Circuit to soft-start 300 watts of lighting?

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NewUser

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Something to bring them from off to full brightness in around a second would be spiffy.

I already have a 500 watt dimmer - is there such a beast as an automatic and time-adjustable "electronic pot" that I could build and just substitute for this dimmer's pot?
 
How does the 500 watt dimmer work? If it runs off of a signal from a POT then you could replace the POT with a large capacitor and a resistor tuned to go from 0 volts to full voltage over a short period of time.
 
Sceadwian said:
How does the 500 watt dimmer work? If it runs off of a signal from a POT then you could replace the POT with a large capacitor and a resistor tuned to go from 0 volts to full voltage over a short period of time.
Click to expand...

http://www.geocities.com/tomzi.geo/dimmer/dimmer.gif

Assuming this circuit is similar to what I have, would it be as simple as
replacing the 470k pot with a series-connected capacitor and resistor?
 
You still have to control the FET even if you can find one that works high voltage.

I would suggest a simple and effective solution using optical feedback. It involves using an additional small wattage(1~3W) 230V filament lamp and a LDR(light dependent resistor). Mount the LDR close to the lamp so that the lamp shines onto the LDR, but the setup should be light tight.

Set your dimmer POT so that the small filament lamp just start emitting light. Due to feedback action, the lamp(and the 300W external lights) will eventually goes full brightness.
 

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NewUser said:
Would a FET work in the above 230V circuit as a substitute for the pot?
Click to expand...

hi,
No.
Have a look at this circuit I have added a NTC thermistor and a 'heating' resistor, this would give an initial slow start.
 
Last edited:
Digikey sells "inrush current limiters", basically very low resistance NTC thermistors. For 300W 230V you might get KC013L-ND which starts out cold at 50 ohms and then heats up to about 0.75 ohms. $2 or less.

There's a similar device in most PC SMPSes that I've taken apart.
 
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