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  1. #1
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,834. Received thanks 192 times, giving thanks to others 5 times.
    Just been working on how I'm going to wire up my control box, and was wondering if a solid state relay would be upto switching the 625VA toroidal transformer.

    Had a quick look on RS, and part no. 346-918 is rated at 25A continous, with a surge rating of 250A.
    What kind of surge rating would be needed for switching a Toroidal?

    Main reason for wanting to use a Solid State Relay, is I plan on switching it from the E-stop circuit (hitting an e-stop will kill power to the main transformer, and trigger the e-stop signal to the breakout board, while the 5v control supply is kept live).
    Being able to switch the transformer direct from 5v will eliminate the need for an extra booster relay to switch the transformer, as I can't find any 5v relays capable of switching the transformer direct.

  2. Quote Originally Posted by m_c View Post
    Just been working on how I'm going to wire up my control box, and was wondering if a solid state relay would be upto switching the 625VA toroidal transformer.

    Had a quick look on RS, and part no. 346-918 is rated at 25A continous, with a surge rating of 250A.
    What kind of surge rating would be needed for switching a Toroidal?

    Main reason for wanting to use a Solid State Relay, is I plan on switching it from the E-stop circuit (hitting an e-stop will kill power to the main transformer, and trigger the e-stop signal to the breakout board, while the 5v control supply is kept live).
    Being able to switch the transformer direct from 5v will eliminate the need for an extra booster relay to switch the transformer, as I can't find any 5v relays capable of switching the transformer direct.
    A 625VA transformer has a primary current rating of 2.6A and would normally be fused with a 5A fuse so that SSR is massively overspec'd . Unless you are planning to switch the secondary? If you are using it to switch the secondary then you will need to allow for the 1.6v approx drop across the SSR. This is irrelevant for switching the primary.

    I use a 24v relay for switching my transformer but it comes in a 5v equivalent, with 10A 250v AC rated contacts. Costs 1.50, much cheaper than an SSR...

  3. #3
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,834. Received thanks 192 times, giving thanks to others 5 times.
    I know the SSR is majorly overated for continous current, but I don't want it to get cooked the first time I try turning the transformer on!

    A standard relay is an option, but switching a transformer on is going to provide quite a surge, and still need a reasonably sized relay to handle the surge current.

    I know the 3kw 240-415 step-up transformer I use for my 3 phase converter does on occasion trip it's 32A circuit breaker on power up, and that's without any load on it.
    The Toroidal is going to be starting under slight load (it'll take a short period to charge the smoothing capacitor), so I want to make sure whatever I use to turn it on isn't going to go up in smoke.

  4. #4
    It trips the circuit breaker "occasionally" because you "occasionally" switch it on when the mains is at peak volts.

    Any SSR worth it's salt will delay switching until the mains crosses zero volts. Problem solved :D

  5. #5
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,834. Received thanks 192 times, giving thanks to others 5 times.
    I know about the circuit breaker/peak volts, but never knew about that feature of SSRs.
    You learn something new everyday!

    This would be my first use of a SSR, so I just want to make sure I get it right.
    I like to try and avoid puffs of smoke on the first power on!

    I've been doing some calculations, and the SSR should be capable, unless the secondary side of the transformer has too big a load. It is borderline given it's going to be a 22,000uF capactitor peaking at 77V charging from zero volts, but going by my rough calculations and the datasheets, the bridge rectifier is going to die before the SSR.

    If the BR does die, then I'll add an aditional relay/resistor to reduce the Bridge Rectifier load briefly until the relay closes and bypasses the resistor (a suitable relay will have a delay of around 20ms, which should be enough to overcome the initial surge)

  6. I have been experimenting with this circuit as an alternative to my Thermistor/Relay inrush limiter
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  7. #7
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,834. Received thanks 192 times, giving thanks to others 5 times.
    That looks interesting.

    I did consider something similar, but using a relay.
    However, making the voltage sensitive switching is a bit beyond my electronic abilities, so that's why I was considering using a relay.

  8. #8
    I use a 25A Crydom SSR to switch on the 2HP motor on my mill, got fed up with it pulling the RCB and killing the PC driving the steppers. It really has been a fit and forget solution, never given me a moments worry.

    I liked it so much I used a small potted SSR to switch a transformer output on to a bridge for the DC suds pump.

    I'd never willingly use a mechanical relay to switch AC again :D

  9. #9
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,834. Received thanks 192 times, giving thanks to others 5 times.
    Now that I know more about SSRs, I'm already thinking about what else I can use them for.

    A couple may find there way into the phase converter to reduce the surges on switch on/start up. But my priority just now is to get the wiring diagram sorted for the mill control box.

  10. #10
    Quote Originally Posted by m_c View Post
    Now that I know more about SSRs, I'm already thinking about what else I can use them for.
    If you want to make your own, Google for the MOC3040

    An opto-isolated triac with zero crossover switching.

    Add one resistor to limit the gate current and use it to drive the fat triac of your choice.

    Not all SSR's feature zero cross over, but that's what data sheets are for :D

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