Soft Start Advice

[GTJim]

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

[EddyCurrent]

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!

[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...

[JAZZCNC]

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...

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

[EddyCurrent]

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.

[GTJim]

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

[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.