capacitors in series in PSU

[Wagener]

I need 75V 10,000uF capacitors for a four channel leach amplifier I am building. I will have two 750va toroids. Voltage rails will be + and - 58V dc

I cant seem to find 75V caps. I did find 10,000uF 50V caps at a very cheap price thought. I am thinking - wiring two caps in series for 100V.

Something just doesn't feel right about it. Please comment on this!

[Rhesus]

If you put two 10000uF caps in series, the total cap value will be 5000uF. Place two of these pairs in paralell, and you end up with a total of 10000uF. You will need 4 10000uF/50V to make 1 10000uF/100V.

Make sure that you put bleeder resistors in paralell with each cap, say 1-10k or something. You figure out how much power you will be wasting.
That way, you make sure that the DC-voltage will be the same over each cap.

[Eva]

You may connect them in series but there are two points to take into account :

- Resulting capacitance of 10mF + 10mF in series will be 5mF and, for example, eight 10mF 50V units would be required to get 20mF 100V [and another eight units for the other rail]

- Each capacitor has his own leakage current and you would need to place bleeder resistors of equal value [low enough] in paralell with each half of each bank to ensure DC balance. This is a common thechnique employed in power supplies

[dmfraser]

I use 3K 3W resistors for a design where I needed 12000 uFd at 120V. I use 4 12000 uFd 63V caps in series parallel. However, if you submit the amplifier for regulatory approval, they will short one capacitor. The other one in series will vent and they will fail the unit.

You can only series caps if the unit will not be sent in for regulatory approval.

[Eva]

You may add a protection circuit, disconnecting power supply in case of overvoltage on any capacitor bank, to pass such nonsense tests [some people in this world seem to have nothing better to do than shorting 12mF charged to 120V, ie : 86 Joules and they even charge thousands of $$$ for doing so ]

[dmfraser]

Those people who do these nonsense tests have the organizational names of CSA and UL and unfortunately, as the result of such a nonsense test, can deny you the ability to sell your product. I have had since then to redesign a perfectly good power supply to use 4 x 2700 uFd 160V capacitors in ra egular parallel arrangement. However the 160V parts have much longer lead times but that's not their problem. The extra circuitry to cut off the power in case of an really serious unbalance was an alternative but they consider that a crutch and if you use it, they will find a different reason to fail you.

[Eva]

Most PC AT and ATX power supplies use series capacitors for 120-230V mains rectification. I have half a dozen of units laying out here and at least three of them have CSA, DVE, UR, etc.. seals

I wonder how could these seals got there?

[dmfraser]

Exactly. I wonder about that too but CSA rejected me for having series capacitors and I had to change the design to be accepted. Maybe because my unit was over a certain power threshold. My amplifier is over 1000W. Regardless, I got the impression thet were making up tests until it failed on something as if they were obliged to fail us on something. This after we spent thousands of dollars on a consulting engineer who supposedly knew what was needed for us to pass and he vetted the design, had me make some changes and pronounced it as ready to pass on first try.
While no one wants to make an unsafe product, it seems to me that the inspectors must get brownie points for failing products then they charge us a pile more money for retests.

They also reject many of the parts I selected in spite that all these parts are already CSA/UL approved. My transformer primary connector was rejected for 230V operation in spite of the fact it was already CSA approved for 600V AC operation. I use UL approved relays on the secondary side and they were rejected because they wanted a different UL approval on them. In spite of the fact the relays I was using were being used within spec.

Quite frustrating. Agency approval, for a smaller company with limited engineering resources can add over a year to getting the product out the door. It seems that no matter what we do, the product will be rejected the first time through. However, we have learned to not argue and do whatever they say. If you do argue, you are told that they don't make the rules and you can appeal it to a rules commitee that meets once a year or something and they have their docket filled for several years. Instead, you just do what they say and incorporate everything they say into the next product and see what they come up with next time to reject you.

[Eva]

This point is where I was triying to go

Regulatory agencies are more focused in money issues than in safety issues. If they didn't make enough profit, then the would disappear, products of small companies never get approved at the first time

In the other hand, these agencies are a mechanism employed by big manufacturers to Eliminate Minor Competitors [this is the actual meaning of E.M.C. initials, they have nothing to do with electromagnetic stuff as most people is fooled into thinking]

[Wagener]

so which is going to be best in the end? one 75V cap or 4 50V caps in series parallel setup.

I can get 11 x 50V 10,000uF caps for the price of one 63V 10,000uF.

I can't believe that it is so cheap. Don't suppose they are of the same quality though.