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12.6V Valve (Tube) Heater Regulator Help Needed Please!

Discussion in 'General Electronics Discussion' started by Ned, Aug 18, 2011.

  1. Ned

    Ned

    3
    0
    Aug 18, 2011
    Hello all, would really appreciate some help.
    I'm designing a valve heater supply and need to raise the output of a 12V LDO regulator (278R12) by 0.6V. I tried using a 1N4148 diode on the earth leg, which works on standard regulators (78xx), but it just shuts down. I then tried using two shotky diodes(1N5158) in series in the same way and it works but only gives 12.4V.
    The input voltage is 14Vdc (rectified from tx, with a 2200u smoothing cap) so I have to use an LDO regulator, the load takes around 1.2A.
    I could use the circuit as is, but was wondering why it doesn't work with a normal silicon diode on the earth leg? Is it because the earth leg current on an LDO reg is higher?
    Many thanks in advance,
    Ned.
     
  2. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Tube valve circuit

    Hi there, silicone diodes on the common leg of the regulator should be fine, as many as you need, each one will give an increase of 0.7 volts, it needs to be silicone diodes, the current does not increase, 14 volts input is a bit low, 15+ would be a bit better. Dave.
     
  3. Ned

    Ned

    3
    0
    Aug 18, 2011
    Cheers Dave, unfortunately I'm stuck with 14V on the input as it's from a heater winding from a mains tx, which is why I'm using the low drop out regulator (0.5V) rather than a standard one (2.5V). I did try a silicon diode but it just shut down and I was wondering why, when it works fine on a standard 78xx regulator? Thanks though.
    Ned.
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    You may need to add a decoupling capacitor across the diode, and perhaps add some current through it with a (~2.2k) resistor from the output.
    The stability criteria are quite different for LDO regulators, for example they usually require a much larger capacitance on the output.
     
  5. davelectronic

    davelectronic

    1,079
    12
    Dec 13, 2010
    Valve regulator problem

    Hi again, what the previous post says makes sence, i always use decoupling capacitors, one circuit i built using a similar regulator was in a remote location from the main circuit, and on loading the regulators volts droped massively, i used a 470uf 25 volts radial electrolytic on the out put, there was an led indicator series resistor load anyway, although a light load it was enough for my circuit, the extra capacitor on the output maintained a good load line regulation. Dave.
     
  6. Ned

    Ned

    3
    0
    Aug 18, 2011
    Well I finally got round to experimenting and found that a 47u cap across the diode seemed to do the job fine. Thanks again fellas, much appreciated.
    Ned.
     
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