Tap changer on load

Discussion in 'Homework Help' started by Cesar Paredes, Nov 20, 2006.

  1. Cesar Paredes

    Thread Starter New Member

    Nov 20, 2006
    2
    0
    Hi:

    I am designing a lab power supply(variable) with LM317 (2-30 volts, 2 Amperes) so I need to switch the taps at the transformer to get and optimal work from the LM317, I am planning to compare the out voltage with a window voltage comparator to trigger a relay to switch a tag, but I don´t know how to handle 3 tag synchronous.

    Would you give me a hand about it.

    Thanks

    César
    Lima Perú
    :)
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    let me clarify the issue first.
    change tap: are you using a multi secondary output transformer
    Questions:
    1. What's the maximum secondary output voltage
    2. What's a 3 tag synchronous

    moz
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I think you're up against the limits of the LM317. My spec sheet says 1.5 amps is the max draw. There are several methods given to increase the output current with external devices. If you use one of these methods, dissipation becomes a minor consideration (heat sink the 317 and pass transistor), so you don't have to worry about switching transformer taps.

    For really spiffy work, though, I've seen a really big regulated supply (50 amps) that had an SCR preregulator that kept the voltage on the filter caps about 2 volts above the desired output so the pass transistors could run near saturation and stay out of the ohmic region.
     
  4. Cesar Paredes

    Thread Starter New Member

    Nov 20, 2006
    2
    0
    Hi Mozikluv;

    I really appreciate your help:
    Let me explain better:

    1. Yes, I am using a transformer with a multi secondary output
    The maximum secondary output voltage is 25 Vrms (3Amp)
    The next secondary output voltage is 18 Vrms (3Amp)
    The next secondary output voltage is 11 Vrms.(3Amp)
    The next is central tap ( 0 Vrms )
    The next secondary output voltage is 11 Vrms
    The next secondary output voltage is 18 Vrms
    The next secondary output voltage es 25 Vrms
    (I am planning to built a symmetrical power supply (+/-30v dc))

    2. I am sorry about the term ¨3 tag synchronous¨ what I mean is that I don´t know if I will need some additional circuit to control automatic taps changing, because I need to avoid 2 or 3 tags working at the same time.ok?

    I am looking for the circuit to get the automatic taps changing, according with the output voltage of the regulator LM317.
    I hope you will get the all picture, if you have any questions, let me know

    Thanks for you help

    Cesar
     
  5. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Perhaps you could use an LM3914 in "Dot" mode calibrated for say...24 v FSD, sensing the Output voltage and using the most appropriate LED output pin(s) via a Buffer, to switch a relay(s).
     
  6. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Hi Cesar,
    there are other voltage regulators available, e.g. the LM338 which is a 5 Amp device (mounted on heatsink, of course). Then there is also a LM350 (3Amps). They are all adjustable positive volt.regulators, similar to the LM317.

    When they are used in a power supply, it is not wise to have the input voltage to the regulator close to the output voltage, because the circuit will suffer proper regulation, therefor it is best not to use the lower voltage taps on the transformer. Rather use the centre tap as your common rail and put a bridge rectifier between the two 25V tags of the trf. Put an electrolytic capacitor between the pos. output of the bridge and the common rail. Also put another identical electrolytic capacitor between the common rail and the negative output of the bridge. First cap POS. to bridge +. and - of cap to common rail. Second cap POS. to common rail and NEG. to NEG of bridge rectifier). You will now have an UNREGULATED, SYMMETRICAL power supply with 3 wires: 1) POSITIVE
    2) COMMON RAIL
    3) NEGATIVE

    In line with the POSITIVE you can wire your LM317 ADJUSTABLE POSITIVE VOLTAGE REGULATOR and associated components with the common rail as the negative (or rather 0V).

    In line with the NEGATIVE you can wire an LM337 ADJUSTABLE NEGATIVE VOLTAGE REGULATOR and associated components with the common rail positive (0V) with respect to the NEGATIVE output of the regulator.

    The above regulators are only rated 1,5Amps and can only adjust down to +1,2V and -1,2V respectively in this circuit.

    I don't know whether a 5 Amp Negative voltage regulator is available. In this case, you can use it together with the previously mentioned LM338 Positive regulator.

    Just mount the regulators on a proper heatsink and forget about using different taps of the transformer!
    I hope this will help you. Alternatively,you can use a completely different circuit for the lab psu.

    Johann.
     
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