Hi, in a board, my around 1997, among other things I have two 1000uf 100v electrolytic capacitors. Both the dmm and a component tester tell me that the current value is around 800uf, Esr 0.39, loss 1.2. due to shape and size this capacitor is difficult for me to find. Can I just add the difference in parallel to get to 1000uf? Obviously with 100v capacitor.

 

Hi, in a board, my around 1997, among other things I have two 1000uf 100v electrolytic capacitors. Both the dmm and a component tester tell me that the current value is around 800uf, Esr 0.39, loss 1.2. due to shape and size this capacitor is difficult for me to find. Can I just add the difference in parallel to get to 1000uf? Obviously with 100v capacitor.

You can add the difference, but the result will only be temporary. The capacitors are losing capacitance because the electrolyte is slowly drying out. It will continue to do so and may get to the point where the circuit becomes damaged. I suggest that you either increase your search for replacements of similar size and shape, or get creative and come up with a way of mounting other shaped replacements.

 

You should be able to mount smaller ones with the same spec. Assuming they are through hole, this just means bending the leads to fit the larger hole spacing. I would definitely replace rather than adding.

 

You can go up in voltage if need to due to availability or price. The question is quality. What are they used for and how long do you want them to last? Do you need high quality "computer grade" or audio caps? How many hours do you want them to last? Or can you get by with cheap caps?

 

Yes I Need High quality components because this is a professional CNC control board. Maybe I will use normal quality components to repair it than if the board is working well I will replace the components with high quality components.

 

Then you should look for Rubycon capacitors and select the long hours ones. Traveling ATM or I'd give you a model number.

 

Near where I live I can get this type ( https://docs.rs-online.com/ef2a/0900766b80fe5e5e.pdf ) they are rated 3000h @105°C Cornell-Dubilier brand. What do you think about?

 

Running electrolytic capacitors at lower temperatures can greatly extend the life.

On many occasions capacitors that have not been energized during long storage times can be reformed by the applied power.
And one question: are these capacitors in a power supply filter application?? If that is the case, then a check for ripple voltage will tell you if they need replacement.

 

are these capacitors in a power supply filter application??

yes, they are in parallel after the rectifier, then I have a bank of 6 N-Chanel mosfet 33A 100V. they are driving a bldc motor.

 

If the board designer was any good, there was enough extra capability built into the original design to allow for capacitor aging, as well as for production tolerances. Not every product application is required to provide the absolute minimum quality possible that may come close to meeting the requirements. At least that was the way our company produced the ,machines we delivered.
But if the filtering is not quite adequate any more, or if the new application is mission critical, replacement as opposed to adding on would be the choice because attaching additional capacitors would probably add complexity that would reduce reliability.

 

The ones you picked are fine for rectifier filtering IF they fit the board. Double check the lead spacing and capacitors diameter for fit. I do like the "headroom" that high temperature and long hours gives. If you can check the operating specs of what's the board now against what you picked and don't go over them. Especially if they are powering a microprocessor. There should be a voltage regulator after them but doesn't hurt to be careful up front. You can go over on hours, temp rating,and voltage. Even capacitance for ripple filtering a bit. I buy long leads for through hole typically.

 

It sounds like the capacitors would see huge ripple current if the are supplying a motor through 35A MOSFETs. I would measure it and get caps rated for that ripple.