Some power supply design help?

Discussion in 'General Electronics Chat' started by Mike33, Dec 6, 2009.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Hi,
    I'm throwing together a power supply for a tube preamp. I was fortunate enough to find a transformer from an old Sencore VOM, which has secondaries for the heater and HT (140VAC). I will just re-use the heater secondary, of course, but need some advice on designing for my HT.

    My circuit requires one 12AU7 dual triode, and this trafo actually powered one in the meter, and the total current draw will not exceed 6 to 8 mA. I'll use a conservatively rated bridge for rectification.

    FILTERING:
    I used Duncan's Power Supply Designer, and came up with a pi filter (C, R, C) using 100uF, 220 ohms, and another 100uF. Now, here's the question, which I am embarrassed to admit my noobishness around: inrush current. With the filter caps appearing as a dead short at start-up, my R has to take something like 11.5W for a very brief time; then it settles down nicely to a dissipation of 1.5 to 2W. What rating should my dropping resistor be: 10W or the average, 2-3W? I'd like to get this part of power supply design straight, it's been bothering me for a long time!
    Thanks a lot!

    ~Mike
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    the inrush is a fraction of a second. it is always nice to over-rate resistors, I would use a 5 watt unit mounted off the circuit board where air can circulate. If you have a 10 watt unit just use it.
     
  3. SgtWookie

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    Jul 17, 2007
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    Are you using a full wave bridge rectifier (two diodes) on a secondary w/o a center tap? Or does the secondary have a center tap, and you're using a 4-diode fullwave bridge?

    You're going to wind up with something like 198v out.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Unless I misunderstood your post, your resistor will only dissipate P=I^2*R=(.008)^2*220=14mW. Unless you are designing for an accidental short to ground, you shouldn't need to plan on 2 Watts.
     
  5. SgtWookie

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    Well, there will be a fair amount of power dissipation in the first few moments of startup, but exactly how much and how long depends on the secondary winding of the transformer.

    I threw together a quick LTSpice simulation; adding in an extra resistor between the bridge and the 1st cap. Didn't know the impedance of the transformer, so arbitrarily threw in 100 Ohms.

    Feel free to experiment with it. If I made some dumb mistake somewhere, you're free to ridicule. ;)
     
  6. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Hi Guys,
    Thanks for the great suggestions!! Yes, of course I mislabeled my steady-state draw, it is not 2 to 3 WATTS but 2 to 3 mW!! I added a bit for safety...

    Yes, I'd planned on an encapsulated bridge (no CT), SgtW....thank you for the LTSpice .asc, I have messed around with it, and can see a little clearer what should be going on! 198V output +/- is what I'd hoped for (guitar preamp, with only 1 tube, that's a good B+ to be at for a 12AU7).
    What's the extra 1R resistor for between rect. and 1st cap?
    Transformer's secondary DC resistance is 150 ohms, you were close!
    I
     
  7. SgtWookie

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    Jul 17, 2007
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    Yes, of course I mislabeled my steady-state draw, it is not 2 to 3 WATTS but 2 to 3 mW!! I added a bit for safety...[/QUOTE]
    Always a good plan. ;) It's funny how quickly a power budget gets used up, and then goes right down the tubes... ask me how I know this. :rolleyes:

    OK, had to ask. Lots of those old HV xfmrs had CT secondaries; back when copper was cheap.
    That was the idea. :)

    Yep, that should be pretty good. Except it'll be more like 186V with R1 set to 1 and R2 set to 220, with a 7.44mA load (that's what 186v/25k works out as). You'll have about 20mV ripple no matter what R1 is set to.
    I threw it in there for a bit of extra filtering, but mainly to reduce the maximum power dissipation in R2. You can set it to 1m to simulate a piece of wire, or try setting it to 220 or so, or anywhere in between. See what it does to your ripple voltage on the output - nothing!
    I should've been able to figure that out from what you initially stated, but I figured you would have fun "tweaking" up the simulation.
     
  8. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Cool man, thanks again! :eek:)
     
  9. SgtWookie

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    Jul 17, 2007
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    You're welcome :)

    If you someday want to take a stab at building a flyback converter, take a look at Ronald Dekker's page:
    http://www.dos4ever.com/flyback/flyback.html
    He named the page "Flyback Converters for Dummies", but it's a really good resource for "getting your feet wet" with inductors and switching power supplies.
     
  10. SgtWookie

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    Jul 17, 2007
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    Mike,
    You might want to give this page a read:
    http://gabevee.tripod.com/mypwrsup.html
    Particularly the lower part of the page. He's using some caps across the rectifiers in his tube supply; he claims that when they turn off, there's an audible pop created. He's using some small caps across them, plus a large inductor between the filter caps to remove the HF pops the rectifiers cause.
     
  11. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Interesting approach from a 'reactance point of view'. Probably more than I want to do with my circuit...I'm just adding 1 more tube stage to the input of my amp via an off-board box. Might get fancy later and add a tone stack after a CF, but I'd probably have to invest in new iron since this trafo is not going to be rated for the extra current! I'll have to see what happens if I increase the draw from it. That's another topic, relating to primary current when secondary current rises!!! Voltage sag, but then what? Smoke?

    I've seen the 'equalizing caps' before, in ham radio circuits, actually. Mostly for really high voltage, but they might also be useful for lower B+, I don't know! There are so many ways to implement rectification...
     
  12. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    One other thing I am thinking about, is would adding another filter section reduce ripple and provide a "cleaner" supply? Just another 220R and filter cap?
    Thx!
     
  13. SgtWookie

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    Well, yes - but you'll get to a point of rapidly diminishing return.

    You'd be better off to split the 220 Ohms between R1/R2, and add a 3rd cap at the bridge.

    See the attached. This change reduced the ripple from around 20mV to about 5mV.
     
  14. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Awesome, man! Thanks again! I have other ?'s, but I'll make a new topic so's to keep this one on-track..... :)
     
  15. SgtWookie

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    Jul 17, 2007
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    Mike,
    Still on this same topic, I'm afraid I've neglected to bring up safety, which I'll now address.

    Once the supply is operating, you'll have around 185v on the caps. If the 25k Ohm load were constant, once the supply was turned off, the caps would discharge to about 3v within 30 seconds. However, the only load I used, the 12AU7 tube, isn't really a constant load, and that load will go away when the power is turned off.

    You will need to use a bleeder resistor to discharge the caps (which will dissipate power and add to ripple).

    Or, you can use a DPDT switch for power; one side for post-fuse HOT and the amplifier supply, and the other side switches the 185v supply to chassis ground via a resistor; perhaps 300 Ohms in value. This won't waste power and cause ripple while the amplifier is on, and will discharge the HV supply very quickly and safely when you turn it off.
     
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