Soft Start Advice

I need a bit of advice as I’m getting slightly confused.
As you can see from the sketch below I want to include a basic soft start circuit into my control panel, but I have the main toroidal transformer, a 24 volt power supply and a 5 volt power supply.

If I ignore the 24v and 5v power supplies for now, I calculated the resistor required as HS50 33R. From the following:

I = VA / V
I = 750 / 230 = 3.26 amps
With a limit of 200% of the power current = 6.52 amps

R = V / I
R = 230 / 6.52 = 35.28 ohms

As I need both the 24 and 12 volt supplies, my questions are:

• Should I connect them up as shown, after the soft start or after the safety relay?
• Do I need to include the additional power supplies in the calc for a suitable resistor, and if so how?
• The HS50 refers to watts, I have only used this as it has been suggested in an article I read. Is this correct?
• I am hoping to find a suitable relay with a timer, can anyone point me in the right direction?

Regards,
Jim

I would connect the 24v and 5v supplies after the safety relay but before the soft start.
Resistor value is not that critical, you might have to try a few values to get the best for your setup.
That's the maximum wattage, and hence current at the rated voltage, the unit can supply so when it first powers up the load current will be less because the steppers will be drawing very little current.

This seller has some good timers at great prices and keeps some in the UK, then you just need to select a relay with correct coil voltage and amps rating on the contacts. I didn't look through the list of timers but there may be one with suitable contacts already built in, you'll have to look.
timer switch, 60 Sec items in timer store on eBay!

If the 5 and 24V supplies are only being used to power sensors/switches and no motion, I'd personally put them before the saftey relay, as you usually want to have inputs active before you power up the safety relay...

Quote:

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Originally Posted by m_c

If the 5 and 24V supplies are only being used to power sensors/switches and no motion, I'd personally put them before the saftey relay, as you usually want to have inputs active before you power up the safety relay...

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Agree with M_C put the aux Psu's before so the BOB etc stays powered.

It's true the safety relay needs power but from the attached drawing it wasn't clear how the relay coils were supplied, also I thought it a good idea to kill everything when the safety relay was tripped.

Thanks for the advice. Now to start laying it out to see what size cabinet I need.

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

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.

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

http://2.bp.blogspot.com/-siNKt-GYwg...SoftStart1.JPG

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.

Quote:

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Originally Posted by 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.

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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.

Quote:

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Originally Posted by Jonathan

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

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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.

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

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?

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 :stupid:

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.

Quote:

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Originally Posted by paulus.v

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

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Have a read through this :- http://sound.westhost.com/project39.htm there are quite a few posts on this forum regarding " soft start " I have had many in my time :joker: usually after a :beer: .. Clive

Steady now, remember what happened last time http://www.mycncuk.com/threads/8251-...5238#post65238
I'm currently on with that myself but it's black nectar :beer:

Quote:

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Originally Posted by EddyCurrent

Steady now, remember what happened last time http://www.mycncuk.com/threads/8251-...5238#post65238
I'm currently on with that myself but it's black nectar :beer:

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I just can't think what you are on about !! ..Clive