How do I wire a toggle switch for an HF amplifier power supply ?

Discussion in 'Amateur Radio Amplifiers' started by KC5SCK, Jun 17, 2017.

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  1. KC5SCK

    KC5SCK Ham Member QRZ Page

    okay thanks
     
  2. KC5SCK

    KC5SCK Ham Member QRZ Page

    It's a 240-volt Supply, tube amp. Want to use main power switch to turn on. Did have a variac in line took variac out as voltage is low enough not to use variac 2500 to 3000 volt to plate.
     
  3. AD4U

    AD4U Ham Member QRZ Page

    If 240 VAC mains, you must use a 2PST switch, You must "break" both sides of the 240 VAC line.

    Dick AD4U
     
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  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    ^ ^ ^ THIS ^ ^ ^
    Further, the switch should be capable of handling not only the steady-state current but also, the inrush current. Most ham amplifiers use a capacitor input filter. That is, immediately after the rectifier, is a capacitor(s). At the moment you turn the power switch ON, the capacitors appear as almost a short circuit (there's a yuuge inrush current pulse). Switches don't like that and can quickly wear out.

    The answer is either a humongous switch/relay, or limit the inrush current pulse. Typically, a resistance is connected in series with the ON/OFF switch. A brief time period after the ON/OFF switch is turned ON, a pair of relay contacts shorts across the resistance. This is how I modified my Heathkit SB-220 to have "step-start":
    [​IMG]
    All new/added circuitry is shown in red. At moment the switch is turned ON, K1's contacts are open and R2 + R3 limit the current into T1. Relay K1 has a 24 VDC coil. D1 rectifies 120 VAC and R1 limits the current so that, after C1 charges up, K1 pulls in (about 1 second or so). K1's contacts then short across R2 + R3, applying full voltage to the transformer primary windings. Since I'll never operate my amplifier on 120V, I didn't bother configuring it for other than 240V.
     
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  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    DPST switch rated for both running current and switching current.

    Henry used 240V 20A double pole circuit breakers as on-off switches in many of their amps, and that was a fairly good choice.
     
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  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    If you're starting from scratch, that works. Otherwise, a workaround as I showed is appropriate.
     
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  7. AC8AC

    AC8AC Ham Member QRZ Page

    what tube choice may help too some tubes require a warm up time good stuff so far . I try to use high current relays to turn on plate transformers after .5kva with some sort of soft start . add on from there if I need a timer I use the timer to hold off soft start as to keep high currents off of the timer module and keep relays and complexity low . not hard just have to draw it out . prior planning goes a long way

    keep in mind 2 piece amplifiers require a bit of wiring challenges for safety and proper metering
     
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  8. G3YRO

    G3YRO Ham Member QRZ Page

    Why? Most equipment only breaks the live wire !

    Including the swtiches on the mains sockets on the wall.

    Roger G3YRO
     
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  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    In the U.S.A. they are both Hot/Live.
     
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  10. AA7QQ

    AA7QQ Ham Member QRZ Page

    Rodger, did you not know that when we use 240VAC here, it is 2 hot legs? I know it's inferior to your British methods, but, that's how it is. A DP switch/breaker/relay Is needed for safety over here.

    Ed
     
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