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Old 12th September 2010, 12:21 PM   #1
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Default soft start help

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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Old 12th September 2010, 12:23 PM   #2
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Default original scheme

This is the original scheme.
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Old 12th September 2010, 01:41 PM   #3
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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.
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Old 12th September 2010, 02:02 PM   #4
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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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Old 12th September 2010, 02:14 PM   #5
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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.
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Old 12th September 2010, 02:41 PM   #6
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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
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Old 12th September 2010, 02:54 PM   #7
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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.
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Old 12th September 2010, 03:19 PM   #8
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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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Old 12th September 2010, 03:35 PM   #9
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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.
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Old 12th September 2010, 03:52 PM   #10
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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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