Wiring toroidal transformer in parallel

[chillybo]

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

[chillybo]

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

[Muzzer]

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.

[chillybo]

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

[Muzzer]

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

[Gytis]

what about those soft starters for the toroidal transformers ?

www.ebay.ie/itm/Inrush-current-limiter-230V-16A-for-transformers-power-supplies-LED/231650038237

www.ebay.ie/itm/Soft-start-inrush-current-limiter-for-toroidal-transformers-assembled-tested/132765338869

[chillybo]

Thanks, guys. I have just wired the AC to DC transformer bits together for the first time and connected it to the mains. I have not yet used a 'delay on' timer relay and just wired an NTC thermistor on the live between mains in and the primary winding. On first switch on nothing dramatic happened and I am getting 53 volts at the DC output which is close to what I wanted. I was surprised that the thermistor did not get hot like I thought they were supposed to. Could this be because there is no load on the DC output? Similarly the bridge rectifier does not seem hot or even warm. Is this normal? The thermistor is a SL15 2R507 that Airlink transformers recommend for toroidals up to 1kVA
Regards ...Chris

[Muzzer]

The second one looks like a bit of a home project, TBH. The first (German) one looks pretty sensible, containing an NTC, timer relay and overtemp protection.