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  1. #91
    Rogue's Avatar
    Lives in MyCity, United Kingdom. Last Activity: 16 Hours Ago Has been a member for 8-9 years. Has a total post count of 214. Received thanks 1 times, giving thanks to others 17 times.
    Quote Originally Posted by Jonathan View Post
    If the rating of your transformer is less than around 600VA, I wouldn't bother with any soft start circuit.
    Thermistor is an easy option, as it's resistance decreases with temperature so no real need to take it out of the circuit.
    It's 625VA. It might not be very useful but it's more useful than detrimental, I suppose.

    I understood the problems with thermistors to be the time needed to cool down again if they are not taken out of the circuit - if the PSU is shut off but restarted within quite a short time, the thermistor doesn't offer enough resistance to work as an effective soft start.

    Quote Originally Posted by Jonathan View Post
    ...In your case you would just use the capacitor and resistor to make a 1 second delay, so calculate it such that it takes 1 second for the capacitor voltage to reach the threshold voltage of the relay coil. ... You can power the relay coil from the 70V output via a potential divider.
    There's 24V available for such electrickery, though I suppose the more self-contained the system the better.

  2. #92
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 17 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,831. Received thanks 192 times, giving thanks to others 5 times.
    Alternatively, use a zero crossing point SSR to power on the transformer, which greatly reduces any switch on surge from the transformer itself. And if you really find a series resistor necessary, just use a resistor bypass relay powered from the DC side. By the time the DC is high enough to power the relay, any surge from bypassing the resistor should be pretty minimal.

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  4. #93
    I'm reading everywhere that zero crossing is the worst thing with regard to inrush.

  5. #94
    Quote Originally Posted by Rogue View Post
    It's 625VA. It might not be very useful but it's more useful than detrimental, I suppose.

    I understood the problems with thermistors to be the time needed to cool down again if they are not taken out of the circuit - if the PSU is shut off but restarted within quite a short time, the thermistor doesn't offer enough resistance to work as an effective soft start.
    You would be fine with 625VA and no soft start circuit, so if you restart the PSU with the thermistor still hot the current is still going to be less than without anything, so either way it's not a problem.

    Quote Originally Posted by EddyCurrent View Post
    I'm reading everywhere that zero crossing is the worst thing with regard to inrush.
    Reference?

    Quote Originally Posted by m_c View Post
    if you really find a series resistor necessary, just use a resistor bypass relay powered from the DC side. By the time the DC is high enough to power the relay, any surge from bypassing the resistor should be pretty minimal.
    I used that strategy recently to charge a 0.06F capacitor bank from the mains via voltage doubler ... nice and simple. Pity what it was powering blew up.
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  7. #95
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 17 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,831. Received thanks 192 times, giving thanks to others 5 times.
    Quote Originally Posted by EddyCurrent View Post
    I'm reading everywhere that zero crossing is the worst thing with regard to inrush.
    Not when starting something like a transformer. A mains powered transformer essentially has a zero current point 100 times a second, so if you can switch on at exactly that point, you won't see any surge on the input line, unlike if you were to switch it on when mains volatage is at it's peak you'll see a major surge as full peak voltage is essentially shorted out until the magnetic field builds up.

    The issue with surges comes when you add things like capacitors, as these need time to charge. For smaller transformers, the transformer itself will usually limit current enough to avoid any major surge issues, however with larger transformers they can allow that much current to pass that you need to limit it until things get charged up.

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  9. #96
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 17 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,831. Received thanks 192 times, giving thanks to others 5 times.
    Quote Originally Posted by Jonathan View Post
    I used that strategy recently to charge a 0.06F capacitor bank from the mains via voltage doubler ... nice and simple. Pity what it was powering blew up.
    There is a lot to be said for the KISS approach.
    No point complicating things anymore than what you really have to!

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  11. #97
    Quote Originally Posted by Jonathan View Post
    Reference?
    I've known about zero switching for many years and was surprised myself.

    here's 2 examples

    Inrush current - Wikipedia, the free encyclopedia
    Time-delay relay reduces inrush current | EDN

  12. #98
    Rogue's Avatar
    Lives in MyCity, United Kingdom. Last Activity: 16 Hours Ago Has been a member for 8-9 years. Has a total post count of 214. Received thanks 1 times, giving thanks to others 17 times.
    Quote Originally Posted by Jonathan View Post
    You would be fine with 625VA and no soft start circuit, so if you restart the PSU with the thermistor still hot the current is still going to be less than without anything, so either way it's not a problem.
    What is the sort of level where soft start becomes important? I kept hitting the figure 500VA when reading, which is why I was looking at including it. Are there other factors that might impact on the choice, ie running it off a standard domestic wall socket, etc?

  13. #99
    Quote Originally Posted by m_c View Post
    Not when starting something like a transformer. A mains powered transformer essentially has a zero current point 100 times a second, so if you can switch on at exactly that point, you won't see any surge on the input line, unlike if you were to switch it on when mains volatage is at it's peak you'll see a major surge as full peak voltage is essentially shorted out until the magnetic field builds up.
    That is true for a pure resistance, however the issue is the transformer in this case is better modeled as an inductor. I followed Eddy's links but unfortunately they only give 'hand waving' explanations, so instead I've just calculated it and it transpires that if you model the transformer as such, using Faraday's law to find the voltage across the inductor as a function of magnetising flux, then solve this differential equation to find the magnetising flux as a function of applied voltage and switching angle, the result is sinosoidal with a DC-offset. This DC offset depends on the initial flux (i.e. residual flux) and the cosine of the switching angle, so clearly if the angle is pi/2 (i.e. a voltage peak), the cosine term is zero and you get the lowest inrush current. To make matters worse, the relationship between flux and current will be non linear since in normal operation the core operates adjacent to saturation, so when switched on the core is operating well into the saturation region. This means that although the flux implied by Faraday's law is only up to twice the rated value, the current is many times higher. Unfortunately things change a bit when you have capacitors connected to the output via a rectifier, as they essentially present a short circuit to the secondary.

    Quote Originally Posted by Rogue View Post
    What is the sort of level where soft start becomes important? I kept hitting the figure 500VA when reading, which is why I was looking at including it. Are there other factors that might impact on the choice, ie running it off a standard domestic wall socket, etc?
    To be honest my 600VA figure was just a rough estimate based on experience. Yes, you could consider the current capacity of the mains circuit you are connecting it to - e.g. if it's got a 30A RCD with no significant load connected then you're much less likely to have a problem, as the surge current trip is much greater than 30A...
    Last edited by Jonathan; 12-01-2014 at 12:33 AM. Reason: Forgot the A on VA!
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  15. #100
    Quote Originally Posted by Jonathan View Post
    To be honest my 600VA figure was just a rough estimate based on experience. Yes, you could consider the current capacity of the mains circuit you are connecting it to - e.g. if it's got a 30A RCD with no significant load connected then you're much less likely to have a problem, as the surge current trip is much greater than 30A...
    I'll back up this based off experience because I often use 500 & 600Va transformers without any trouble with inrush trips. If there ever is I just use C or D rated MCB and I'm sorted.

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