Load switch with soft-start

Discussion in 'General Electronics Chat' started by TheLaw, Feb 24, 2013.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Hello everyone,

    I am building a small bench supply (I know, not very original).

    It'll be about 500mA max and 20V max.

    I want to be able to switch the output on and off in a way that I can adjust the voltage (and view it on a panel volt meter) and then when I have set my voltage, I can flick the switch and give power to the circuit.

    So the switch wouldn't be the mains switch, but an output switch.

    I'm not sure the science behind it but I know that the act of toggling a switch can induce all sorts of crap into a signal, including overshoot.

    So what is a good way to reduce that overshoot and/or have it ramp-up or "soft-start"?

    I was thinking to put a capacitor or two AFTER the switch so that when I toggle the switch, they must first charge up and act as a filter. But is there a better way? Also I wouldn't know what values to use. I was thinking of maybe using a 0.1uF and a 10uF in parallel.

    Thanks.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Push Sw → MC14584 RC debounce → CD4013 divided by 2 Toggle circuit → NPN Bjt → Relay → Vout
    Push Sw → NE555 Toggle Switch → Relay → Vout (I haven't tried this, you can get it from google)
    About the capacitor that I will use 0.1uf/50V.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Not sure what Scott's circuit is supposed to do.:confused:

    The circuit in the attachment starts up at about 1.5v, rising with an RC ramp to the final voltage. It will rise more slowly when the set voltage is higher. Change C1 to change the rate of rise.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    here's another scheme. It requires a DPST switch, instead of a DPDT as in the previous scheme. When the switches are closed, the output starts at about 2V, and ramps up linearly to the final output.
    You can add caps on the output, which most circuits require anyway.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    The circuit that what I mentioned as below, there is no switch, and it can modify to use a push switch.

    [​IMG]
     
  6. crutschow

    Expert

    Mar 14, 2008
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    3,234
    I don't see any problem with simply putting a SPST switch in series with the output. The output will rise rapidly to the set voltage but there should be only a small overshoot (from the small parasitic line inductance) which should be easily absorbed by the input decoupling capacitors that all properly designed circuits have. It's no different then switching on an circuit from a battery.
     
  7. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Thanks everyone. All circuits attached are very interesting.

    Ron, are those your own circuits or is there a site/resource that might have an explanation for them?

    In the second one, what does the 2N3906 in the feedback loop do? And how about R5? The transistor seems to work as some sort of switch.

    I guess my real question is why is there a transistor in the DPST model and not in the DPDT model?

    Thanks.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I tend to agree with Carl that you probably don't need soft start, although, if you have a lot of capacitance on both sides of the switch. the inrush current might be hard on the switch.
    I designed these circuits because I like the challenge. They may be overkill.
    Attached are explanations of how each one works.
    To answer your question about the switches, the DPDT is not needed in the one with the transistor.
     
  9. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    Thank you thank you. You are very kind to write such nice explanations up. I appreciate it.
     
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