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How does it affect power supply operation if filter caps are doubled in size?

Discussion in 'Electronic Basics' started by at, Aug 30, 2004.

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  1. at

    at Guest

    I've got a power supply that has 2 470uF 400V filter caps in series. The
    power supply puts out 400VDC.

    Now if I change the caps to 1000uF 400V, what will happen? Will the output
    voltage of the supply change?

    (this is just a basic supply that fullwave rectifies AC with diodes, then
    comes the filter stage between the supply voltage and ground)

  2. Tom Biasi

    Tom Biasi Guest

    With the supply that you describe the result will most likely be a little
    better filtering. This may raise the average output under load but you
    mention no regulation anyway. Are you sure the caps are in series? Series
    filter caps are not the norm.
  3. at

    at Guest

    Thanks. There are 2 caps in series, I guess it's to give better voltage
    handling capacity.

    I've got one other question.

    Now if I change the diodes into a rectifier tube, that has a max capacitance
    in the first filter as 60uF, and after that I place a resistor or a choke,
    what is the max capacitance the second filter stage can have (it's after the
    resistor/choke)? I've heard people tell me it can be bigger than the first
    stage, and I've heard it cannot. So I'm somewhat confused what's the truth.
    If I use a resistor, does it isolate the second filter stage from the first?
    If I use a choke, does it do the trick?

    If you've got a regulated power supply, does that make things different?

  4. CFoley1064

    CFoley1064 Guest

    Subject: How does it affect power supply operation if filter caps are doubled
    Good morning. You didn't say if you've got 120VAC or 240 VAC input. Here's
    one way you can get "400VDC" from 120VAC using two caps in series (view in
    fixed font or M$ Notepad):

    | +|
    120VAC | ---
    | C ---
    | |
    o-----|---------o "400VDC"
    | +|
    | ---
    | C ---
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    As was mentioned in another post, putting 2 470uF caps in series will give you
    a net capacitance of 470/2 = 235uF. Doubling up the caps to 1000uF will get
    you a net capacitance of 1000/2 = 500uF.

    In an unregulated supply, your DC voltage will be dependent on ripple voltage
    (each of the 2 caps is recharged 50 or 60 times a second). Doubling the
    capacitance will cut the ripple voltage in half, which will result in a higher
    DC voltage, and that could cause problems. Look at the load and evaluate
    whether it can handle the higher voltage. Sometimes it makes a difference.

    Oh, yes. Do be careful here. In this type of unregulated supply, both + and -
    are at line voltage potential, which could be dangerous.

    Good luck
  5. Randy Day

    Randy Day Guest

    Having the 2 in series gives you an 800V cap.
    Otherwise the OP would be running too close
    to the limit and be risking a blowout...
  6. at

    at Guest

    Thank you for your reply.

    It's good to know how it works. If there's a problem with the higher DC
    voltage, I'll increase the resistor size in the power supply. That will drop
    the voltage some. Or is there a better way?

    Don't the filter stages also drop voltage? Or do they increase it? Is it the
    resistors between the filter stages that make the voltage drop, and not the
    If I put resistors parallel with the caps (as is sometimes used), the
    resistors will also drop some voltage, is this correct?

  7. Tom Biasi

    Tom Biasi Guest

    Hi at,
    It would help if you told more about your application for this supply.
    You sound as though you have little experience with high voltage supplies,
    be careful.
    You are describing very old design charistics. If you are working on and old
    radio or other old device tell us what it is.
  8. at

    at Guest

    This is just an example supply that's like many used in old tube-based audio
    equipments. I know the dangers with high voltages, I'm not going to get
    hurt. It's true I don't know much about the theory details, that's why I'm
    writing to this basics newsgroup. I would like to know how the capacitors
    work so I could see if I could benefit from better filtering. Big cans are
    expensive so it's better to ask questions before deciding. I just would like
    to know how big capacitors you can put into an tube amp with a tube
    rectifier. I would want to get good filtering, so I'm interested to know if
    I could put bigger caps in the other filtering positions, than what the tube
    rectifier allows for in the first filter position.

  9. CFoley1064

    CFoley1064 Guest

    Subject: Re: How does it affect power supply operation if filter caps are
    I believe in your response to another answer you mentioned you've got one of
    those "old tyme tube-type" setups. In days of yore, back when "men were men
    and women were glad of it" (Three Stooges), they did something like this (view
    in fixed font or M$ Notepad):

    1N4006 ___
    | +| R |
    120VAC | --- ---
    | C --- C ---
    | | | "400VDC"
    | +| |
    | --- ---
    | C --- C ---
    | | ___ |
    1N4006 R
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    except they used tubes as rectifiers instead of silicon. You've got an R-C
    filter at the output, which reduces ripple voltage as well as lowering the
    output voltage somewhat. A good rule of thumb commonly followed was to make
    all four caps the same size, but nowhere is it written in stone one way or
    another. Do the math, or just try it and see. Just make sure you've got high
    enough wattage for your power resistors.

    Using a series R as a filter element is much smarter than just using the
    resistor to load down the cap. You're just wasting capacitance that way, and
    at 400V, uF get pretty expensive. But, if you have an existing layout and
    don't want to rewire, it's a safe way to get the voltage back to where it was.
    Double the capacitance, double the load, and you have the same output voltage
    as the original, just more heat.

    Good luck, and thanks for a good question
  10. Tom Biasi

    Tom Biasi Guest

    "> This is just an example supply that's like many used in old tube-based
    Back in the day, we liked the time constant discharging the caps to be at
    least three times that of the charging time constant.
    What do those terms mean? You should be able to find something and bring
    your questions back.
    Good Luck,
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