Author Topic: Designing a capacitive AC voltage dropper  (Read 2583 times)

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Offline 3DogsTopic starter

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Designing a capacitive AC voltage dropper
« on: December 12, 2024, 06:06:24 pm »
I am using a potted power supply module that takes 90-305VAC and outputs 12V (at 1.8A). I would really like to be able to run it on 480VAC if necessary. I’m thinking that the right value capacitor in series with the input will drop the voltage from 480V to something more reasonable - let’s say 50%, or 240VAC.
The problem is - the power supply is (literally) a black box. I don’t know what the input circuit looks like, but I don’t think they have any power factor correction, so it’s probably just a full-wave bridge and a capacitor.
I’m looking for suggestions on the best way to make this capacity voltage dropper.
 

Offline IanB

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Re: Designing a capacitive AC voltage dropper
« Reply #1 on: December 12, 2024, 06:26:29 pm »
This is probably not the right solution. A series capacitor is not really going to provide any regulated voltage reduction, it would just work as a current limiter. It could still allow you to blow up your power supply with an over-voltage input.

The better solution is a step-down transformer, from say 480 V to 240 V. This will give you proper isolation of the 480 V and a regulated output voltage.
 

Offline 3DogsTopic starter

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Re: Designing a capacitive AC voltage dropper
« Reply #2 on: December 12, 2024, 07:28:55 pm »
That’s not going to work.
Nobody makes transformers small enough, and the big ones are enormously expensive (and too big).

I wonder if I could make a TRIAC phase-angle chopper...
« Last Edit: December 12, 2024, 07:31:47 pm by 3Dogs »
 

Offline 3DogsTopic starter

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Re: Designing a capacitive AC voltage dropper
« Reply #3 on: December 13, 2024, 12:56:09 am »
I think I’m going to use a simple leading-edge Triac dimmer, and pick the RC values to keep the peak voltage under 240V or so (well below the maximum voltage the SMPS can withstand). I’ll add a power resistor in series to keep the inrush current to something reasonable (I’ll need to take some measurements with the power supply fully loaded). This is only a handful of components, and should do what I need.
I think.
 


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