# soft start help

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#### pmchoong

Hi All

I have this simple soft start scheme which I modify to include a LED. My question is the resistor 220R sufficient for the LED?

Thanks

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#### pmchoong

original scheme

This is the original scheme.

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#### Frank Berry

If the power supply is putting out 9.1 volts, the resistor value is a bit low.
A resistor in the range of 820 ohms to 1k ohms would probably be better.

#### pmchoong

Hi Frank

Thanks for helping. The soft start is for a500Va transformer. The main supply is from 220v. I have remove the zener and replace with only a 20k resistor. Will this work?
Thanks again.

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#### Frank Berry

Measure the voltage of the power supply before you install the LED.
Make sure that the relay coil is in the circuit or the voltage will be very high.
What is the d.c. resistance and operating voltage of the relay coil? That will determine the resistance of R1 and R2.
I'm guessing that 20k is too much resistance for the LED.

#### pmchoong

Hi Frank

Below is the spec:
R1, R2*.............................470K 0.25W 1% mf
R3*..................................220R 0.25W 1% mf
R4, R5, R6, R7...................10R 5W
C1...................................330n 250V (for direct mains connection)
C2, C3............................. 470uF 40V
B1.................................. B250C1500
Re1.................................24V (250V-8A contact)
F1*..................................2A slow blow

#### Frank Berry

I've simulated the circuit. Depending on the relay coil resistance, you may need to trim R1 and/or R2 to obtain something near 24 volts on the relay coil.
The resistor feeding the LED should be about 1.2k/1.5k.
A .25 or .5 watt resistor is good.

#### pmchoong

Hi Frank

Thanks again. I'll change it to 1.5k 5w. What value is suitable for R1 and R2? The mains is at between 220-230v. I have connect a voltage regulator which will output at close to 240v to the soft start.

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#### Frank Berry

Don't need a 5 watt resistor. .5 (1/2) watt is big enough.
The value of R1 and R2 depends on the coil resistance of the relay.
Increasing the value of C1 will also increase the voltage to the relay coil.
Experiment with the values to obtain something near 24 volts on the relay coil.

#### pmchoong

Hi Frank.

Thanks for your time and advice. I appreciate your help. Will try other value with R1/R2 to obtain 24v.

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#### sathit dokmadue

Add 24v ZANER DIODE to control the voltage constant.

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#### metal

I am interested to know much limitation do these resistors provide before the load is connected by relay.

#### Frank Berry

I am interested to know much limitation do these resistors provide before the load is connected by relay.

That would depend upon the amount of current being drawn by the load through the resistors.

#### metal

I see, then we have to calculate the bypass resistors values before we use the circuit because resistors might end up flamed if wrong values used. For a transformer, the power rating is enough to be used for these calculations?

Also what happens if I use 1000uF/50V capacitor instead of two 470uF/40V capacitors?

#### AndrewT

you have an extra 40r in the primary circuit.

#### pmchoong

Thanks Sathit.
Metal, is that a loudspeaker protection circuit for a mono? What is the ac require to run it?
Thanks

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#### pmchoong

It doesn't work with the zener and 1.5k resistor in. How much do i need to trim on R1 and R2?

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#### AndrewT

Pm,
be very careful with this circuit.
It is connected directly to the mains.
Don't touch any parts while it is plugged into the mains supply.

The 470k+470k do not supply much current.
If your mains is 230Vac and you use 24V for the relay drive, you have 215Vac to drive current through the 940k, I=V/R = ~0.23mA

The main current to drive the circuit comes through the capacitor, it must be X rated.
Find their AC impedance at your mains frequency and using the same formula find out how much AC current passes the capacitor.
Is that sufficient to drive the LED and to drive the relay?

#### peranders

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