Rectifying AC mains

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Thread Starter


Joined Oct 27, 2015
Hello! This is my first post here and i am building a power supply by rectifying AC mains. The mains electricity 220vac will be connected to a 20A variac and then a bridge rectifier then a capacitor bank consisting of 8 1500uf 400v capacitors, giving around 12000uf in total. I am trying to use this to power a solenoid with around 15 A current. Do i need to add any current limiting resistor or inductor? Thanks in advance!


Joined Feb 5, 2010
You might consider a 10 ohm high wattage resistor for power up purposes. That would limit in rush current to about 20 amps. After the caps charge it can be bypassed with a switch or by relay.


Joined Dec 1, 2012
How about some NTC thermistors for inrush limiting? I've read app notes that recommend it but never built it.


Joined Mar 30, 2015
If you don't want to be bothered with switching out a surge limiting resistor, NTC thermistors are the way to go. I have some 10 ohm 4A that drop down to less than half an ohm at 50% of rated current and 0.22Ω at 100%.

Hope this answers your question because transformerless AC mains circuits are prohibited and this thread will be shutdown soon.


Joined Nov 23, 2012
Hope this answers your question because transformerless AC mains circuits are prohibited and this thread will be shutdown soon.
I'm not so sure. He said he was connected to a variac. Not completely isolated but, I think he is within the rules.


Joined Mar 14, 2008
A variac is a non-isolated supply and likely not within the rules.
To minimize the chance of electrocution when experimenting with a variac output without an isolation transformer, connect it to a ground-fault isolator (GFI) outlet or with a GFI cord such as this.
You can still get zapped, but it's not as likely to be fatal.
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Joined Jun 17, 2014

Yes good idea, about the isolation transformer. They also make variacs that are isolated.

But any voltage over 20 volts could be dangerous, and 220v certainly is over 20 volts so much care in handling of the circuit components with the power off is a good idea.

Also, and i cant stress this enough, wear eye protection and stand away from the electrolytic caps especially when the power is turned on. I've seen caps blow their tops like little cannons, spewing all kinds of materials out. They sound like a gun going off too, so maybe ear protection as well. It will scare the **** out of you if it happens :)


Joined Sep 9, 2010
I am trying to use this to power a solenoid with around 15 A current.
You didn't mention what your solenoid voltage is. If you don't need the variable voltage given by the variac, it might be simpler and safer to just use a simple step-down transformer.


Joined Nov 4, 2013
I would say your design for a line powered variable DC power supply will work just fine.

I see no reason for any inrush current limiting devices due to the amount of capacitance you are working with simply being you have a Variac and can start each power up at zero volts.


Joined Nov 4, 2013
At that power level I tend to consider them as just starting to be fun to play with! :D


Joined Aug 23, 2012
Welcome to the AAC forum.

The owners of All About Circuits has elected not to host discussions of transformerless threads.

This thread is against the AAC forum rules, Terms of Service (ToS).

Any kind of over-unity devices and systems
Devices designed to electrocute or shock another person
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Transformer-less power supplies
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