they are good units you will have 10 years plus of life from them if they are kept cool. At 60c they probably will make 2 years.

[rbola35618]

Generally, in switching power supplies the ESR sets the ripple output voltage of power supplys. so if you what low Vripple, you want to us low ESR capacitors. If the ripple current is too high for the capacitor, then you add more capacitor in parrallel to divide the ripple current between all the capacitors. Hope this helps. Best Regards Robert Bolanos