Soft start for power amplifier

Discussion in 'The Projects Forum' started by williamthomas, Aug 4, 2013.

  1. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    Hi i have an idea how this soft start works, but can i have a clear explanation?
    That would be great. Thanks.
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    A linear power supply will typically have some large filter caps which draw a large current for a short time when power is first applied.

    The thermistors provide a resistance to limit this inrush current when AC power is first applied. The thermistors resistance will then drop as current is drawn through them, but there will still be a small resistance.

    That's were the relay circuit comes in. The 24 volts DC charges the 470uf capacitor through a resistor, after quite a few seconds the transistor will be turned on which will turn on the relay. The relay contacts close and are intended to bypass the thermistors. But, the schematic as drawn has an issue, The relay contacts are drawn incorrectly. They will short out the mains and the transformer primary :eek:

    The respective contacts should be wired in parallel with each thermistor.

    This is a nice feature because it removes the resistance of the thermistors. Otherwise this resistance would cause an unwanted voltage drop when the power amp is operating.

    BTW. Did you have any luck with the power amplifier ?
     
    Last edited: Aug 4, 2013
  3. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    Nice thanks. the RC network is about 30 sec. this is the time that is specified in the data sheet for the thermistor. i have made some changes to the attached circuit. is it correct now?
    I got help with the power amplifier and i am now ready to do the pcb with EAGLE. the power amplifier is for my final school project.
     
  4. LDC3

    Active Member

    Apr 27, 2013
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    You still have a problem with the mains circuit. When the relay closes, I can connect both mains connections without any resistance. Also with the transformer connections. I believe you need to change the relay for a DPDT type.
     
  5. LDC3

    Active Member

    Apr 27, 2013
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    I believe that this is the circuit you want for the mains side of your drawing.
     
  6. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    the relay that i have is omron g2rl-2. it says that it's a dpdt. are my connections then okay?
    farnell link is: http://dk.farnell.com/omron-electro...c/relay-pcb-dpco-12vdc/dp/9950176?Ntt=3650585
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You need to make the connections using the NO contacts as shown by LDC3 in post #5. Your relay type is fine.
    But the coil voltage is 12VDC. If you want to run it using 24VDC then you should put a resistor in series with the coil to drop the voltage. Since you need 1/2 the 24 volts, a resistor close to the relay coil ohms (360) would be fine. A 330 ohm would be OK. I suggest the resistor should be at least a 2 watt rating. The dissipation will be ~ 0.4 watts. A 1 watt would work but get quite warm.
     
    Last edited: Aug 4, 2013
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Why would you need two thermistors?
     
  9. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    i spoke with my class mate and he thought that two thermistors will be fine. i thought that only one thermistoris necessary for the inrush current. now i have to figure out how to connect it with a relay.
     
  10. #12

    Expert

    Nov 30, 2010
    16,345
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    It goes like this:
    (assuming the values in the original circuit are valid)
    For the nit pickers: it's a concept drawing. It can be turned into a schematic by adding the part numbers and values.
    You can use one or two thermistors, depending on what you can buy and how much current/power each one can survive.
    The 68K resistor has been balanced to the gain of the transistor and the load current. If it was trying to do a saturated switch at 3.33ma, it would use up 226 volts. Forget the 360 ohm resistor and adjust the 68K to get the voltage on the relay right.
    Don't forget to add a fuse.
    I don't know what, "TRAFO" means, so I faked it.
     
    Last edited: Aug 4, 2013
  11. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    What about now? i've attached the circuit.
     
  12. LDC3

    Active Member

    Apr 27, 2013
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    Damn, it's my own fault for not labeling my figure. :eek:

    What does the 230V supply power in your circuit? As far as I can see, only the RTC. It does not connect to the rest of the circuit. You probably want one line of the 230V to go through the RTC and onto the transformer primary coil and the other line to do the same.
     
  13. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    the 230v is the mains power.
     
  14. #12

    Expert

    Nov 30, 2010
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    Be sure to see my correction in post#10. The relay driver is a finicky circuit.
     
  15. williamthomas

    Thread Starter New Member

    Jul 26, 2013
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    trafo means transformer. see my edited circuit on page 3 of the thread.
     
  16. #12

    Expert

    Nov 30, 2010
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    Here is a different way to make a delayed relay driver, or you could use an op-amp, or a mosfet or a j-fet, if you know how to design that sort of thing.
     
    Last edited: Aug 4, 2013
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