Replacing tantalum caps

Discussion in 'General Electronics Chat' started by Zeebit, Nov 17, 2013.

  1. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Hello all.

    All the local stores in my place do not carry any tantalum caps. Buying from overseas is too expensive for me.

    Here is the circuit I'm building: http://www.quan-diy.com/projects/benchps/benchps.htm

    I would like to replace the tantalum caps with aluminum electrolytic. What valued would be suitable?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,796
    Same value and rating. The thing that might cause an issue is the higher ESR of the Aluminum cap, so you might put a 0.1uF film or polymer in parallel with it.
     
  3. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Does the ESR matter when dealing with DC?
     
  4. tvtech

    New Member

    Mar 18, 2012
    6
    14
    Hi Zeebit

    Be very wary of the circuit you are building. I believe it comes originally from the Horowitz and Hill book "The Art of Electronics". I built this exact same PSU circa 1990 and had issues that sometimes a short circuit on the output would cause the bypass transistors to go short circuit and drama.

    The Tants would also, from time to time, fail too when the the rest of the PSU was working.

    Maybe I did not lay my PCB out 100%....I don't know. I scrapped the whole thing a few years ago after many strange failures.

    All the best,
    tvtech

    Edit: It also on National Semiconductor LM317 application notes.
     
    Last edited: Nov 17, 2013
  5. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    tvtech, I've contacted the owner of the webpage and ge said the supply is still working to date(more than 2 years now).

    ******************
    So, I could replace the tantalum with the same value aluminum cap and a 100nf in parallel?
     
  6. tvtech

    New Member

    Mar 18, 2012
    6
    14
    Hello Zeebit

    There is a big difference between:

    1. Yes it's working ..and
    2. Yes it's working properly and being used all the time.

    And has been put through it's paces...

    Like I say, I have been there.

    Have fun,
    tvtech
     
  7. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Well apparently you did something that messed it up. This guy has been using it for more than two years, so yes, it has been tested.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Since it is the capacitor itself that is supposed to make this point a pure DC... you tell me. ;)
     
  9. tvtech

    New Member

    Mar 18, 2012
    6
    14
    Was going to guide and help here...

    Why bother when the OP is stubborn and has no idea. And won't listen.
    That is mainly why I am considering bailing out from the Internet and helping anymore.

    I know that circuit. Backwards. Otherwise, I would not of commented...

    Better I sit right next to the individual I am trying to teach....not anywhere in the World.
     
  10. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    LOL.

    Okay let's hear it. What's wrong with the circuit? What do you suggest that needs to be changed in order for it to work properly?
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    If you read the datasheet, section on "external capacitors" says...

    In general, the best type of capacitors to use is solid tantalum. Solid tantalum capacitors have low impedance even at high frequencies. Depending upon capacitor construction, it takes about 25 μF in aluminum electrolytic to equal 1μF solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies; but some types have a large decrease in capacitance at frequencies around 0.5 MHz. For this reason, 0.01 μF disc may seem to work better than a 0.1 μF disc as a bypass.

    Although the LM117 is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1 μF solid tantalum (or 25 μF aluminum electrolytic) on the output swamps this effect and insures stability. Any increase of the load capacitance larger than 10 μF will merely improve the loop stability and output impedance.
     
  12. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Okay I have little idea about this capacitor stuff but this supply is linear and I think I'm dealing with 60hz here.

    What am I missing?
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    Battery or mains powered?

    Wallwart or discrete transformer?

    If transformer, half wave or full wave rectification?

    What size decoupling caps do you have after the rectifier?

    If full wave, do you have any long wires connecting parts of your circuit that can act like an antenna and pick up signals from fluorescent lamps or dimmer switches?
     
  14. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    Mains powered, R-core transformer, full wave rectifier, wires ranging from 4" to more than 6".

    What replacement values would you recommend for the two 10uf and one 1uf tantalums?
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    What is the current draw (load)?

    What is the unloaded voltage at the rectifier (before the voltage regulator)?

    What is the regulated voltage (or voltage range).
     
  16. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    It's an adjustable CC/CV supply.

    I'm currently deciding if I set it to either 3A or 4A. So that's 0A to either 3A or 4A.

    Traffo secondary voltage is 18v minus 1.4v drop on the bridge rectifier and multiplied by 1.41 to get the peak dc voltage of 23.4V.

    Capacitor filter is made up of eight 2200uf caps in parallel.

    Output voltage range of the regulator is adjustable from 0V to 23.4V(dc peak after capacitor filter) minus the regulator dropout, I think.
     
    Last edited: Nov 17, 2013
  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    It seems like your input is well filtered, I am guessing your 60hz issue is looping lead wires picking up 60hz from something else in your room. Try twisting any pairs of wires in pairs and keeping them equal lengths. This prevents pick-up.

    What type of circuit are you powering that has 60hz interference?
     
  18. Zeebit

    Thread Starter Member

    Nov 8, 2013
    72
    0
    What? No. I do not have issues whatsoever. I was simply referring to the mains frequency. My bad.

    The only problem is I can't figure out the values of the alumin caps to be substituted for the tantalums.
     
  19. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    25 to 100uF should be fine. If you are amplifying high power at high frequencies (ultrasonic), you may want to go higher.

    The biggest problem with your circuit will likely be heat sinking your pass transistor. When outputting 3 volts, the pass transistor drops 20 volts x 4 amps is 80 watts.
     
  20. tvtech

    New Member

    Mar 18, 2012
    6
    14
    Listen to me carefully Zeebit..

    The Tants used in the circuit are going to cause problems...replacing them with Electrolytics will cause problems too.

    Not a nice circuit...as I told you. Or is that not what you want to hear??
     
Loading...