Image Map Image Map
Page 3 of 3 FirstFirst 123
Results 21 to 27 of 27

Thread: Non polarity cap questions on CRT monitor repair.

  1. #21

    Default

    Quote Originally Posted by Dwight Elvey View Post
    Although, I've never done so, you can make a non-polar capacitor from two placed in series of opposite polarity. I'd guess you'd need to use values of 2X capacitance but of equal voltage. In this case 25V each.
    Dwight
    Folks, the correct way to make an electrolytic NP capacitor is to connect a +--+ series of polarized capacitors of the SAME capacity as the original one and same or higher voltage rating. This isn't a normal capacitor series because the capacitor that is reverse polarized (at any time) of the two has effectively a large equivalent capacitance (much larger than the rated one), so in the series what counts is the forward polarized one. This also explain why most of the voltage is localized on the forward polarized capacitor. In a capacitor series, the voltage division is inversely proportional to the capacitance, the lower the capacitance, the higher is the voltage drop on any of the capacitors in a series.
    I've done this many times, works fine. However, you should always try to first measure the original capacitors and determine it actually needs replacing, second: try to obtain the right part, third: make a +--+ series only if the right part isn't obtainable easily.
    HTH
    Frank IZ8DWF

  2. #22
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    30,641
    Blog Entries
    20

    Default

    There have long been discussions on eevblog and AAC about the resultant capacity. It's been verified a couple of times with real cases that the resultant is 0.5C, whether you connect + to + or - to -. A polarized capacitor with a reverse voltage impressed on it still exhibits capacitance--it is not purely resistive. If you think about it a bit, you can see why--you're essentially doubling the distance between the outer connections.

    The big confusion is usually over how much capacitance is exhibited in the reverse direction. That's an interesting subject in itself. I'll submit that commercial equipment using polar caps to get a nonpolar use 2C rated caps to get C exist and have been doing so for years.

  3. #23

    Default

    Quote Originally Posted by Chuck(G) View Post
    There have long been discussions on eevblog and AAC about the resultant capacity. It's been verified a couple of times with real cases that the resultant is 0.5C, whether you connect + to + or - to -. A polarized capacitor with a reverse voltage impressed on it still exhibits capacitance--it is not purely resistive. If you think about it a bit, you can see why--you're essentially doubling the distance between the outer connections.

    The big confusion is usually over how much capacitance is exhibited in the reverse direction. That's an interesting subject in itself. I'll submit that commercial equipment using polar caps to get a nonpolar use 2C rated caps to get C exist and have been doing so for years.
    Ok, I stand corrected. Sorry

    Frank

  4. #24

    Default

    Quote Originally Posted by Chuck(G) View Post
    There have long been discussions on eevblog and AAC about the resultant capacity. It's been verified a couple of times with real cases that the resultant is 0.5C, whether you connect + to + or - to -. A polarized capacitor with a reverse voltage impressed on it still exhibits capacitance--it is not purely resistive. If you think about it a bit, you can see why--you're essentially doubling the distance between the outer connections.

    The big confusion is usually over how much capacitance is exhibited in the reverse direction. That's an interesting subject in itself. I'll submit that commercial equipment using polar caps to get a nonpolar use 2C rated caps to get C exist and have been doing so for years.
    I was just about to pull out my RLC bridge. This is actually just similar to the diode case. The leaking back current will charge each capacitor to the maximum swing of the voltage. On the first cycle one will charge to the maximum while the other is still about 0V. On the next swing, the voltage on the first capacitor gets pushed to 0V. That current gets pushed into the other capacitor as forward current. So say the swing was +- 50V. As long as things were equal, each capacitor would hold an average of 25V and act just like two capacitors in series. So, imagine the maximum voltage shuttling back and forth between the two capacitors.
    Any slight imbalance in capacitance will cause the smaller one to experience a little more back current. This is what causes them to get warm. I would think that adding diodes will help to shunt the current from being back current.
    Dwight

  5. #25
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    30,641
    Blog Entries
    20

    Default

    This reminds me a bit of the old capacitor web page (it may still be out there) and the matter of theory vs. practice.

    You have two identical capacitors (large electrolytics will do) Charge one to a voltage of V, then bridge it to the other cap. What's the voltage in each capacitor? (W = (CV2)/2) Why does it never work out the way that theory predicts?

    Don't take my word for it--try it.

  6. #26

    Default

    Much of the power went up in smoke. You need to have two perfect capacitors with not resistance and no inductance. It will surely end with only half the voltage as you can not create charges were they don't exist. Once things settle down, the voltage will be 1/2 the starting voltage. Half of the original power went up in smoke or electromagnetic waves.
    Dwight

  7. #27
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    30,641
    Blog Entries
    20

    Default

    Yup--that really puzzles the students the first time in the lab. "But, this is what theory says..." Lesson: we live in an imperfect world.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •