Soft Start Advice

[paulus.v]

I do not want to start another thread for the same question...

I am building a soft start for my 630VA toroid. I want to go with 3 resistors in parallel. Which value to choose from these: 5W 100R; 5W 150R or 10W 100R?

Regards,
Paul

[m_c]

You'll need to check the datasheet for the resistors.

The simplist option is to treat the load at switch on as a pure short circuit, which then gradually reduces to normal load.
So assuming, 240V, with a 100R resistor, you're going to have a 2.4A flow producing 576W of heat.
At 150R, current and power is reduced to 1.6A and 384W.
These loads should only remain for under a second, so you need to look at the datasheet and see what the surge/intermittent max dissipation is for each resistor.
The 5 and 10W is the continuous disipation rating, but they will have a much higher intermittent rating, typically given as 5, 10 and 60second intervals.

Also keep on eye out for any maximum current ratings, otherwise your resistor will pop like a fuse before it even warms up.

[paulus.v]

It seems there is a lot of power involved...

The resistors are the most common and cheap ones and I cannot find any current or intermittent ratings.

The resistors are looking like these

[cropwell]

I am a little perplexed why a soft start is required, if it is to avoid a spike at switch on/off then a triac with a zero crossing detector would seem to be better.

[Jonathan]

I am a little perplexed why a soft start is required, if it is to avoid a spike at switch on/off then a triac with a zero crossing detector would seem to be better.

For 630VA you'd probably get away with it. More interestingly, the best point to switch on is actually when the mains voltage is at a peak - see here.

[paulus.v]

For 630VA you'd probably get away with it.

I thought the same but for no apparent reasons I am unable to turn on my router. My 16A main fuse is tripping. After a few attempts I managed to get it on but later on trying again I manage to blow the T4A fuse at the toroid.

The only reasons I could think about are the temperature drop in the workshop (7°C) or some alterations on the mains electricity supply.

[EddyCurrent]

Take a look at this;
http://www.ft-transformers.co.uk/wp-...ction-50VA.pdf

[paulus.v]

The T4A fuse was just fine, I'll change it with 6.3

I have to check if the MCB is type D. Until now I did not know that there are different types of mcb's based on the instantaneous tripping current.

But my question remains, if I want to implement a soft start will the cheap 10W resistors work?

[cropwell]

Now, after a little bit of digging around t'Interweb, I realise the problem- it's big toroidal transformers needing a high-current fix at switch on. As I haven't used a toroid for anything other than power supplies for pre-amps and mixers I have not had a problem like that to solve. I must pull the case off one of my 1KW PA amps to see what that uses to switch on the power to the toroid in it.

Never got round to that Laplace stuff at Uni, it was over 50 years ago and I studied Geomorphology

[m_c]

A contributing issue is CNC power supplies have a nice big capacitor to charge from nothing at switch on, which contributes quite heavily to the surge.
Placing a resistor inline, limits the surge until everything reaches working voltage.