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  1. #1
    Hi everyone, this may seem like a daft question that I think I already know the answer to, but so as not to fry anything can anyone confirm that to wire the secondary coils in parallel from the attached diagram it would be red with yellow and black with orange?
    Thanks in advance .....Chris
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    Last edited by chillybo; 09-07-2019 at 11:14 AM.

  2. #2
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,243. Received thanks 234 times, giving thanks to others 5 times.
    Looks about right to me.

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  4. #3
    Next question......Im using a 750VA toroidal with 36 volt output. Smoothing capacitor is 22,000 microfarads. Would it be a good idea to use an NTC thermistor on the primary circuit to guard against inrush current on initial switch on?
    Thanks in advance ....Chris
    Last edited by chillybo; 09-07-2019 at 12:59 PM. Reason: incorrect predictive correction

  5. #4
    Often a good idea to do so. However, you may find that every so often you trip the breaker when you switch the power to a transformer. It's not usually due to the capacitive load so much as the primary saturating. But a nice surge current can also stress the rectifiers. If you are suffering from tripping breakers, read on....

    When you disconnect the mains, there is usually some residual magnetisation remaining in the cores ("remanence") at the moment of disconnection. The polarity and magnitude of that remanence varies randomly depending on the conditions when you disconnected the supply but that magnetic field persists while the equipment is powered off. When you come along next time and reconnect the supply, if the conditions are right you can further add to that residual field such that the cores saturate and the primary current is then limited only by the low resistance of the wiring and winding. You can prove to yourself that the secondary circuit isn't the cause by completely removing it and finding that the problem persists. The magnitisation flux within the core of a typical mains transformer is often very close to the saturation flux.

    You can buy soft starters for transformers but they tend to be industrial ie f expensive. The simple solution is a fixed resistor or NTC with a timer relay (to short it out after it has had time to get the core flux balanced, eg 1/2 second or so) in series with the primary. If you go the fixed resistor route, you have to get the resistance low enough to generate a useful AC flux in the transformer but high enough to avoid the problem you are trying to address. For my 10kW single phase TIG welder, I found 2.2 Ohms was about right. I added a short delay to the primary contactor using a standard timer (delay) relay.
    Last edited by Muzzer; 09-07-2019 at 04:13 PM.

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  7. #5
    The timer relay along with the NTC sounds interesting. I will research timer relays to try and find what I need but aren't they a fairly standard component for this purpose? i.e. taking over from the thermistor once it has done its job so that the thermistor isn't always on and running hot. I'm guessing that the relay kicks in when the thermistor resistance drops as it gets hot (and current increases to trigger it) then it runs in parallel with the thermistor but breaks the thermistor circuit? Ive just about finished the mechanical side to my build and now im trying to get my head around the electrical / electronics side of things.
    Kind regards ...Chris

  8. #6
    Just had a quick look and found adjustable delay relays such as this
    https://uk.rs-online.com/web/p/products/8966810/
    So i'm guessing that if the thermistor is wired parallel to this, then when the delay relay kicks in the thermistor is then 'shorted' and no longer works until the next time the PSU is switched on?

    Regards ...Chris

  9. #7
    They look pretty good. I don't think RS sold those when I did mine. I used a Finder one which costs more https://uk.rs-online.com/web/p/timer-relays/0221086/

    Yes, that's how they work. The nice thing about an NTC is that you don't need to worry so much about the value. If you aren't getting nuisance tripping of breakers, it should be fairly straightforward.

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  11. #8
    I think the price difference could be related to the current ratings 16 amp versus 6 amp.

  12. #9
    No, it's because RS Pro is an "in house" brand which by definition is less expensive than a main line industrial product. The cost difference between 6A and 16A contacts is only a small part of the total.

    Here's a 6A nd 16A, both by RS Pro. Not such a bit difference. https://uk.rs-online.com/web/c/autom...949,4294572463

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