Author Topic: Question about using mains power and variacs to power this circuit  (Read 425 times)

Offline Michelle_

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So for DC this makes sense to me, but say one wants to power this with a variac from mains... Am I right in understanding that:

1.) You need an isolation transformer

2.) You would hook "line" [after the isolation transformer] (either of the wires) to where DC+ would go

3.) You would hook "neutral" [after the isolation transformer] (the other wire) Up to the grounded point of the circuit, which would also be hooked up to mains GND

4.) Question: Would you want to put the isolation transformer before or after the variac?

5.) Is there anything dangerous to be aware of if attempting this?

Offline unrealcrafter2

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Re: Question about using mains power and variacs to power this circuit
« Reply #1 on: May 09, 2024, 12:48:57 AM »
I don't think you Need to use an isolation transformer. It just helps with safety. Unless you are using an oscilloscope you can probably get away without an isolation traffo. Id personally just use a lab bench power supply instead.

Mainly if you run it un isolated make sure you don't hook the Hot to the GND

If you already have an isolation transformer it would probably be better before the variac so you aren't underpowering it. Not sure about this

Also general note only touch a HV circuit with one hand at a time
« Last Edit: May 10, 2024, 03:33:35 PM by unrealcrafter2 »

Offline Michelle_

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Re: Question about using mains power and variacs to power this circuit
« Reply #2 on: May 09, 2024, 02:00:28 AM »
I don't think you Need to use an isolation transformer. It just helps with safety. Unless you are using an oscilloscope you can probably get away without an isolation traffo. Id personally just use a lab bench power supply instead.

Mainly if you run it un isolated make sure you don't hook the Hot to the GND

If you already have an isolation transformer it would probably be better before the variac so you aren't underpowering it

Also general note only touch a HV circuit with one hand at a time


I have a lab bench power supply, it outputs DC, I'm talking about powering this with 48VAC by lowering mains (110) voltage with a variac. Hooking the variac up with neutral to ground trips the GFCI, and the video where the designer of the circuit sort of explains what's going on here mentions using an isolating transformer, which is why I'm trying to make sure I understand what's going on instead of trying a circuit without GFCI (if I even have one which I may not).

I already know about touching things with one hand I'm asking for information specifically about running neutral from an isolated power supply to mains ground and whether or not to hook the variac up before (where it is grounded to mains) or after the isolation transformer (which has a 2 prong outlets with no ground).

I've never seen a circuit like this before and don't typically mess with mains power so I'm trying to learn more.

« Last Edit: May 09, 2024, 02:02:01 AM by Michelle_ »

Offline Michelle_

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Re: Question about using mains power and variacs to power this circuit
« Reply #3 on: May 09, 2024, 02:16:30 AM »
I was intrigued by this circuit when I stumbled across it so I built it since I had almost all the parts. It works great with DC so I want to try AC now...

Might post a thread on it later but for now it's just in parts and the top load/base isn't finished but it works well especially for lighting up tubes.

(also I switched to a WAY bigger heat sink and a fan)

Offline Mads Barnkob

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Re: Question about using mains power and variacs to power this circuit
« Reply #4 on: May 09, 2024, 01:05:26 PM »
A variac / auto transformer is not isolated from mains and you can therefore NOT ground the neutral. Grounding neutral or negative on a isolation transformer or DC power supply, is to get all voltages to refer to a common earth. So you are sure to have a path to ground for fault currents. This is called PELV or SELV.

Isolation transformer after variac, so that peak loads are at a higher voltage, low voltage high current tends to damage the rollers in a variac.

All work with electricity is dangerous.
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Offline Michelle_

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Re: Question about using mains power and variacs to power this circuit
« Reply #5 on: May 09, 2024, 03:06:19 PM »
A variac / auto transformer is not isolated from mains and you can therefore NOT ground the neutral. Grounding neutral or negative on a isolation transformer or DC power supply, is to get all voltages to refer to a common earth. So you are sure to have a path to ground for fault currents. This is called PELV or SELV.

Isolation transformer after variac, so that peak loads are at a higher voltage, low voltage high current tends to damage the rollers in a variac.

All work with electricity is dangerous.

Thanks Mads I see what's going on now I think. I didn't know there was a name for that practice but that makes it easier to look into. Now that I think about it I've seen neutral to ground in control cabinets sometimes after trasnsformers and wasn't sure what to make of it, but now I realize why they did it and why it works.

Makes sense about putting the transformer after the variac to not damage it, that's why I asked before I went and just did it.

Thanks
« Last Edit: May 09, 2024, 03:53:20 PM by Michelle_ »

Offline Michelle_

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Re: Question about using mains power and variacs to power this circuit
« Reply #6 on: May 09, 2024, 03:42:49 PM »
Also just some feedback, I don't know what you guys do for work, but if someone asks you a question about the safety of a specific task they're doing, please give them specific actionable advice because a generic answer is unhelpful as far as keeping that person safe. If they are asking you, then they already know there's something to be aware of and they need specific and actionable information. I would usually say "there's nothing specific to watch out for besides following typical safety procedures" just so the person isn't wondering if there's something they could be missing, and it reassures them that they understand what they're doing which helps their confidence.

- Someone that manages and trains engineers and technicians
« Last Edit: May 09, 2024, 03:46:09 PM by Michelle_ »

Offline klugesmith

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Re: Question about using mains power and variacs to power this circuit
« Reply #7 on: May 09, 2024, 07:10:19 PM »
On the topic of isolation, since oscilloscopes came up...

Let's say someone had a battery powered oscilloscope with an ethernet port for communicating with a PC.
The scope ground could be connected to hot side of AC line, and copper-wired Ethernet would still work, completely within spec. 
Thanks to its mandatory isolation transformers, usually built into the board mounted "RJ45" connectors.
It's an exercise I have never personally done or seen done.  :)
[edit]
https://www.we-online.com/en/components/products/WE-LAN-RJ45
I had forgotten about power over Ethernet, but I bet that would also work and be within spec.
« Last Edit: May 09, 2024, 07:13:52 PM by klugesmith »

Offline Michelle_

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Re: Question about using mains power and variacs to power this circuit
« Reply #8 on: May 10, 2024, 05:08:38 AM »
On the topic of isolation, since oscilloscopes came up...

Let's say someone had a battery powered oscilloscope with an ethernet port for communicating with a PC.
The scope ground could be connected to hot side of AC line, and copper-wired Ethernet would still work, completely within spec. 
Thanks to its mandatory isolation transformers, usually built into the board mounted "RJ45" connectors.
It's an exercise I have never personally done or seen done.  :)
[edit]
https://www.we-online.com/en/components/products/WE-LAN-RJ45
I had forgotten about power over Ethernet, but I bet that would also work and be within spec.

That's interesting. Ethernet standards are pretty amazing actually. What a great invention.


Anyway I ran the circuit on AC. I guess I'll post a thread on it in case anyone in the future tries to build one. I'm ultimately going to repurpose the secondary though.

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Re: Question about using mains power and variacs to power this circuit
« Reply #8 on: May 10, 2024, 05:08:38 AM »

 


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