Power Supply resister died out, half output after change

Discussion in 'The Projects Forum' started by Romes, Oct 14, 2014.

  1. Romes

    Thread Starter Member

    Sep 30, 2014
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    So, I was building a power supply for a 6111 subminy pedal. I have a 120-12v 750 transformer going into a rectifier. Then a 47ohm resister into a LM317. I adjusted the trimmer to get 6.3 v for the filament. Then I smelled something and the voltage dropped in half. Turns out the resister got very hot and burned out. Since then I changed out the resistor yet the voltage is still at half.

    Any ideas would be greatly appreciated.

    [​IMG]
     
  2. wayneh

    Expert

    Sep 9, 2010
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    By "the resistor", you mean R1? How much current is drawn by the heater?

    And do you really connect the heater between "Vin" and the regulator output (not ground)? That's an odd configuration.
     
  3. bertus

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    Apr 5, 2008
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  4. Romes

    Thread Starter Member

    Sep 30, 2014
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    Thats correct, 6.3 heater and ground and 12v for the rest of the circuit, also I mentioned the 47ohm resister fried and its the only 47ohm in the power supply. I will take a look at the links you provided, but I am pretty sure I took a look at them already (I google stuff before I come here). I',m just curious as to why it worked for a little bit until the resistor fried, and then even with a fresh resistor, it wont go back up to 6.3. Also, When I play with the trimmer, it will reach a point and then plummet.

    PS: I didnt design the circuit, so what should I replace the 47ohm with?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    We appreciate that effort. It separates you from 90% of the other posts here!
    I missed the concept of the circuit. The heater is powered thru the LM317 to ground, right? The "Vin" is not related to the heater, it's for the rest.

    The 47Ω resistor is OK but you should probably up the power rating from whatever it was. A fuse might be a good idea though - the blowing of the resistor might have spared another part of the circuit. Or something else blew along with the resistor.

    So what voltage do you now see at the input to the LM317? If something in the circuit is drawing too much current, it will drop the voltage across that resistor and the LM317 won't be able to hold its output voltage. Any chance you misread the 47Ω, maybe it's 4.7Ω?
     
  6. Romes

    Thread Starter Member

    Sep 30, 2014
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    :)
    Yes the heater is powered by lm317. The Vin is separate. Currently, the power supply is not connected to the rest of the circuit. I am trying to get that 6.3v before I hook it up. Voltage going into the LM317 is 18v (Not sure why, the transformer said it was 120 to 12. And according to the literature, I have it hooked up correct) coming out, its at 3.63ish. Like I said, before the resistor fried, I had it at 6.3 for a little bit. And, as for the resistor, Y-V-Bl, I tried a few different ones, and diffrent wat ratings.

    Thanks for the help.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you use a 47 Ohms resistor and use a current of 0.3 Amps, the voltage drop of 14.1 volts would be expected.
    This is not possible as the powersupply is only 12 Volts.
    As said by wayneh a resistor of 4.7 Ohms would be more likely.

    Bertus
     
  8. wayneh

    Expert

    Sep 9, 2010
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    The 18V is expected. When you rectify the AC out of the transformer without much load, it will act a bit like a peak detector and you get 1.4 times the nominal AC voltage.

    So the LM317 is seeing 18V and is only putting out 3.6V. What voltage do you see on the adjust pin? Your trimmer or your R2 may have failed. Check them both. If they check out, your LM317 is likely toast.
     
  9. Romes

    Thread Starter Member

    Sep 30, 2014
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    Alright, thanks for the responses. Interesting to know about the transformer as well . I will try the 4.7ohm. The trimmer is putting out min .88v max at the 3.6v. I will go through it again as soon as I finish the home work I am working on.

    Thanks you folks again for the help and patience.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    Correct me if I'm wrong, but isn't the heater of a vacuum tube supposed to be 6.3 VOLTS AC -- NOT DC??!!??
    I always used the 6.3V filament winding directly to heater -- there is no need for regulation on a heater. Where are you getting your information and what in the name of all the hollies was the 47 ohm resistor supposed to be for -- warming the room?

    12.6 VAC * 1.414 = 17.81 Volts Peak
     
  11. Romes

    Thread Starter Member

    Sep 30, 2014
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    The data sheet says AC or DC, and like I said, I didn't draw the circuit. If it would make it easier, can you or anyone show me a circuit that will produce 6.3 and 12v (dc, but I got that far). Thanks !
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you use one or two tubes?
    When you got two tubes, you can put the heaters in series and put them on 12 Volts as in the schematic in the first link I gave.

    Bertus
     
  13. crutschow

    Expert

    Mar 14, 2008
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    DC is often used for the filament in high end tube audio circuits to minimize hum pickup.
     
  14. Romes

    Thread Starter Member

    Sep 30, 2014
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    I was thinking about that, (using two in a series) but according to ESP, its not a good idea to use two in a series in audio.
     
  15. Papabravo

    Expert

    Feb 24, 2006
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    Yeah, I've read differing opinions on the effectiveness of the technique, and I'm not convinced that it is a universally accepted principle. Then of course it is impossible to refute the claims of the Golden Ears on any audio related topic. Gold connectors and shielded speaker cables are my favorites though.

    So the 6111 is a sub-miniature tube, I had it confused with something a bit different. So you're trying to get both 6.3VDC for the heater and 12.6VDC for the B+ from a supply with a single transformer? The current requirement for the 6.3V portion is 150 mA which is almost 1.7 watts ((17.8V-6.3V)*.15=1.725). The 317 should do that with a suitable heat sink. Check the 317 datasheet for the "thermal resistance". The units are degrees Centigrade per watt. Multiply this number by you expected power dissipation and it will tell you what temperature rise to expect. Don't let the junction to ambient temperature rise exceed about 80 degrees C. I still don't know what the 47 or 4.7 ohm resistor is for.
     
    Last edited: Oct 15, 2014
  16. Romes

    Thread Starter Member

    Sep 30, 2014
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    Ahh, I got it. Thanks for all your help.
     
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