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Old 9th March 2015, 08:58 PM   #1
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Default Cold start inrush limiter.

I have just come up with a design for a inrush limiter.
It uses a PIC micro to control a triac.
The PIC increases the phase angle the triac is on until reaches full power.
If the mains disappears it holds the triac off until it reappears then goes through power up phase again.

I have added a resistor across the output to ensure triac holding current is always met even on small loads.

I have snubbered the triac to help control it.
Click the image to open in full size.

Last edited by nigelwright7557; 9th March 2015 at 11:18 PM.
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Old 9th March 2015, 11:18 PM   #2
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Just ordered some pcb's so will have to wait a couple of weeks before I can play with it.
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Old 27th March 2015, 01:16 AM   #3
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PCB's arrived OK.
Built one up.
It works fine with a mains lamp. I can see the lamp getting brighter over 2 seconds then it stays on full brightness.
Tried it with a 250VA transformer on an amplifier and after the initial phase ramp up
period the transformer rattles as the triac doesn't latch with gate pulses.
I looked up my triac datasheet and its holding current is 50mA.
This means the mains voltage has to reach 120V before the triac will latch properly.

I have ordered some more mains triacs but with 15mA holding current.
Should be here in a day or two.
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Old 27th March 2015, 10:36 AM   #4
AndrewT is online now AndrewT  Scotland
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Is it possible to change the background BLACK to background WHITE in your PCB software?

A new Thread with some user guidance would be good.
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Old 27th March 2015, 10:37 AM   #5
AndrewT is online now AndrewT  Scotland
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Is there a way to post your circuit (an export facility).
Can we import that file to see your circuit on our PCs?
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Old 27th March 2015, 10:01 PM   #6
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The circuit is in PCBCAD360.
It can output a bitmap but thats it.
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Old 27th March 2015, 10:04 PM   #7
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Got some triacs with lower latching current but the transformer still rattles on full power.

So I have changed the software from give a 1ms triac pulse to giving continuous 15uS pulses until the end of the current mains phase. The circuit now works a treat.

I have tested the circuit with a 2000watt heater and it ramps up and works fine.
I have also tested it wit ha 250 watt amplifier and it behaves very well.
Click the image to open in full size.

Last edited by nigelwright7557; 27th March 2015 at 10:06 PM.
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Old 28th March 2015, 12:29 AM   #8
Zero D is offline Zero D  United Kingdom
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Hi, glad you got it sorted ! Why not replace the 3k3 7W dropper resistors etc with a simple reactive PS

I converted the pic to a white background
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File Type: png Zero D coldstart.png (224.4 KB, 49 views)
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Old 28th March 2015, 12:32 AM   #9
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A capacitive dropper is quite often used in this type of circuit.
I was a little worried about what happens if there is HF noise on the mains and it will go straight through the dropper capacitor and might take out the zener.
A 470nF high voltage capacitor would have worked OK.
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Old 28th March 2015, 12:38 AM   #10
Zero D is offline Zero D  United Kingdom
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I've made reactive PS's with full a wave bridge & regulator in the past, with no issues
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